# Patent

## Title
Read XF instruction for processing vectors

## Abstract

The described embodiments include a processor that handles faults. The processor first receives a first input vector, a control vector, and a predicate vector, each vector comprising a plurality of elements. For each element in the first input vector for which a corresponding element in the control vector and the predicate vector are active, the processor then performs a read operation using an address from the element of the first input vector. When a fault condition is encountered while performing the read operation, the processor determines if the element is a first element where a corresponding element of the control vector is active. If so, the processor handles/processes the fault. Otherwise, the processor masks the fault for the element.

## Background
RELATED APPLICATIONS
This application is a continuation in part of, and hereby claims priority under 35 U.S.C. §120 to, pending U.S. patent application Ser. No. 12/873,063, entitled “Non-Faulting and First-Faulting Instructions for Processing Vectors,” by inventor Jeff Gonion and Keith E. Diefendorff, filed 31 Aug. 2010. This application is also a continuation in part of, and hereby claims priority under 35 U.S.C. §120 to, pending U.S. patent application Ser. No. 12/541,546, entitled “Running-Shift Instructions for Processing Vectors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed 14 Aug. 2009. This application further claims priority under 35 U.S.C. §120 to U.S. provisional patent application No. 61/089,251, entitled “Macroscalar Processor Architecture,” by inventor Jeffry E. Gonion, filed 15 Aug. 2008, to which the parent application Ser. Nos. 12/541,546 and 12/873,063 also claim priority. These applications are each herein incorporated by reference.


This application is related to: (1) pending application Ser. No. 12/419,629, entitled “Method and Apparatus for Executing Program Code,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed on 7 Apr. 2009; (2) pending application Ser. No. 12/419,644, entitled “Break, Pre-Break, and Remaining Instructions for Processing Vectors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed on 7 Apr. 2009; (3) pending application Ser. No. 12/419,661, entitled “Check-Hazard Instructions for Processing Vectors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed on 7 Apr. 2009; (4) pending application Ser. No. 12/495,656, entitled “Copy-Propagate, Propagate-Post, and Propagate-Prior Instructions For Processing Vectors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed on 30 Jun. 2009; (5) pending application Ser. No. 12/495,643, entitled “Shift-In-Right Instructions for Processing Vectors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed on 30 Jun. 2009; (6) pending application Ser. No. 12/495,631, entitled “Increment-Propagate and Decrement-Propagate Instructions for Processing Vectors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed on 30 June 2009; (7) pending application Ser. No. 12/541,505, entitled “Running-Sum Instructions for Processing Vectors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed on 14 Aug. 2009; and (8) pending application Ser. No. 12/541,526, entitled “Running-AND, Running-OR, Running-XOR, and Running-Multiply Instructions for Processing Vectors” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed on 14 Aug. 2009.
This application is also related to: (1) pending application Ser. No. 12/873,043, entitled “Running-Min and Running-Max Instructions for Processing Vectors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed 31 Aug. 2010; (2) pending application Ser. No. 12/873,063, entitled “Non-Faulting and First-Faulting Instructions for Processing Vectors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed 31 Aug. 2010; (3) pending application no. 12/873,074, entitled “Vector Test Instruction for Processing Vectors” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed 31 Aug. 2010; (4) pending application Ser. No. 12/907,471, entitled “Select First and Select Last Instructions for Processing Vectors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed 19 Oct. 2010; (5) pending application Ser. No. 12/907,490, entitled “Actual Instruction and Actual-Fault Instructions for Processing Vectors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed 19 Oct. 2010; (6) pending application Ser. No. 12/977,333, entitled “Remaining Instruction for Processing Vectors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed 23 Dec. 2010; (7) pending application Ser. No. 13/006,243, entitled “Remaining Instruction for Processing Vectors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed 13 Jan. 2011; (8) pending application Ser. No. 13/189,140, entitled “GetFirst and AssignLast Instructions for Processing Vectors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed 22 Jul. 2011; (9) pending application Ser. No. 13/291,931, entitled “Vector Index Instruction for Processing Vectors,” by inventor Jeffry E. Gonion and Kieth E. Diefendorff, filed 8 Nov. 2011; (10) pending application Ser. No. 13/343,619, entitled “Predicate Count and Segment Count Instructions for Processing Vectors” by inventor Jeffry E. Gonion, filed on 4 Jan. 2012; (11) pending application Ser. No. 13/414,606, entitled “Predicting Branches for Vector Partitioning Loops when Processing Vector Instructions” by inventor Jeffry E. Gonion, filed on 7 Mar. 2012; (12) pending application Ser. No. 13/456,371, entitled “Running Unary Operation Instructions for Processing Vectors” by inventor Jeffry E. Gonion, filed on 26 Apr. 2012; (13) pending application Ser. No. 13/463,454, entitled “Running Multiply Accumulate Instruction for Processing Vectors” by inventor Jeffry E. Gonion, filed on May 3, 2012; (14) pending application Ser. No. 13/479,097, entitled “Confirm Instruction for Processing Vectors” by inventor Jeffry E. Gonion, filed on May 23, 2012; (15) pending application Ser. No. 13/484,666, entitled “Conditional Extract Instruction for Processing Vectors” by inventor Jeffry E. Gonion, filed on May 21, 2012; and (16) pending application Ser. No. 13/484,709, entitled “Value Check Instruction for Processing Vectors” by inventor Jeffry E. Gonion, filed on May 31, 2012.
This application is also related to: (1) pending application Ser. No. 12/237,212, entitled “Conditional Data-Dependency Resolution in Vector Processors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed 24 Sep. 2008; (2) pending application Ser. No. 12/237,196, entitled “Generating Stop Indicators Based on Conditional Data Dependency in Vector Processors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed 24 Sep. 2008; (3) pending application Ser. No. 12/237,190, entitled “Generating Predicate Values Based on Conditional Data Dependency in Vector Processors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed 24 Sep. 2008; (4) application Ser. No. 11/803,576, entitled “Memory-Hazard Detection and Avoidance Instructions for Vector Processing,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed 14 May 2007, which has been issued as U.S. Pat. No. 8,019,976; and (5) pending application Ser. No. 13/224,170, entitled “Memory-Hazard Detection and Avoidance Instructions for Vector Processing,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed 14 May 2007.
BACKGROUND
1. Field
The described embodiments relate to techniques for improving the performance of computer systems. More specifically, the described embodiments relate to a ReadXF instruction for processing vectors.
2. Related Art
Recent advances in processor design have led to the development of a number of different processor architectures. For example, processor designers have created superscalar processors that exploit instruction-level parallelism (ILP), multi-core processors that exploit thread-level parallelism (TLP), and vector processors that exploit data-level parallelism (DLP). Each of these processor architectures has unique advantages and disadvantages which have either encouraged or hampered the widespread adoption of the architecture. For example, because ILP processors can often operate on existing program code that has undergone only minor modifications, these processors have achieved widespread adoption. However, TLP and DLP processors typically require applications to be manually re-coded to gain the benefit of the parallelism that they offer, a process that requires extensive effort. Consequently, TLP and DLP processors have not gained widespread adoption for general-purpose applications.
One significant issue affecting the adoption of DLP processors is the vectorization of loops in program code. In a typical program, a large portion of execution time is spent in loops. Unfortunately, many of these loops have characteristics that render them unvectorizable in existing DLP processors. Thus, the performance benefits gained from attempting to vectorize program code can be limited.
One significant obstacle to vectorizing loops in program code in existing systems is dependencies between iterations of the loop. For example, loop-carried data dependencies and memory-address aliasing are two such dependencies. These dependencies can be identified by a compiler during the compiler's static analysis of program code, but they cannot be completely resolved until runtime data is available. Thus, because the compiler cannot conclusively determine that runtime dependencies will not be encountered, the compiler cannot vectorize the loop. Hence, because existing systems require that the compiler determine the extent of available parallelism during compilation, relatively little code can be vectorized.
SUMMARY
The described embodiments include a processor that handles faults. The processor first receives a first input vector, a control vector, and a predicate vector, each vector comprising a plurality of elements. For each element in the first input vector for which a corresponding element in the control vector and the predicate vector are active, the processor then performs a read operation using an address from the element of the first input vector. When a fault condition is encountered while performing the read operation, the processor determines if the element is a first element where a corresponding element of the control vector is active. If so, the processor handles/processes the fault. Otherwise, the processor masks the fault for the element.
In some embodiments, when masking the fault for the element, the processor ignores the fault condition that was encountered while performing the read operation using the address from the element. The processor can continue at least one of the read operations that use an address from an element of the first input vector (e.g., for at least one earlier element of the input vector).
In some embodiments, when masking the fault, the processor clears at least one corresponding bit position in a fault-status register (FSR) to indicate that the fault was masked.
In some embodiments, when clearing at least one corresponding bit position in the FSR, the processor clears: (1) a first bit position in the FSR for which a read that used an address from a corresponding element from the input vector encountered a fault condition and (2) any bit positions subsequent to the first bit position in the FSR.
In some embodiments, when at least one fault has been masked, the processor sets a processor status flag to indicate that at least one fault has been masked.
In some embodiments, the processor receives returned data for at least one of the read operations that used an address from an element of the first input vector and writes the returned data to a corresponding element of the result vector when the corresponding element of the result vector is located before any element position for which a fault was masked. However, the processor ignores the returned data when a corresponding element of the result vector is located at or after any element position for which a fault was masked.
In some embodiments, the first input vector comprises one of an address or an address pointer in each element.



BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 presents a block diagram of a computer system in accordance with the described embodiments.
FIG. 2 presents an expanded view of a processor in accordance with the described embodiments.
FIG. 3 presents an expanded view of a vector execution unit in accordance with the described embodiments.
FIG. 4 presents a flowchart illustrating a process for executing program code in accordance with the described embodiments.
FIG. 5 presents a flowchart illustrating a process for executing a ReadXF instruction in accordance with the described embodiments.



In the figures, like reference numerals refer to the same figure elements.
DETAILED DESCRIPTION
The following description is presented to enable any person skilled in the art to make and use the described embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the described embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the described embodiments. Thus, the described embodiments are not limited to the embodiments shown, but are to be accorded the widest scope consistent with the principles and features disclosed herein.
The data structures and code described in this detailed description are typically stored on a computer-readable storage medium, which may be any device or medium that can store code and/or data for use by an electronic device with computing capabilities. For example, the computer-readable storage medium can include volatile memory or non-volatile memory, such as flash memory, random access memory (RAM, SRAM, DRAM, RDRAM, DDR/DDR2/DDR3 SDRAM, etc.), magnetic or optical storage mediums (e.g., disk drives, magnetic tape, CDs, DVDs), and/or other mediums capable of storing data structures or code. Note that in the described embodiments, the computer-readable storage medium does not include non-statutory computer-readable storage mediums such as transitory signals.
The methods and processes described in this detailed description can be included in one or more hardware modules. For example, the hardware modules can include, but are not limited to, processors, application-specific integrated circuit (ASIC) chips, field-programmable gate arrays (FPGAs), and other programmable-logic devices. When the hardware modules are activated, the hardware modules perform the methods and processes included within the hardware modules. In some embodiments, the hardware modules include one or more general-purpose circuits that are configured by executing instructions (program code, firmware, etc.) to perform the methods and processes.
The methods and processes described in the detailed description section can be embodied as code and/or data that can be stored in a computer-readable storage medium as described above. When computer system (e.g., a processor in the computer system) reads and executes the code and/or data stored on the computer-readable storage medium, the computer system performs the methods and processes embodied as data structures and code and stored within the computer-readable storage medium.
The following description refers to “some embodiments.” “Some embodiments” describes a subset of all of the possible embodiments, but does not necessarily always specify the same subset of the embodiments.
Macroscalar Architecture
The embodiments described herein are based in part on the Macroscalar Architecture that is described in U.S. patent application Ser. No. 12/873,063, entitled “Non-Faulting and First-Faulting Instructions for Processing Vectors,” by inventor Jeff Gonion, filed 31 Aug. 2010, and U.S. patent application Ser. No. 12/541,546, entitled “Running-Shift Instructions for Processing Vectors,” by inventors Jeffry E. Gonion and Keith E. Diefendorff, filed 14 Aug. 2009, (hereinafter, “the '063 and '546 applications”), the contents of which are (as described above) incorporated by reference.
As recited in the '063 and '546 applications, the described embodiments provide an instruction set and supporting hardware that allow compilers to generate program code for loops without completely determining parallelism at compile-time, and without discarding useful static analysis information. Specifically, these embodiments provide a set of instructions that do not mandate parallelism for loops but instead enable parallelism to be exploited at runtime if dynamic conditions permit. These embodiments thus include instructions that enable code generated by the compiler to dynamically switch between non-parallel (scalar) and parallel (vector) execution for loop iterations depending on conditions at runtime by switching the amount of parallelism used.
These embodiments provide instructions that enable an undetermined amount of vector parallelism for loop iterations but do not require that the parallelism be used at runtime. More specifically, these embodiments include a set of vector-length agnostic instructions whose effective vector length can vary depending on runtime conditions. Thus, if runtime dependencies demand non-parallel execution of the code, then execution occurs with an effective vector length of one element. Likewise, if runtime conditions permit parallel execution, the same code executes in a vector-parallel manner to whatever degree is allowed by runtime dependencies (and the vector length of the underlying hardware). For example, if two out of eight elements of the vector can safely execute in parallel, the described embodiments execute the two elements in parallel. In these embodiments, expressing program code in a vector-length agnostic format enables a broad range of vectorization opportunities that are not present in existing systems.
In the described embodiments, during compilation, a compiler first analyzes the loop structure of a given loop in program code and performs static dependency analysis. The compiler then generates program code that retains static analysis information and instructs processor 102 how to resolve runtime dependencies and process the program code with the maximum amount of parallelism possible. More specifically, the compiler provides vector instructions for performing corresponding sets of loop iterations in parallel, and provides vector-control instructions for dynamically limiting the execution of the vector instructions to prevent data dependencies between the iterations of the loop from causing an error (which can be called “vector partitioning”). This approach defers the determination of parallelism to runtime, where the information on runtime dependencies is available, thereby allowing the software and processor to adapt parallelism to dynamically changing conditions (i.e., based on data that is not available at compile-time).
Vectorized program code can comprise vector-control instructions and vector instructions forming a loop in the vectorized program code that performs vector operations based on a corresponding loop in program code. The vector control instructions can determine iterations of the loop in program code that are safe to execute in parallel (because, e.g., no runtime data dependencies have occurred), and the vector instructions can be executed using predication and/or other dynamic controls to limit the elements of the vector instruction that are processed in parallel to the determined-safe iterations. (Recall that, in the described embodiments, each element of a vector instruction can perform an operation (or operations) for corresponding iterations of a loop in the program code.)
Terminology
Throughout the description, we use the following terminology. These terms may be generally known in the art, but are described below to clarify the subsequent descriptions.
The term “active” or “active element,” as used in this description to refer to one or more elements of a vector, indicates elements that are operated on during a given operation. Generally, the described embodiments enable a vector execution unit to selectively perform operations on one or more available elements in a given vector in parallel. For example, an operation can be performed on only the first two of eight elements of the vector in parallel. In this case, the first two elements are “active elements,” while the remaining six elements are “inactive elements.” In the described embodiments, one or more other vectors can be used to determine which elements in a given operand vector are active (i.e., are to be operated on). For example, a “predicate vector” or “control vector” can include “active” elements that are used to determine which elements in the operand vector to perform operations on. In some embodiments, elements that contain data of a predetermined type are active elements (e.g., true, false, non-zero, zero, uppercase/lowercase characters, even/odd/prime numbers, vowels, whole numbers, etc.).
The terms “true” and “false” are used in this description to refer to data values (e.g., a data value contained in an element in a vector). Generally, in computer systems true and false are often represented by 1 and 0, respectively. In practice, a given embodiment could use any value to represent true and false, such as the number 55, or the letter “T.”
In the following examples, “corresponding elements” may be described. Generally, corresponding elements are elements at a same element position in two or more different vectors. For example, when a value is copied from an element in an input vector into a “corresponding element” of a result vector, the value is copied from an nth element in the input vector into an nth element in the result vector.
In the following examples, “relevant” elements may be described. In the described embodiments, a relevant element is an element in a given vector for which the corresponding element in one or more other vectors (e.g., a control vector and/or predicate vector) is/are active. For example, given an input control vector for which only a fourth element is active, a second input vector only has one relevant element—the fourth element.
In this description, for clarity, operations performed for “vector instructions and/or operations” may be described generally as operations performed for “vector instructions,” however, in the described embodiments “vector operations” can be handled in similar ways.
Notation
In describing the embodiments in the instant application, we use the following formats for variables, which are vector quantities unless otherwise noted:

p5=a<b; 

    
    
        Elements of vector p5 are set to 0 or 1 depending on the result of the comparison operation a<b. Note that vector p5 can be a predicate vector that can be used to control the number of elements of one or more vector instructions that execute in parallel.

{tilde over ( )}p5; a=b+c; 

        Only elements in vector a designated by active (i.e., non-zero) elements in the predicate vector p5 receive the result of b+c. The remaining elements of a are unchanged. This operation is called “predication,” and is denoted using the tilde (“{tilde over ( )}”) before the predicate vector.

!p5; a=b+c; 

        Only elements in vector a designated by active (e.g., non-zero) elements in the predicate vector p5 receive the result of b+c. The remaining elements of a are set to zero. This operation is called “zeroing,” and is denoted using the exclamation point (“!”) before the predicate vector.

if (FIRST ( )) goto . . . ; Also LAST ( ), ANY ( ), ALL ( ), CARRY ( ), ABOVE ( ), or NONE ( ), (where ANY ( )==!NONE ( ))

        These instructions test the processor status flags and branch accordingly.

x+=VECLEN; 

        VECLEN is a value that communicates the number of elements per vector. The value is determined at runtime by the processor 102 (see FIG. 1), rather than being determined by the compiler/assembler.

// Comment

        In a similar way to many common programming languages, the examples presented below use the double forward slash to indicate comments. These comments can provide information regarding the values contained in the indicated vector or explanation of operations being performed in a corresponding example.
    
    


In these examples, other C++-formatted operators retain their conventional meanings, but are applied across the vector on an element-by-element basis. Where function calls are employed, they imply a single instruction that places any value returned into a destination register. For simplicity in understanding, all vectors discussed herein are vectors of integers, but alternative embodiments support other data formats.
Instruction Definitions
The described embodiments comprise a ReadXF instruction. The ReadXF instruction takes an input vector, a predicate vector, and a control vector as inputs, each vector comprising N elements (where N equals, e.g., 32, 64, 86, or another number of elements). When executed, the ReadXF instruction causes processor 102 to perform read operations using a value in each element of the input vector for which corresponding elements of the predicate vector and the control vector are both active as the address for the read. When at least one fault condition is encountered when performing the reads (e.g., a page fault, an illegal address, etc.), processor 102 determines if a fault condition was encountered when performing the read using an address from a key element position of the first input vector. In the described embodiments, the key element position is based on a location of a first active element of the control vector. More specifically, the key element position is found in an element position where the first active element of the control vector and a corresponding element of the predicate vector are both active. In these embodiments, if the predicate vector is inactive in the first element where the control vector is active, or if no element of the control vector is active, there is no key element position. If a fault condition was encountered when performing the read using an address from the key element position of the first input vector, processor 102 processes/handles the fault for the read normally. Otherwise, if a fault condition is encountered when performing the read using an address from an element other than the element at the key element position, processor 102 masks (i.e., ignores) the fault and proceeds with processing at least one of the read operations (e.g., a read operation for the key element) from the ReadXF instruction normally. Note that, in these embodiments, all faults may be masked if there is no key element position.
The key element position is herein described as a “first” element position, if any, where the element of the control vector is active (when a corresponding element of the predicate vector is also active). Generally, a “first” element position is an element position with a lowest element number. For example, assuming that the elements in the input vector are numbered 0-N from left to right, the first element position where a condition occurs (e.g., the element of the control vector is active) is the leftmost element position. As another example, assuming that the elements in the input vector are numbered 0-N from right to left, the first element position where a condition occurs (e.g., the element of the control vector is active) is the rightmost element position.
In some embodiments, when a fault is masked for a given element in the vector instruction, processor 102 can clear a corresponding bit position and subsequent bit positions in FSR 214 in order to record the element position where a fault was encountered. Clearing the bit position(s) of the FSR 214 in this way enables processor 102 to maintain a record of elements that may be effected by a faulting read. Subsequent operations in processor 102 can read FSR 214 to determine elements for which faults were masked.
In the embodiment presented in the example below, the inputs to the ReadXF instruction include two input vectors, a first input vector (src1) and a second input vector (src2). As can be seen in the example, each element of src1 contains a base value and each element of src2 contains an offset value. The base value and offset in each element can be used in computing a corresponding address as shown. However, in alternative embodiments, the inputs to the ReadXF instruction may comprise a single input vector that contains complete addresses, pointers, etc. More generally, the described embodiments can use any number of input vectors that enable ReadXF to compute or otherwise determine addresses for read operations.
Although certain arrangements of instructions are used in describing the ReadXF instruction, a person of skill in the art will recognize that these concepts may be implemented using different arrangements or types of instructions without departing from the spirit of the described embodiments. Additionally, the ReadXF instruction is described using a signed-integer data type. However, in alternative embodiments, other data types or formats are used. Moreover, although Macroscalar instructions may take vector, scalar, or immediate arguments in practice, vector arguments are described herein.
In the example below, the BadAddr function is an exemplary function that causes processor 102 to determine if the address is “bad” (e.g., if a check of the page table for an input address returns a fault, if the address is illegal/malformed/etc., or if the address is bad for another reason). If so, the BadAddr function returns a 1. Although this example is presented to illustrate a portion of the operations performed by the ReadXF instruction, in some embodiments, processor 102 does not use the BadAddr function. Instead, a read operation that uses the address in each element of an input vector for which corresponding elements of the predicate vector and control vector are active is simply forwarded to a cache or another memory in the memory hierarchy for processor 102, and processor 102 automatically handles faults arising from the reads so that the outcome is the same as the outcome reached by performing the operations described below. Note that automatically handling faults can comprise masking the fault and clearing a corresponding bit and any subsequent bits in FSR 214, or processing the fault when the fault occurs in the key element position (which, as described above, is a first element position where a corresponding element of the control vector and the predicate vector are active).
In addition, in the example below, the Read function is an exemplary function that causes processor 102 to perform a read operation using an address input to the function. Specifically, when executing the Read function, processor 102 forwards a read of the address to a cache or another memory in the memory hierarchy for processor 102. In the example, because nff is set to 1 after the key element position (i.e. the first element position where a corresponding element of the control vector and the predicate vector are active), processor 102 does not perform a read of a bad address (or any subsequent addresses) after the first element where the control vector and the predicate vector are active. However, in alternative embodiments, processor 102 does not perform at least some of the operations shown (e.g., the check of nff; etc.). Instead, processor 102 simply sends a read and automatically handles faults arising from the read as described above—so that the outcome is the same as the outcome reached by performing the operations described below.
For the purposes of explanation, the vector data type is defined as a C++ class containing an array v[ ] of elements that comprise the vector. Within these descriptions, the variable VECLEN indicates the size of the vector. In some embodiments, VECLEN is constant.
Note that the format of the following instruction definitions is a statement of the instruction type followed by a description of the instruction that can include example code as well as one or more usage examples.
ReadXF
The ReadXF instruction causes processor 102 to determine when a fault condition has been encountered during a read operation using an address from a key element of an input vector, where the key element is a first element position where a corresponding elements of a control vector and a predicate vector are active. When a fault is encountered when performing the read operation using the address from the key element, processor 102 can process/handle the fault normally. For elements other than the key element, processor 102 can mask the fault (by, e.g., not performing the read for the element).
In the described embodiments, processor 102 can clear a bit position in FSR 214 (i.e., set the bit position to zero) to indicate that a fault has been masked for a given element or a preceding element. In some embodiments, processor 102 automatically clears a given bit position of FSR 214 and any subsequent bit positions when a fault condition has been encountered (with exception of the key element position, as described above) when performing a read using an address from a corresponding position of the input vector. Processor 102 can also set processor flags (“none”) to indicate when no fault conditions were encountered.















vector ReadXF (vector gp, vector src1, vector src2,



vector p)



{



  vector r;



  int x, offset, count = 0, nff = 0, bad = 0;



  uint address;



  for (x=0; x<VECLEN; ++x)



  {



    if (gp.v[x])



    {



      if (p.v[x])



      {



        offset = src2.v[x] * 4;



        address = src1.v[x]+offset;



        if(nff) bad |= BadAddr(address);



        if (bad) FSR[x] = 0;



        else r.v[x] = Read(address);



        count += !bad;



        nff = 1;



      }



      else



      {



        r.v[x] = 0;



        if (bad) FSR[x] = 0;



      }



    }



    else



    {



      r.v[x] = <Z/M>; // Z = zero (for



      zeroing), M = mask (for predication)



      if (bad) FSR[x] = 0;



      if (p.v[x]) nff = 1;



    }



  SetNone(count == 0);



  return(r);



}










Example















~p; ReadXF(pred, control, src1, src2)




















On Entry:
FSR
= {
1
1
1
1
1
1
1
1
}




cntl
= {
0
0
0
0
1
1
1
1
}




pred
= {
0
0
0
1
1
1
1
0
}




src1
= {
1
2
3
4
5
6
7
8
}




src2
= {
1
2
3
4
5
6
7
8
}


















Bad address?
=
N
N
N
N
Y
N
Y
N




















Result:
FSR
= {
1
1
1
1
1
1
0
0
}









SetNone = 0



















r
= {
M
M
M
0
D
D
?
?
}









// where D = data returned from read



and ? = unknown, M = Zero/Mask



predication results, and 0 = Zero.










In this description, the expression “Z/M” denotes the use of zeroing (Z) or predication (M) depending on the call of the ReadXF instruction. As described above, when zeroing is used (e.g., for a call of the form !p; ReadXF (pred, control, src1, src2), elements of the result vector for which corresponding elements of the predicate vector are inactive are set to zero. When predication is used (e.g., for a call of the form {tilde over ( )}p; ReadXF (pred, control, src1, src2), elements of the result vector for which corresponding elements of the predicate vector are inactive are left unchanged.
Computer System
FIG. 1 presents a block diagram of a computer system 100 in accordance with the described embodiments. Computer system 100 includes processor 102, L2 cache 106, memory 108, and mass-storage device 110. Processor 102 includes L1 cache 104.
Processor 102 can be a general-purpose processor that performs computational operations. For example, processor 102 can be a central processing unit (CPU) such as a microprocessor, a controller, an application-specific integrated circuit (ASIC), or a field-programmable gate array (FPGA). In the described embodiments, processor 102 has one or more mechanisms for vector processing (i.e., vector execution units).
Mass-storage device 110, memory 108, L2 cache 106, and L1 cache 104 are computer-readable storage devices that collectively form a memory hierarchy that stores data and instructions for processor 102. Generally, mass-storage device 110 is a high-capacity, non-volatile memory, such as a disk drive or a large flash memory, with a large access time, while L1 cache 104, L2 cache 106, and memory 108 are smaller, faster semiconductor memories that store copies of frequently used data. Memory 108 is typically a dynamic random access memory (DRAM) structure that is larger than L1 cache 104 and L2 cache 106, whereas L1 cache 104 and L2 cache 106 are typically comprised of smaller static random access memories (SRAM). In some embodiments, L2 cache 106, memory 108, and mass-storage device 110 are shared between one or more processors in computer system 100. Such memory structures are well-known in the art and are therefore not described in more detail.
In some embodiments, the devices in the memory hierarchy (i.e., L1 cache 104, etc.) can access (i.e., read and/or write) multiple cache lines per cycle. These embodiments enable more effective processing of memory accesses that occur based on a vector of pointers or array indices to non-contiguous memory addresses. In addition, in some embodiments, the caches in the memory hierarchy are divided into a number of separate banks, each of which can be accessed in parallel. Banks within caches and parallel accesses of the banks are known in the art and hence are not described in more detail.
Computer system 100 can be incorporated into many different types of electronic devices. For example, computer system 100 can be part of a desktop computer, a laptop computer, a tablet computer, a server, a media player, an appliance, a cellular phone, a piece of testing equipment, a network appliance, a personal digital assistant (PDA), a hybrid device (i.e., a “smart phone”), or another electronic device.
Although we use specific components to describe computer system 100, in alternative embodiments, different components may be present in computer system 100. For example, computer system 100 may not include some of the memory hierarchy (e.g., memory 108 and/or mass-storage device 110).
Alternatively, computer system 100 may include video cards, video-capture devices, user-interface devices, network cards, optical drives, and/or other peripheral devices that are coupled to processor 102 using a bus, a network, or another suitable communication channel. Computer system 100 may also include one or more additional processors, wherein the processors share some or all of L2 cache 106, memory 108, and mass-storage device 110.
Processor
FIG. 2 presents an expanded view of processor 102 in accordance with the described embodiments. As shown in FIG. 2, processor 102 includes L1 cache 104, fetch unit 200, decode unit 202, dispatch unit 204, branch execution unit 206, integer execution unit 208, vector execution unit 210, floating-point execution unit 212 (branch execution unit 206, integer execution unit 208, vector execution unit 210, and floating-point execution unit 212 as a group are interchangeably referred to as “the execution units”). Processor 102 also includes fault status register (FSR) 214.
Fetch unit 200 fetches instructions from the memory hierarchy in computer system 100 and forwards the fetched instructions to be decoded in decode unit 202 for eventual execution in the execution units. Generally, fetch unit 200 attempts to fetch instructions from the closest portion of the memory hierarchy first, and if the instruction is not found at that level of the memory hierarchy, proceeds to the next level in the memory hierarchy until the instruction is found. For example, in some embodiments, fetch unit can request instructions from L1 cache 104 (which can comprise a single physical cache for instructions and data, or can comprise physically separate instruction and data caches). Aside from the operations herein described, the operations of fetch units are generally known in the art and hence are not described in more detail.
Decode unit 202 decodes the instructions and assembles executable instructions to be sent to the execution units, and dispatch unit 204 receives decoded instructions from decode unit 202 and dispatches the decoded instructions to the appropriate execution unit. For example, dispatch unit 204 can dispatch branch instructions to branch execution unit 206, integer instructions to integer execution unit 208, etc.
Each of execution units 206-212 is used for performing computational operations, such as logical operations, mathematical operations, or bitwise operations for an associated type of operand or operation. More specifically, integer execution unit 208 is used for performing computational operations that involve integer operands, floating-point execution unit 212 is used for performing computational operations that involve floating-point operands, vector execution unit 210 is used for performing computational operations that involve vector operands, and branch execution unit 206 is used for performing operations for resolving branches. Integer execution units, branch execution units, and floating-point execution units are generally known in the art and are not described in detail.
In the described embodiments, vector execution unit 210 is a single-instruction-multiple-data (SIMD) execution unit that performs operations in parallel on some or all of the data elements that are included in vectors of operands. FIG. 3 presents an expanded view of vector execution unit 210 in accordance with the described embodiments. As is shown in FIG. 3, vector execution unit 210 includes a vector register file 300 and an execution unit 302. Vector register file 300 includes a set of vector registers that can hold operand vectors and result vectors for execution unit 302. In some embodiments, there are 32 vector registers in the vector register file, and each register includes 128 bits. In alternative embodiments, there are different numbers of vector registers and/or different numbers of bits per register.
Vector execution unit 302 retrieves operands from registers in vector register file 300 and executes vector instructions that cause execution unit 302 to perform operations in parallel on some or all of the data elements (or, simply, “elements”) in the operand vector. For example, execution unit 302 can perform logical operations, mathematical operations, or bitwise operations on the elements in the vector. Execution unit 302 can perform one vector operation per cycle (although the “cycle” may include more than one cycle of a clock used to trigger, synchronize, and/or control execution unit 302's computational operations).
In the described embodiments, execution unit 302 supports vectors that hold N data elements (e.g., bytes, words, doublewords, etc.). In these embodiments, execution unit 302 can perform operations on Nor fewer of the data elements in an operand vector in parallel. For example, assuming an embodiment where the vector is 256 bits in length (i.e., 32 bytes), the data elements being operated on are four-byte words, and the operation is adding a value to the data elements, these embodiments can add the value to any number of the eight words in the vector.
In the described embodiments, execution unit 302 includes at least one control signal that enables the dynamic limitation of the data elements in an operand vector on which execution unit 302 operates. Specifically, depending on the state of the control signal, execution unit 302 may or may not operate on all the data elements in the vector. For example, assuming an embodiment where the vector is 512 bits in length and the data elements being operated on are four-byte words, the control signal can be asserted to prevent operations from being performed on some or all of 16 data words in the operand vector. Note that “dynamically” limiting the data elements in the operand vector upon which operations are performed can involve asserting the control signal separately for each cycle at runtime.
In some embodiments, based on the values contained in a vector of predicates or one or more scalar predicates, execution unit 302 applies vector operations to selected vector data elements only. In some embodiments, the remaining data elements in a result vector remain unaffected (which we call “predication”) or are forced to zero (which we call “zeroing”). In some of these embodiments, the clocks for the data element processing subsystems (“lanes”) that are unused due to predication or zeroing in execution unit 302 can be gated, thereby reducing dynamic power consumption in execution unit 302.
The described embodiments are vector-length agnostic. Thus, a compiler or programmer need not have explicit knowledge of the vector length supported by the underlying hardware (e.g., vector execution unit 302). In these embodiments, a compiler generates or a programmer writes program code that need not rely on (or use) a specific vector length (some embodiments are forbidden from even specifying a specific vector size in program code). Thus, the compiled code in these embodiments (i.e., binary code) runs on other embodiments with differing vector lengths, while potentially realizing performance gains from processors that support longer vectors. Consequently, as process technology allows longer vectors, execution of legacy binary code simply speeds up without any effort by software developers.
In some embodiments, vector lengths need not be powers of two. Specifically, vectors of 3, 7, or another number of data elements can be used in the same way as vectors with power-of-two numbers of data elements.
In the described embodiments, each data element in the vector can contain an address that is used by execution unit 302 for performing a set of memory accesses in parallel. In these embodiments, if one or more elements of the vector contain invalid memory addresses, invalid memory-read operations can occur. In these embodiments, invalid memory-read operations that would otherwise result in program termination instead cause any elements with valid addresses to be read and elements with invalid elements to be flagged, allowing program execution to continue in the face of speculative, and in hindsight illegal, read operations.
In some embodiments, processor 102 (and hence execution unit 302) is able to operate on and use vectors of pointers. In these embodiments, the number of data elements per vector is the same as the number of pointers per vector, regardless of the size of the data type. Instructions that operate on memory may have variants that indicate the size of the memory access, but elements in processor registers should be the same as the pointer size. In these embodiments, processors that support both 32-bit and 64-bit addressing modes may choose to allow twice as many elements per vector in 32-bit mode, thereby achieving greater throughput. This implies a distinct throughput advantage to 32-bit addressing, assuming the same width data path. Implementation-specific techniques can be used to relax the requirement. For example, double-precision floating-point numbers can be supported in 32-bit mode through register pairing or some other specialized mechanism.
FSR 214 is a memory element that includes a number of bit positions (e.g., individual memory circuits) that can be used by processor 102 to record element positions for vector operations for which faults were masked. In the described embodiments, processor 102 supports a number of different vector instructions and/or operations that can be configured by default and/or can be overridden (e.g., using a software switch, control element, setting, etc. in processor 102) to enable processor 102 to mask (i.e., at least temporarily ignore) fault conditions encountered for at least some of the elements of the vector instruction/operation (including the herein-described ReadXF instruction). For example, the described embodiments can support fault masking for vector read instructions, vector write instructions, vector mathematical instructions, and/or other vector instructions/operations. In these embodiments, upon masking a fault for an element of a vector instruction/operation, processor 102 clears the corresponding bit position and any subsequent bit positions in FSR to indicate that the fault was masked (in some embodiments, processor 102 clears only the corresponding bit position in FSR 214). Processor 102 can then subsequently read FSR 214 to determine if at least one element of the vector instruction/operation encountered a fault condition that was masked by processor 102.
The number of bit positions/memory elements in FSR 214 is sufficient to record masked faults for the widest possible vector instruction supported in processor 102. For example, assuming that the widest vector instruction supported in processor 102 is N elements wide (where N can equal, e.g., 64, 128, 357, etc.), FSR 214 can include N bit positions/individual memory circuits. FSR 214 may also include other control mechanisms that are used when performing operations using FSR 214. For example, in some embodiments, FSR 214 can include status bits such as a valid bit that indicates that the value in FSR 214 is valid/invalid.
In some embodiments, a bit position in FSR 214 is set to zero (or “cleared”) to record that a fault was masked for a corresponding vector element, and is otherwise set to 1. In alternative embodiments, a bit position can be set to another value to indicate a masked fault (e.g., 1, T, etc.). Generally, any value that can be used to indicate that a fault was masked can be used in the described embodiments.
Although we describe processor 102 as including a particular set of units, in alternative embodiments, processor 102 can include different numbers or types of units. In addition, although vector execution unit 210 is describe using particular mechanisms, alternative embodiments may include different mechanisms. Generally, vector execution unit 210 (and, more broadly, processor 102) comprises sufficient mechanisms to perform vector operations, including the operations herein described.
Executing a ReadXF Instruction
FIG. 4 presents a flowchart illustrating a process for executing program code in accordance with the described embodiments. As can be seen in FIG. 4, when executing program code, processor 102 receives a predicate vector, a control vector, and an input vector that each include N elements (step 400). Next, using the received predicate vector, control vector, and input vector, processor 102 executes a ReadXF instruction (step 402).
In the description of the ReadXF instruction above, exemplary functions are used to generally describe the operations performed by the ReadXF instruction. Although the example above is presented for illustrative purposes, alternative embodiments may function differently. For example, in some embodiments, read operations are automatically performed using an address from each element of the input vector for which a corresponding element of the predicate vector and the control vector are active. Processor 102 uses active elements of the control vector (in combination with active elements of the predicate vector) to determine the fault conditions (if any) that are encountered during the read operations which are handled/processed by processor 102 or are masked by processor 102 (as described below).
In the following examples, “corresponding elements” of vectors and “corresponding bit positions” in FSR 214 may be described with respect to elements of other vectors. As used herein, as an example, a corresponding element of a given vector (i.e., one of the control vector, the predicate vector, or the input vector) for a bit position in FSR 214 is an nth element in the predicate vector for an nth bit position in FSR 214. As another example, a corresponding element for a given vector (i.e., one of the control vector, the predicate vector, or the input vector) for an element of another vector (i.e., one of the control vector, the predicate vector, or the input vector) is an nth element in the given vector for an nth element of the other vector.
FIG. 5 presents a flowchart illustrating a process for executing a ReadXF instruction in accordance with the described embodiments. In these embodiments, the operations shown in FIG. 5 are performed as part of step 402 in FIG. 4. Thus, for the purposes of describing the operations shown in FIG. 5, it is assumed that the predicate vector, the control vector, and the input vector have been received, as shown in step 400 in FIG. 4.
When executing the ReadXF instruction, for each element of the input vector for which a corresponding element of the predicate vector and the control vector are active in parallel (step 500), processor 102 attempts to perform a read operation using a value from the element as an address for the read operation (step 502). In attempting to perform the read operation, processor 102 can send a read request using the address to L1 cache 104, L2 cache 106, and/or to another level of the memory hierarchy. Note that, in some embodiments, for elements of the input vector for which a corresponding element of the predicate vector is active, but the corresponding element of the control vector is inactive, a read operation is not attempted, and the corresponding element of the result vector can be set to 0 or another predetermined value. In addition, for elements of the input vector for which the predicate vector is inactive (regardless of a state of the corresponding element of the control vector), the corresponding element of a result vector generated by the ReadXF instruction is left in an original value (predication) or is set to zero (zeroing).
In the described embodiments, by performing the read operations for each of these elements of the input vector in parallel, processor 102 may read from one or more addresses that may not have been read during ordinary serial (non-vectorized) program flow. For example, processor 102 may operate on a vector of N addresses (e.g., 32 addresses, 64 addresses, etc.) from N iterations of a loop in parallel, but a loop termination condition may have caused the termination of processing of loop iterations at the N-Mth loop iteration (where M<N) if the loop iterations had been processed serially. Because this is true, processor 102 may encounter fault conditions when processing vectorized code that would not have been encountered when processing the underlying serial/non-vectorized code. This is one reason why the described embodiments can process/handle faults for read operations for only the first element where the control vector is active (and the corresponding element of the predicate vector is active)—this may enable a fault condition encountered while performing the read for a desired (e.g., first) iteration of a loop to be handled normally, while fault conditions for subsequent (perhaps speculative) reads are masked.
If no fault condition is encountered for the element when attempting to perform the read operation (step 504) (i.e., if no fault condition is encountered when performing the read using the address from the element), processor 102 completes the read operation (step 506). Otherwise, if a fault condition is encountered for the element, processor 102 determines if a corresponding element of the control vector is the first active element of the control vector (step 508). Recall that a “first” element position is an element position with a lowest element number. For example, assuming that the elements in the input vector are numbered 0-N from left to right, the first element position is the leftmost element position. Additionally recall that the corresponding element of the predicate vector is known to be active because read operations are only attempted for elements where the predicate vector and the control vector are both active.
When the corresponding element of the control vector is the first active element of the control vector (step 508), processor 102 processes the fault (step 510). For example, if the fault is a page fault, processor 102 can perform operations for processing/handling the page fault. As another example, the address in the element can be an illegal address, and processor 102 can perform operations for processing/handling the corresponding fault. When the corresponding element of the control vector is not the first active element of the control vector (step 508), processor 102 masks the fault for the element (step 512). When masking the fault, processor 102 can terminate the read operation for the element and/or can ignore any result returned from the read operation and can ignore the fault condition (e.g., not immediately process/handle the fault), but can allow a read operation for one or more other elements of the ReadXF instruction to complete normally.
For example, assuming that the input vector inpt (for which exemplary address values are represented using single characters), the control vector ctrl, and the predicate vector pred contain the following values, and that the read of corresponding addresses from the input vector encounter fault conditions as follows (with Y representing a faulting element and N representing a non-faulting element), the ReadXF instruction can have the following result:


























On entry:
inpt
= {
A
B
C
D
E
F
G
H
}




ctrl
= {
1
1
1
1
1
1
1
1
}




pred
= {
0
0
1
1
1
1
1
1
}




fault
=
X
X
Y
Y
Y
N
N
N










Result:
fault processed (third element)










As can be seen in this example, processor 102 processes the fault condition encountered when performing the read using the address from the third element of the input vector, because the third element is in the key element position (i.e., an element location where the corresponding element of the control vector is the first active element of the control vector and the corresponding element of the predicate vector is active). In the described embodiments, when processing the fault, processor 102 may perform one or more operations for handling the fault such as loading pages, terminating the program, etc. depending on the nature of the fault. Note that reads are not performed for the first and second elements because the predicate vector is inactive in the corresponding elements—which is why an X is shown in the fault listing. In addition, assuming that processing the fault does not cause the termination of the ReadXF instruction and/or the termination of subsequent read operations, the fault conditions encountered in the fourth and fifth elements may be masked by processor 102.

As another example, assuming that the input vector inpt (for which exemplary address values are represented using single characters), the control vector ctrl, and the predicate vector pred contain the following values, and that the read of corresponding addresses from the input vector encounter fault conditions as follows (with Y representing a faulting element, and N representing a non-faulting element), the ReadXF instruction can have the following result:


























On entry:
inpt
= {
A
B
C
D
E
F
G
H
}




ctrl
= {
1
1
1
1
1
1
1
1
}




pred
= {
0
0
1
1
1
1
1
1
}




fault
=
X
X
N
Y
Y
N
N
N










Result:
faults masked (fourth and fifth elements)










As can be seen in this example, the read for the third element, which is at a key element position (i.e., an element location where the corresponding element of the control vector is the first active element of the control vector and the corresponding element of the predicate vector is active), does not encounter a fault condition. Thus, because the read for the key element position does not encounter a fault condition, the ReadXF instruction will process no fault conditions for the ReadXF instruction. Hence, the fault conditions encountered for the fourth and fifth elements are masked by processor 102, and at least the read for the third element completes normally.

Processor 102 can then perform an operation to clear one or more bits in FSR 214 to indicate elements where faults were masked. Specifically, processor 102 can clear a corresponding bit position of FSR 214 for an element where a fault was masked (step 514). Processor 102 can also optionally discard results of completed read operations and clear corresponding bit positions in FSR 214 for any elements following an element where a fault condition was encountered and masked (step 516). For example, assuming that the control vector ctrl and the predicate vector pred contain the following values, and that the read of corresponding addresses from the input vector inpt (for which exemplary address values are represented using single characters) encounter fault conditions as follows (with Y representing a faulting element, and N representing a non-faulting element), the ReadXF instruction in the described embodiments can clear bit positions in FSR 214 as follows:


























On entry:
inpt
= {
A
B
C
D
E
F
G
H
}




ctrl
= {
1
1
1
1
1
1
1
1
}




pred
= {
0
0
1
1
1
1
1
1
}




FSR
= {
1
1
1
1
1
1
1
1
}




fault
=
X
X
N
Y
N
N
N
N



On exit:
FSR
= {
1
1
1
0
0
0
0
0
}










As this example shows, a fault condition encountered while performing a read using the address from the fourth element was masked because the fault condition was not encountered in the key element position. In this embodiment, the corresponding fourth bit position of FSR 214 is cleared. In addition, the fifth through eighth bit positions of FSR 214 are cleared, despite the fact that no fault conditions were encountered while performing a read using the address from these elements. In this way, processor 102 can clear each bit position after a bit position in which a fault condition was encountered. In some embodiments, the read operations for the fifth through eighth elements may complete, but the data returned from read operations for the fifth through eighth elements can be discarded, invalidated, or otherwise ignored by processor 102.

In the described embodiments, the operation of setting the bit positions in FSR 214 can be performed as a single operation. Specifically, when a fault condition is encountered while performing a read using the address from a given element, processor 102 can mask/ignore the fault, and can subsequently determine that the fault condition occurred for the element and can clear the corresponding bit position and any subsequent bit positions of FSR 214 in a single operation. This can mean that the operations shown in 514 and 516 are performed as a single operation, despite being shown separately in FIG. 5.
In some embodiments, processor 102 sets one or more processor status flags to indicate that a given set of the read operations for active elements have been masked. For example, processor 102 can set a “none” flag to indicate that no faults were masked.
In some embodiments, processor 102 receives returned data for at least one of the read operations from step 502. Processor 102 can then write the returned data to a corresponding element of a result vector. In some embodiments, only returned data from reads based on addresses from elements before an element for which a fault was masked are written to the result vector (where an element is before another element when the element has a lower element number). For example, if reads are sent using the addresses from the first through fifth elements of the input vector, but a fault is encountered and masked when performing the read using the address from the fourth element, only the returned data from the read from the addresses from the first through third elements of the input vector is written to the corresponding element of the result vector. In some embodiments, data returned from the read for the address from the fifth element of the input vector is ignored or otherwise discarded.
The foregoing descriptions have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the described embodiments to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the described embodiments. The scope of these embodiments is defined by the appended claims.

## Claims

1. A method for handling faults in a processor, comprising:
receiving a first input vector, a control vector, and a predicate vector, each vector comprising a plurality of elements; and
for each element in the first input vector for which a corresponding element in the control vector and the predicate vector are active,
performing a read operation using an address from the element of the first input vector;
when a fault condition is encountered while performing the read operation, determining if the element is a first element where a corresponding element of the control vector is active; and
if so, processing the fault;
otherwise, masking the fault for the element.

2. The method of claim 1, wherein masking the fault for the element comprises:
ignoring the fault condition that was encountered while performing the read operation using the address from the element; and
continuing at least one of the read operations that use an address from an element of the first input vector.

3. The method of claim 1, wherein masking the fault comprises clearing at least one corresponding bit position in a fault-status register (FSR) to indicate that the fault was masked.

4. The method of claim 3, wherein clearing at least one corresponding bit position in the FSR comprises:
clearing a first bit position in the FSR for which a read that used an address from a corresponding element from the input vector encountered a fault condition; and
clearing bit positions subsequent to the first bit position in the FSR.

5. The method of claim 1, wherein the method further comprises:
when at least one fault has been masked, setting a processor status flag to indicate that at least one fault has been masked.

6. The method of claim 1, wherein the method further comprises:
receiving returned data for at least one of the read operations that used an address from an element of the first input vector;
writing the returned data to a corresponding element of the result vector when the corresponding element of the result vector is before any element position for which a fault was masked; and
ignoring the returned data when a corresponding element of the result vector is at or after any element position for which a fault was masked.

7. The method of claim 1, wherein the first input vector comprises one of an address or an address pointer in each element.

8. A processor that handles faults, comprising:
the processor, wherein the processor:
receives a first input vector, a control vector, and a predicate vector, each vector comprising a plurality of elements; and
for each element in the first input vector for which a corresponding element in the control vector and the predicate vector are active,
performs a read operation using an address from the element of the first input vector;
when a fault condition is encountered while performing the read operation, determines if the element is a first element where a corresponding element of the control vector is active; and
if so, processes the fault;
otherwise, masks the fault for the element.

9. The processor of claim 8, wherein, when masking the fault for the element, the processor:
ignores the fault condition that was encountered while performing the read operation using the address from the element; and
continues at least one of the read operations that use an address from an element of the first input vector.

10. The processor of claim 8, wherein, when masking the fault, the processor clears at least one corresponding bit position in a fault-status register (FSR) to indicate that the fault was masked.

11. The processor of claim 10, wherein, when clearing at least one corresponding bit position in the FSR, the processor:
clears a first bit position in the FSR for which a read that used an address from a corresponding element from the input vector encountered a fault condition; and
clears bit positions subsequent to the first bit position in the FSR.

12. The processor of claim 8, wherein, when at least one fault has been masked, the processor sets a processor status flag to indicate that at least one fault has been masked.

13. The processor of claim 8, wherein the processor:
receives returned data for at least one of the read operations that used an address from an element of the first input vector;
writes the returned data to a corresponding element of the result vector when the corresponding element of the result vector is before any element position for which a fault was masked; and
ignores the returned data when a corresponding element of the result vector is at or after any element position for which a fault was masked.

14. The processor of claim 8, wherein the first input vector comprises one of an address or an address pointer in each element.

15. A computer system that handles faults, comprising:
a processor; and
a memory coupled to the processor, wherein the memory stores instructions and data for the processor;
wherein the processor:
receives a first input vector, a control vector, and a predicate vector, each vector comprising a plurality of elements; and
for each element in the first input vector for which a corresponding element in the control vector and the predicate vector are active,
performs a read operation using an address from the element of the first input vector;
when a fault condition is encountered while performing the read operation, determines if the element is a first element where a corresponding element of the control vector is active; and
if so, processes the fault;
otherwise, masks the fault for the element.

16. The computer system of claim 15, wherein, when masking the fault for the element, the processor:
ignores the fault condition that was encountered while performing the read operation using the address from the element; and
continues at least one of the read operations that use an address from an element of the first input vector.

17. The computer system of claim 15, wherein, when masking the fault, the processor clears at least one corresponding bit position in a fault-status register (FSR) to indicate that the fault was masked.

18. The computer system of claim 17, wherein, when clearing at least one corresponding bit position in the FSR, the processor:
clears a first bit position in the FSR for which a read that used an address from a corresponding element from the input vector encountered a fault condition; and
clears bit positions subsequent to the first bit position in the FSR.

19. The computer system of claim 15, wherein, when at least one fault has been masked, the processor sets a processor status flag to indicate that at least one fault has been masked.

20. The computer system of claim 15, wherein the processor:
receives returned data for at least one of the read operations that used an address from an element of the first input vector;
writes the returned data to a corresponding element of the result vector when the corresponding element of the result vector is before any element position for which a fault was masked; and
ignores the returned data when a corresponding element of the result vector is at or after any element position for which a fault was masked.

21. The computer system of claim 15, wherein the first input vector comprises one of an address or an address pointer in each element.