Abstract:
This invention prevents illegal memory address faults on speculative data loads. Circular addressing of the address pointer limits memory access to a range of addresses including all addresses used by the address pointer and not including any invalid addresses. The invention uses circular addressing hardware, if available on the data processor. If not available, this invention simulates circular addressing. This invention permits loads to be issued earlier than if predication were used and allows already predicated loads to be speculated without the overhead of a compound predicate. This invention can be used on processors without hardware supporting speculation.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims priority under 35 U.S.C. 119(e)(1) to U.S. Provisional Application No. 60/645,164 filed Jan. 18, 2005. 
     
    
     TECHNICAL FIELD OF THE INVENTION  
       [0002]     The technical field of this invention is speculative loading data in data processing operations.  
       BACKGROUND OF THE INVENTION  
       [0003]     Speculative data processing operations are a technique used to increase speed of operation. An instruction is issued on speculation before the input operands are confirmed. If the input operands are later found to have been correct, then the speculative execution saved any time between the issue of the speculative instruction and the time when the operands are confirmed as valid. If the operands are later found to be incorrect, then the instruction is issued with the correct operands. This results in no increase in speed, but generally there is no loss of processing due to the speculative execution. Speculative execution is generally only used in super-scalar data processors or very long instruction word (VLIW) data processors which have vacant or empty instruction issue slots that can be filled by the speculative execution. Super-scalar and VLIW data processors have the capability of issuing more than one instruction each machine cycle. The fastest processing operation occurs when these data processors issue their maximum number of instructions each cycle. However, data dependencies, resource conflicts and other issues often prevent issuing the maximum number of instructions. Speculative execution is a technique intended to permit these otherwise unused instruction slots to be profitably employed. As described above, speculative execution employing unused instruction slots has the potential for performance gain and no performance loss.  
         [0004]     Data loads generally have the longest latency of any instructions in super-scalar or VLIW data processors. Differences in the speed of operation of high end data processors and large memories can cause length latency in data loads. Thus speculative execution of data loads may be particularly advantageous. The memory address for such a data load is often stored in a data register as an address pointer. The data in the data register is supplied to the memory to specify the address location storing the desired data. Many data processors use a base plus offset address. A base data register stores a base address and an offset data register stores an offset from the base address. The final memory address is the sum of the base address and the offset. Generally the base address is modified infrequently with most of the variability in the offset. The identity of the address register or offset address register must be known at the time of the issue of a data load instruction. A speculative load can be issued using the data in this known address register or offset register at a time before this load is confirmed valid.  
         [0005]     There are two types of possible faults of such speculative loads. The data stored in this address register or offset register may not be proper for the intended data load. This would occur if the data in the address register or data register were changed between the time of the issue of the speculative load instruction and the issue of the actual load instruction. It is also possible that the data stored in the intended memory location is incorrect. This would occur if there were a data write to the memory address between the time of the issue of the speculative load instruction and the issue of the actual load instruction. This invention deals only with the invalid address fault.  
         [0006]     A speculative load to an invalid address may cause problems. On many data processors, load instructions to bogus addresses can cause a memory access fault. That is, a load instruction specifying an address not implemented in the system causes an operating system fault. With 32-bit data registers capable of specifying more than 4 billion data words, it is likely that not all possible data values that can be stored in an address register or an offset register point to a valid memory address. Such memory access faults may cause the application to crash or require hundreds or thousands of machine cycles for the operating system to recover. The possibility of such memory faults prevents more frequent use of speculative loads.  
         [0007]     If such a speculative load can be restricted to a subset of the address space which known to be accessible without faulting, it may be possible to issue a speculative load earlier in the instruction stream. In many cases, program performance could be significantly improved if a speculative load is issued before the address is confirmed valid. Such early data loading may prevent the algorithm from waiting for data following confirmation of the data address.  
         [0008]     There are only two known solutions to this problem. The speculative load may use predication to conditionally prevent an improper load from executing. Using predication delays the load unit the predicate is known. The second known solution uses special purpose hardware. Many data processors do not include special purpose hardware for such speculative loads. Accordingly, another method enabling speculative data loads would be useful in data processors.  
       SUMMARY OF THE INVENTION  
       [0009]     This invention uses circular addressing hardware, if available on the data processor. If not available, this invention simulates circular addressing. Making the address pointer circular restricts the address pointer range. If the circular addressing is set up properly, then the address pointer will always point to a valid memory address. Thus speculative data loads will not cause a memory access fault.  
         [0010]     This invention is advantageous because it permits loads to be issued earlier than if predication were used. This invention also allows already predicated loads to be speculated without the overhead of a compound predicate. This invention can be used on processors without hardware supporting speculation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     These and other aspects of this invention are illustrated in the drawings, in which:  
         [0012]      FIG. 1  illustrates details of a very long instruction word digital signal processor integrated circuit suitable for practicing this invention (prior art);  
         [0013]      FIG. 2  illustrates the pipeline stages of the very long instruction word digital signal processor integrated circuit illustrated in  FIG. 3  (prior art);  
         [0014]      FIG. 3  illustrates the instruction syntax of the very long instruction word digital signal processor integrated circuit illustrated in  FIG. 1  (prior art);  
         [0015]      FIG. 4  illustrates a process to set up circular addressing for speculative loads;  
         [0016]      FIG. 5  illustrates an example of the operation of selection of the circular buffer; and  
         [0017]      FIG. 6  illustrates use of this invention applied automatically by a compiler. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0018]      FIG. 1  is a block diagram illustrating details of a digital signal processor integrated circuit  100  suitable but not essential for use in this invention (prior art). The digital signal processor integrated circuit  100  includes central processing unit  1 , which is a 32-bit eight-way VLIW pipelined processor. Central processing unit  1  is coupled to level  1  instruction cache  2  included in digital signal processor integrated circuit  100 . Digital signal processor integrated circuit  100  also includes level one data cache  3 . Digital signal processor integrated circuit  100  also includes peripherals  4  to  9 . These peripherals preferably include an external memory interface (EMIF)  4  and a direct memory access (DMA) controller  5 . External memory interface (EMIF)  4  preferably supports access to supports synchronous and asynchronous SRAM and synchronous DRAM. Direct memory access (DMA) controller  5  preferably provides 2-channel auto-boot loading direct memory access. These peripherals include power-down logic  6 . Power-down logic  6  preferably can halt central processing unit activity, peripheral activity, and phase lock loop (PLL) clock synchronization activity to reduce power consumption. These peripherals also include host ports  7 , serial ports  8  and programmable timers  9 .  
         [0019]     Central processing unit  1  has a 32-bit, byte addressable address space. Internal memory on the same integrated circuit is preferably organized in a data space including level one data cache  3  and a program space including level one instruction cache  2 . When off-chip memory is used, preferably these two spaces are unified into a single memory space via the external memory interface (EMIF)  4 .  
         [0020]     Level one data cache  3  may be internally accessed by central processing unit  1  via two internal ports  3   a  and  3   b . Each internal port  3   a  and  3   b  preferably has 32 bits of data and a 32-bit byte address reach. Level one instruction cache  2  may be internally accessed by central processing unit  1  via a single port  2   a . Port  2   a  of level one instruction cache  121  preferably has an instruction-fetch width of 256 bits and a 30-bit word (four bytes) address, equivalent to a 32-bit byte address.  
         [0021]     Central processing unit  1  includes program fetch unit  10 , instruction dispatch unit  11 , instruction decode unit  12  and two data paths  20  and  30 . First data path  20  includes four functional units designated L1 unit  22 , S1 unit  23 , M1 unit  24  and D1 unit  25  and 16 32-bit A registers forming register file  21 . Second data path  30  likewise includes four functional units designated L2 unit  32 , S2 unit  33 , M2 unit  34  and D2 unit  35  and 16 32-bit B registers forming register file  31 . The functional units of each data path access the corresponding register file for their operands. There are two cross paths  27  and  37  permitting limited access to one register in the opposite register file each pipeline stage. Central processing unit  1  includes control registers  13 , control logic  14 , and test logic  15 , emulation logic  16  and interrupt logic  17 .  
         [0022]     Program fetch unit  10 , instruction dispatch unit  11  and instruction decode unit  12  recall instructions from level one instruction cache  2  and deliver up to eight 32-bit instructions to the functional units every instruction cycle. Processing occurs in each of the two data paths  20  and  30 . As previously described above each data path has four corresponding functional units (L, S, M and D) and a corresponding register file containing 16 32-bit registers. Each functional unit is controlled by a 32-bit instruction. The data paths are further described below. A control register file  13  provides the means to configure and control various processor operations.  
         [0023]      FIG. 2  illustrates the pipeline stages  200  of digital signal processor integrated circuit  100  (prior art). These pipeline stages are divided into three groups: fetch group  210 ; decode group  220 ; and execute group  230 . All instructions in the instruction set flow through the fetch, decode, and execute stages of the pipeline. Fetch group  210  has four phases for all instructions, and decode group  220  has two phases for all instructions. Execute group  330  requires a varying number of phases depending on the type of instruction.  
         [0024]     The fetch phases of the fetch group  210  are: Program address generate phase  211  (PG); Program address send phase  212  (PS); Program access ready wait stage  213  (PW); and Program fetch packet receive stage  214  (PR). Digital signal processor integrated circuit  100  uses a fetch packet (FP) of eight instructions. All eight of the instructions proceed through fetch group  210  together. During PG phase  211 , the program address is generated in program fetch unit  10 . During PS phase  212 , this program address is sent to memory. During PW phase  213 , the memory read occurs. Finally during PR phase  214 , the fetch packet is received at CPU  1 .  
         [0025]     The decode phases of decode group  220  are: Instruction dispatch (DP)  221 ; and Instruction decode (DC)  222 . During the DP phase  221 , the fetch packets are split into execute packets. Execute packets consist of one or more instructions which are coded to execute in parallel. The relationship between a fetch packet and an execute packet will be explained below. During DP phase  222 , the instructions in an execute packet are assigned to the appropriate functional units. Also during DC phase  222 , the source registers, destination registers and associated paths are decoded for the execution of the instructions in the respective functional units.  
         [0026]     The execute phases of the execute group  230  are: Execute 1 (E2)  231 ; Execute 2 (E2)  232 ; Execute 3 (E3)  233 ; Execute 4 (E4)  234 ; and Execute 5 (E5)  235 . Different types of instructions require different numbers of these phases to complete. These phases of the pipeline play an important role in understanding the device state at CPU cycle boundaries.  
         [0027]     During E1 phase  231 , the conditions for the instructions are evaluated and operands are read for all instruction types. For load and store instructions, address generation is performed and address modifications are written to a register file. For branch instructions, branch fetch packet in PG phase  211  is affected. For all single-cycle instructions, the results are written to a register file. All single-cycle instructions complete during the E1 phase  231 .  
         [0028]     During the E2 phase  232 , for load instructions, the address is sent to memory. For store instructions, the address and data are sent to memory. Single-cycle instructions that saturate results set the SAT bit in the control status register (CSR) if saturation occurs. For single cycle 16×16 multiply instructions, the results are written to a register file. For M unit non-multiply instructions, the results are written to a register file. All ordinary multiply unit instructions complete during E2 phase  222 .  
         [0029]     During E3 phase  233 , data memory accesses are performed. Any multiply instruction that saturates results sets the SAT bit in the control status register (CSR) if saturation occurs. Store instructions complete during the E3 phase  233 .  
         [0030]     During E4 phase  234 , for load instructions, data is brought to the CPU boundary. For multiply extensions instructions, the results are written to a register file. Multiply extension instructions complete during the E4 phase  234 .  
         [0031]     During E5 phase  235 , assuming no memory stall load instructions write data into a register. Load instructions complete during the E5 phase  235 .  
         [0032]      FIG. 3  illustrates an example of the instruction coding of instructions used by digital signal processor integrated circuit  100  (prior art). Each instruction consists of 32 bits and controls the operation of one of the eight functional units. The bit fields are defined as follows. The creg field (bits  29  to  31 ) is the conditional register field. These bits identify whether the instruction is conditional and identify the predicate register. The z bit (bit  28 ) indicates whether the predication is based upon zero or not zero in the predicate register. If z=1, the test is for equality with zero. If z=0, the test is for nonzero. The case of creg=0 and z=0 is treated as always true to allow unconditional instruction execution. The creg field is encoded in the instruction opcode as shown in Table 1.  
                                                         TABLE 1                                       Conditional   creg   z                Register   31   30   29   28                       Unconditional   0   0   0   0           Reserved   0   0   0   1           B0   0   0   1   z           B1   0   1   0   z           B2   0   1   1   z           A1   1   0   0   z           A2   1   0   1   z           A0   1   1   0   z           Reserved   1   1   1   X                        
 Note that “z” in the z bit column refers to the zero/not zero comparison selection noted above and “X” is a don&#39;t care state. This coding can only specify a subset of the 32 registers in each register file as predicate registers. This selection was made to preserve bits in the instruction coding. 
 
         [0033]     The dst field (bits  23  to  27 ) specifies one of the 32 registers in the corresponding register file as the destination of the instruction results.  
         [0034]     The scr2 field (bits  18  to  22 ) specifies one of the 32 registers in the corresponding register file as the second source operand.  
         [0035]     The scr1/cst field (bits  13  to  17 ) has several meanings depending on the instruction opcode field (bits  3  to  12 ). The first meaning specifies one of the 32 registers of the corresponding register file as the first operand. The second meaning is a 5-bit immediate constant. Depending on the instruction type, this is treated as an unsigned integer and zero extended to 32 bits or is treated as a signed integer and sign extended to 32 bits. Lastly, this field can specify one of the 32 registers in the opposite register file if the instruction invokes one of the register file cross paths  27  or  37 .  
         [0036]     The opcode field (bits  3  to  12 ) specifies the type of instruction and designates appropriate instruction options. A detailed explanation of this field is beyond the scope of this invention except for the instruction options detailed below.  
         [0037]     The s bit (bit  1 ) designates the data path  20  or  30 . If s=0, then data path  20  is selected. This limits the functional unit to L1 unit  22 , S1 unit  23 , M1 unit  24  and D1 unit  25  and the corresponding register file A  21 . Similarly, s=1 selects data path  30  limiting the functional unit to L2 unit  32 , S2 unit  33 , M2 unit  34  and D2 unit  35  and the corresponding register file B  31 .  
         [0038]     The p bit (bit  0 ) marks the execute packets. The p-bit determines whether the instruction executes in parallel with the following instruction. The p-bits are scanned from lower to higher address. If p=1 for the current instruction, then the next instruction executes in parallel with the current instruction. If p=0 for the current instruction, then the next instruction executes in the cycle after the current instruction. All instructions executing in parallel constitute an execute packet. An execute packet can contain up to eight instructions. Each instruction in an execute packet must use a different functional unit.  
         [0039]      FIG. 4  illustrates process  400  showing how to set up circular addressing for speculative loads. Process  400  can be implemented manually by a user or automatically by a compiler. Flow chart  400  begins at start block  401 . At block  402  the process considers the next pointer to enable speculative loads. Block  403  determines the address range used by the particular address register or offset register. Block  404  creates a circular buffer including this range and also completely within the memory space implemented by the system.  
         [0040]      FIG. 5  illustrates an example of the operation of block  404 . Memory range  500  is a 32-bit addressable memory range from hex00000000 to hexFFFFFFFF. Address range  501  is the range of legal addresses used by the address pointer or offset pointer in question. Circular buffer  502  is an example circular buffer that can be defined for this address range  501 . Circular buffer  502  starts at a base address p and includes an offset range x. Typical circular address hardware enables an offset range x that is an integral power of 2, i.e. 2 N  where N is an integer. Such an offset range x is achieved by limiting the offset pointer to N bits. Note that offset range  502  includes all of address range  501  of the pointer in question and does not fall into illegal address range  503  corresponding to memory addresses having no physical counterpart in the system. Accordingly, no address within offset range  502  will generate a memory access fault due to an illegal, not implemented address.  
         [0041]     Block  450  then declares the address pointer to be a circular address pointer. This declaration invokes circular addressing hardware that insures that all accesses using this pointer fall within offset range  502 . Any change to the pointer that would tend fall outside this offset range, such as a pointer increment or pointer arithmetic, causes the actual address used to wrap around and alias into offset range  502 .  
         [0042]     Query block  406  determines if the process will operate on another pointer. If so (Yes at query block  406 ), then process flow return to block  402  for the next pointer. If not (No at query block  406 ), flow chart  400  completes at end block  407 .  
         [0043]     The complier then can create speculative loads using the modified pointer. A speculative load using this pointer will always generate a legal address, that is, an address within the range of memory implemented in this system. The circular addressing hardware insures that the actual memory access generated falls within the offset range regardless of the data in the pointer register. This prevents generation of a memory access fault triggered by an illegal, not implemented address. The speculative load is preferably launched early enough so that the return data arrives at the data processor at the same time as pointer register is determined to be valid. The speculative load address is compared with the confirmed valid address. If these are the same, the data processor uses the data from the speculative load. This assumes that the memory data has not changed from the issue of the speculative load and the confirmation of the address. If these are not the same, then the speculative load data is invalid. The speculative load data is discarded and a new memory load issues for the data at the confirmed valid address. If the speculative load address is confirmed correct, then the algorithm speeds up based upon the early issue of the speculative load relative to when the pointer is confirmed valid.  
         [0044]     The above discussion assumes the data processor implements a circular addressing mode in hardware. If circular addressing hardware is not available, it can be simulated. Assume that the address is calculated in the common base/offset manner. That is the actual address used in the sum of the data in a base address register and the date in an offset address register. It will often be the case that a proper circular address buffer of block  404  can be formed by limiting the range of the offset address register. A logical AND of the offset address register data with a mask having N least significant bits  1  and the most significant bits  0  creates a circular buffer address range of 2 N . A proper circular buffer can be implemented by prefacing each speculative load by this AND operation  
         [0045]     Offset register AND mask  
         [0046]     Where: mask is the above described data mask. This process requires an additional operation to condition the speculative load operation. It will often be the case that there will be an unused instruction slot to permit this operation without requiring additional cycles.  
         [0047]     Using a buffer size which is a power of 2 and which encloses the data structure but still lies entirely within the range of memory, loads can always be accessed without faulting. The loads can be performing by masking off high bits of the pointer address that are outside of address range.  
         [0048]     This invention is applicable only for loads that use offset based addressing with p as the base pointer. The use of circular buffers is guarantees that even if an offset is outside the linear range of the array x the address will be wrapped into a legal address using circular addressing. Circular addressing will not apply in a situation where the base pointer p is continuously changing. Fortunately in the domain of irregular loops, while the actual buffer is known, offsets are often computed speculatively and pre-loading these can have a huge benefit on scheduling. Since the buffer has been declared circular, multiple loads can be performed ahead of time, without waiting to make sure which of these offsets lie within the valid range of the buffer.  
         [0049]     Efficient software-pipelined schedules for while loops often depend on being able to speculate loads. For one set of example multimedia applications on the Texas Instruments Incorporated TMS320C6000 digital signal processor, the inability of the compiler to speculate loads with unbounded address ranges, such as when the offset is contained in a register, forces the compiler to generate less efficient software pipelined schedules. Simulations indicate that if the compiler is able to speculate these unbounded loads, an average speedup of greater than 1.3 times can be achieved.  
         [0050]      FIG. 6  illustrates this invention applied automatically by the compiler. Complier process  600  begins at start block  601 . Query block  602  checks to determine it an address pointer has been declared a circular address. If so (Yes at query block  602 ), the compiler assumes that it is safe to speculate loads using this pointer regardless of the actual offset into the pointer by enabling speculative loads in block  603 . This may permit the compiler to speculatively schedule loads in more cases resulting in better performance.  
         [0051]     In block  604  the compiler may suggest a set of loads that, if made circular and enabled for speculative loads, might lead to better performance. The compiler would search the high level code and determine which address pointers if made circular may aid in speculative loading. The user would then be free to follow or disregard the compiler suggestions in query block  605 . Accepted suggestions and previously declared circular pointers flow to block  401  in  FIG. 4  (Yes at query block  605 ). If not accepted (No at query block  605 ), this complier block ends at end block  606 .  
         [0052]     This invention works because making a load circular implies making sure that the offset referenced in the load falls within a circular addressing range. Thus no memory access fault would result from speculative loading.