Abstract:
One embodiment of the present invention provides a system for compiling source code into executable code that performs prefetching for memory operations within regions of code that tend to generate cache misses. The system operates by compiling a source code module containing programming language instructions into an executable code module containing instructions suitable for execution by a processor. Next, the system runs the executable code module in a training mode on a representative workload and keeps statistics on cache miss rates for functions within the executable code module. These statistics are used to identify a set of “hot” functions that generate a large number of cache misses. Next, explicit prefetch instructions are scheduled in advance of memory operations within the set of hot functions. In one embodiment, explicit prefetch operations are scheduled into the executable code module by activating prefetch generation at a start of an identified function, and by deactivating prefetch generation at a return from the identified function. In embodiment, the system further schedules prefetch operations for the memory operations by identifying a subset of memory operations of a particular type within the set of hot functions, and scheduling explicit prefetch operations for memory operations belonging to the subset.

Description:
RELATED APPLICATION 
     The subject matter of this application is related to the subject matter in a co-pending non-provisional application by the same inventors as the instant application and filed on the same day as the instant application entitled, “Method and Apparatus for Performing Prefetching at the Critical Section Level,” having Ser. No. 09/434,714, and filing date Nov. 5, 1999. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to compilers for computer systems. More specifically, the present invention provides a method and an apparatus for compiling source code into executable code that performs prefetching for memory operations within regions of code that tend to generate a large number of cache misses. 
     2. Related Art 
     As processor clock speeds continue to increase at an exponential rate, memory latencies are becoming a major bottleneck to computer system performance. On some applications a processor can spend as much as half of its time waiting for outstanding memory operations to move data from cache or main memory into registers within the processor. A single memory operation can cause the processor to wait for many clock cycles if the memory operation causes a cache miss from fast L1 cache and a corresponding access from slower L2 cache, or worse yet, causes a cache miss from L2 cache and a corresponding access from main memory. 
     It is possible to alleviate some of the performance limiting effects of memory operations by designing a system so that it can initiate a memory operation in advance of instructions that make use of the data returned from the memory operation. However, designing such capabilities into a processor can greatly increase the complexity of the processor. This increased complexity can increase the cost of the processor and can potentially decrease the clock speed of the processor if the additional complexity lengthens a critical path through the processor. Furthermore, the potential performance gains through the use of such techniques can be limited. 
     It is also possible to modify executable code during the compilation process so that it explicitly prefetches data associated with a memory operation in advance of where the memory operation takes place. This makes it likely that the data will be present in L1 cache when the memory operation occurs. This type of prefetching can be accomplished by scheduling an explicit prefetch operation into the code in advance of an associated memory operation in order to prefetch the data into L1 cache before the memory operation is encountered in the code. 
     Unfortunately, it is very hard to determine which data items should be prefetched and which ones should not. Prefetching all data items is wasteful because the memory system can become bottlenecked prefetching data items that are not referenced. On the other hand, analyzing individual memory operations to determine if they are good candidates for prefetching can consume a great deal of computational time. 
     What is needed is a method and an apparatus that selects a set of memory operations for prefetching without spending a great deal of time analyzing individual memory operations. 
     SUMMARY 
     One embodiment of the present invention provides a system for compiling source code into executable code that performs prefetching for memory operations within regions of code that tend to generate cache misses. The system operates by compiling a source code module containing programming language instructions into an executable code module containing instructions suitable for execution by a processor. Next, the system runs the executable code module in a training mode on a representative workload and keeps statistics on cache miss rates for functions within the executable code module. These statistics are used to identify a set of “hot” functions that generate a large number of cache misses. Next, explicit prefetch instructions are scheduled in advance of memory operations within the set of hot functions. 
     In one embodiment, explicit prefetch operations are scheduled into the executable code module by activating prefetch generation at a start of an identified function, and by deactivating prefetch generation at a return from the identified function. 
     In embodiment, the system further schedules prefetch operations for the memory operations by identifying a subset of memory operations of a particular type within the set of hot functions, and scheduling explicit prefetch operations for memory operations belonging to the subset. The particular type of memory operation can include, memory operations through pointers, memory operations involving static data, memory operations from locations that have not been previously accessed, memory operations outside of the system stack, and memory operations that are likely to be executed. 
     In one embodiment, the system schedules the prefetch operations by identifying a subset of prefetch operations with a particular property, and by scheduling the prefetch operations based on the property. For example, the particular property can include having an available issue slot, being located on an opposite side of a function call site from an associated memory operation, being located on the same side of a function call site from the associated memory operation, and being associated with a cache block that is not already subject to a scheduled prefetch operation. 
     One embodiment of the present invention provides a system for compiling source code into executable code that performs prefetching for memory operations within critical sections of code that are subject to mutual exclusion. The system operates by compiling a source code module containing programming language instructions into an executable code module containing instructions suitable for execution by a processor. Next, the system identifies a critical section within the executable code module by identifying a region of code between a mutual exclusion lock operation and a mutual exclusion unlock operation. The system schedules explicit prefetch instructions into the critical section in advance of associated memory operations. 
     In one embodiment, the system identifies the critical section of code by using a first macro to perform the mutual exclusion lock operation, wherein the first macro additionally activates prefetching. The system also uses a second macro to perform the mutual exclusion unlock operation, wherein the second macro additionally deactivates prefetching. Note that the second macro does not deactivate prefetching if the mutual exclusion unlock operation is nested within another critical section. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 illustrates a computer system in accordance with an embodiment of the present invention. 
     FIG. 2 illustrates load operations occurring within regions of executable code in accordance with an embodiment of the present invention. 
     FIG. 3A illustrates macros that enable and disable prefetching in accordance with an embodiment of the present invention. 
     FIG. 3B illustrates nesting of critical sections in accordance with an embodiment of the present invention. 
     FIG. 4 presents an example of prefetching loads that are likely to be executed accordance with an embodiment of the present invention. 
     FIG. 5 is a flow chart illustrating the process of creating code that prefetches loads within hot functions in accordance with an embodiment of the present invention. 
     FIG. 6 is a flow chart illustrating the process of creating code that prefetches loads within critical sections in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed 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 present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is 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 a computer system. This includes, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs) and DVDs (digital video discs), and computer instruction signals embodied in a transmission medium (with or without a carrier wave upon which the signals are modulated). For example, the transmission medium may include a communications network, such as the Internet. 
     Computer System 
     FIG. 1 illustrates the internal structure of computer system  100  in accordance with an embodiment of the present invention. In particular, FIG. 1 illustrates the memory hierarchy for computer system  100 , which includes registers  104  within central processing unit (CPU)  102 , L1 cache  106 , prefetch cache  108 , L2 cache  110 , memory  112  and storage device  116 . 
     CPU  102  can include any type of processing engine that can be used in a computer system, including, but not limited to, a microprocessor, a mainframe processor, a device controller, a processor within a personal organizer and processing circuitry within an appliance. Registers  104  are internal registers within CPU  102  into which data is loaded from L1 cache  106 , prefetch cache  108 , L2 cache  110  or memory  112 . Once data is loaded into registers  104 , CPU  102  can perform computational operations on the data. (Although this disclosure often discusses prefetching for “load” operations, please note that the discussion applies to any memory operations that can benefit from prefetching, including stores and other memory references.) 
     Data is loaded into registers  104  from L1 cache  106 . L1 cache  106  is a high-speed cache memory of limited size that is located in close proximity to CPU  102 . In some embodiments, L1 cache  106  may be located within the same semiconductor chip as CPU  102 . 
     Similarly, data is loaded into registers  104  from prefetch cache  108 . Prefetch cache  108  is also a high-speed cache memory of limited size that is located in close proximity to CPU  102 . The difference between prefetch cache  108  and L1 cache  106  is that prefetch cache  108  holds data that is explicitly prefetched, whereas L1 cache  106  holds data that has been recently referenced, but not prefetched. The use of prefetch cache  108  allows speculative prefetching to take place without polluting L1 cache  106 . 
     Data is loaded into L1 cache  106  and prefetch cache  108  from L2 cache  110 . L2 cache  110  is considerably larger that L1 cache  106  or prefetch cache  108 . However, L2 cache is located farther from CPU  102 , and hence accesses to L2 cache  110  take more time than accesses to L1 cache  106  or prefetch cache  108 . However, note that accesses to L2 cache take less time than accesses to memory  112 . 
     L1 cache  106 , prefetch cache  108  and L2 cache  110  may be designed in a number of ways. For example, they may include direct-mapped caches, fully associative caches or set-associative caches. They may also include write-through or write-back caches. 
     Data is loaded into L2 cache from memory  112 . Memory  112  can include any type of random access memory that can be used to store code and/or data for use by CPU  102 . In the embodiment of the present invention illustrated in FIG. 1, memory  112  contains code with explicit prefetch instructions that are inserted at the function level or at the critical section level as is discussed below with reference to FIGS. 2-6. 
     Data is loaded into memory  112  from files within storage device  116 . Storage device  116  can include any type of non-volatile storage device for storing code and/or data to be operated on by CPU  102 . In one embodiment, storage device  116  includes a magnetic disk drive. 
     FIG. 1 also illustrates how CPU  102  can be coupled to server  122  through network  120 . Network  120  can include any type of wire or wireless communication channel capable of coupling together computing nodes. This includes, but is not limited to, a local area network, a wide area network, or a combination of networks. In one embodiment of the present invention, network  120  includes the Internet. Server  122  can include any computational node including a mechanism for servicing requests from a client for computational or data storage resources. In embodiment of the present invention, server  122  is a file server that contains executable code to by executed by CPU  102 . Also note that although network  120  is illustrated as being directly coupled to CPU  102 , in general network  102  can be coupled to other locations within the computer system illustrated in FIG.  1 . 
     Note that FIG. 1 does not illustrate the many possible ways in which components of the memory hierarchy can be coupled together through various data paths and busses. Also note that the present invention can generally be applied to any type of computer system with prefetch capability, not just the specific computer system illustrated in FIG.  1 . 
     Loads within Regions of Code 
     FIG. 2 illustrates load operations occurring within regions of executable code in accordance with an embodiment of the present invention. FIG. 2 illustrates a section of code that is divided into regions, including region A  202 , region B  204  and region C  206 . These regions include load operations to load data from the memory hierarchy into registers  104  within CPU  102 . These load operations are illustrated in the middle column of FIG.  1 . Note that the section of code also includes many intervening non-load operations, which are not illustrated. These non-load operations manipulate the data that is pulled into registers  104  by the load operations. 
     The right-hand column of FIG. 2 illustrates the results of the load operations. More specifically, the first two load operations from the top of FIG. 2 (which are within region A  202 ) are retrieved from L1 cache  106 . The next four load operations (within region B  204 ) are retrieved from L2 cache  110 , memory  112 , L2 cache  110  and L2 cache  110 , respectively. The last two loads (within region C  206 ) are retrieved from L1 cache  106 . 
     In this example, all of the loads within region B  204  generate cache misses from L1 cache  106  to L2 cache  110 . One of these loads generates an additional cache miss in L2 cache  110  and a corresponding access to memory  112 . Region B  204  is referred to as a “hot” region because a high percentage of the loads within region B  204  generate cache misses. Hence, the loads within region B  204  are good candidates for prefetching. 
     Note that region boundaries can be determined in a number of ways. In one embodiment of the present invention, region boundaries are function boundaries. In another embodiment, region boundaries are critical section boundaries. Note that loads within critical sections tend to generate a large number of cache misses because critical sections typically access shared data, which is prone to cache misses. Region boundaries may also encompass arbitrary “hot” regions of code that are specified by a user. Regions boundaries can also encompass complete source files, which can be specified in a command line. 
     Prefetching for Critical Sections 
     FIG. 3A illustrates mutual exclusion macros that enable and disable prefetching in accordance with an embodiment of the present invention. The first macro at the top of FIG. 3A is a mutual exclusion lock macro that turns on a prefetching feature of the compiler with specific prefetch properties before locking a mutual exclusion variable. This prefetching feature attempts to perform prefetching for all load operations unless the prefetch operations are filtered out as is discussed below with reference to FIGS. 5 and 6. Note that the mutual exclusion variable can generally include any type of mutual exclusion variable, such as a mutual exclusion variable associated with a spin lock, a semaphore, a read-writer lock, a turnstile, a mutex lock, an adaptive mutex lock, or any other mutual exclusion mechanism. 
     Also note that the prefetching feature can have specific prefetch properties for associated load and prefetch instructions. These properties are discussed in more detail below. Hence, different mutual exclusion macros can activate different prefetching properties. In other embodiment of the present invention different prefetching properties can be activated at the function level, the file level or within an arbitrary region of code. These different prefetching properties can be activated and deactivated by different regions markers (such as mutual exclusion macros) that are specific to particular properties. Note that these different region markers can be nested. 
     The second macro in FIG. 3A illustrates a corresponding mutual exclusion unlock macro that unlocks the mutual exclusion variable and turns off the prefetching feature. In one embodiment of the present invention, the system checks for an unmatched second macro that deactivates prefetching and is not preceded by a matching first macro that activates prefetching. If such an unmatched second macro is encountered, the may system signal an error condition. 
     FIG. 3B illustrates nesting of critical sections in accordance with an embodiment of the present invention. In many applications, critical sections are nested. For example, in FIG. 3B, critical section B  304 , which is bounded by a mutex_lock(B) and mutex_unlock(B), is nested within critical section A  302 , which is bounded by a mutex_lock(A) and mutex_unlock(A). In this case, the turnoff prefetch( ) function keeps track of the number of nested critical sections and does not turn off prefetching at the end of a nested critical section. For example, the mutex_unlock(B) call within FIG. 3B does not turn off prefetching because it is associated with nested critical section B  304 . However, the mutex_unlock(A) call does turn off prefetching because subsequent code is outside of any critical section and is not subject to prefetching. 
     FIG. 6 is a flow chart illustrating the process of creating code that prefetches loads within critical sections in accordance with an embodiment of the present invention. The system starts by compiling a source code module into executable code instructions to produce a corresponding executable code module (step  602 ). In doing so, the system identifies critical sections (step  604 ). This can be done by using the mutex_lock( ) and mutex_unlock( ) macros illustrated in FIG.  3 A. Alternatively, the compiler can be modified to look for mutual exclusion lock and unlock operations in order to enable and disable prefetching. 
     Next, the system examines the load operations within the critical sections and schedules prefetch operations for certain types of load operations (step  606 ). This can greatly reduce the number of prefetch operations. For example, the system can choose to prefetch, loads through pointers, loads of static data, loads through pointer and loads of static data, loads from outside the system stack, or loads that are likely to be executed. Note that loads that are likely to be executed can be identified by running the executable code in a training mode. Also note that loads within the system stack or loads from locations that have been previously loaded are unlikely to generate cache misses and are hence bad candidates for prefetching. 
     The system can also schedule prefetch operations that appear within critical sections based upon properties of the prefetch operations (step  608 ). For example, the system can choose to schedule a prefetch operations only if there exists an available load issue slot and available outstanding loads for the prefetch operation. Note that a typical load store unit in a processor has a small number of load issue slots available as well as a limited number of outstanding loads. If these load issue slots are filled, it makes little sense to schedule a prefetch because no load issue slots are available for the prefetch. The system can also schedule a prefetch operation on an opposite side of a function call site from an associated load operation (or alternatively on the same side of the function call site). This can be useful if the call site is for a function that is unlikely to affect the cache, such as a mutex lock function. For other types of functions it makes little sense to issue a prefetch before the function call, because the function call is likely to move the flow of execution to another region of the code for a long period of time. The system can also schedule a prefetch for a cache block that is not already subject to a scheduled prefetch operation. 
     At this point, the source code is ready for normal program execution. 
     Prefetching Loads that are likely to be Executed 
     FIG. 4 presents an example of prefetching loads that are likely to be executed in accordance with an embodiment of the present invention. Function  400  is divided into four basic blocks  402 - 405 . A basic block is a section of code that executes without a change in control flow. Hence, a basic block contains at most one branch or function call at the end of the block. In FIG. 4, there is a conditional branch at the end of basic block  402 , which goes to either basic block  404  or basic block  403 . Later on, these separate branch paths rejoin in basic block  405 . 
     Each of the illustrated basic blocks  402 - 405  includes load operations. More specifically, basic block  402  includes loads A and B. Basic block  403  includes loads D and E. Basic block  404  includes loads F and G. Finally, basic block  405  includes loads H, I, J and K. 
     In the example illustrated in FIG. 4, assume that function  400  is a “hot” function that has exhibited a large number of cache misses while running on a representative workload. In this example, the system starts by filtering out loads that are directed to the system stack, because these loads are unlikely to generate cache misses. This eliminates loads C, G and H. 
     Next, the system eliminates loads that are not likely to be executed. Assume that basic blocks  402 ,  404  and  405  contain likely executed load operations. This eliminates loads D and E. Note that the system can identify the load instructions that are likely to be executed by running a program containing function  400  in a “training mode” on a representative workload and by keeping statistics on which instructions are executed through function  400 . 
     Next, the system schedules prefetches up the likely execution path. In doing so, the system ensures that the number of outstanding prefetches does not exceed the number of available load issue slots in the system&#39;s load store unit and the maximum number of outstanding loads. The example illustrated in FIG. 4 assumes there are four outstanding loads available. Hence, at the beginning of basic block  402 , the system prefetches loads B, F and I prior to load A. (Note that the three prefetches for B, F and I plus the load of A will take up the four load issue slots). Next, assuming that the prefetch of B completes immediately after the load of A completes, another outstanding load becomes available and the system prefetches load J. Later on, assuming the prefetch of F completes before load F is encountered, the system prefetches load K. 
     Note that the technique of prefetching loads that are likely to be executed can be performed for any region of code, and is not limited to a function. For example, the system can also prefetch loads that are likely to be executed within a critical section, or any other arbitrary section of code. 
     Prefetching for Selected Functions 
     FIG. 5 is a flow chart illustrating the process of creating code that prefetches loads within hot functions in accordance with an embodiment of the present invention. The system starts by compiling a source code module into executable code instructions to produce a corresponding executable code module (step  502 ). 
     Next, the system determines which functions within the executable module tend to create a large number of cache misses. We refer to these functions as “hot functions.” The system does so by running the executable module in a training mode on a representative workload (step  504 ), and by keeping statistics on cache miss rates at the function level (step  506 ). Next, the system uses these statistics to identify functions that tend to generate a large number of cache misses (step  508 ). 
     Next, the system examines all load operations within the hot functions and schedules prefetch operations for certain types of load operations (as was done above for critical sections) (step  510 ). The system can also schedule prefetch operations that appear within hot functions based upon properties of the prefetch operations (step  512 ). At this point, the source code is ready for normal program execution. 
     The foregoing descriptions of embodiments of the invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the invention 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 invention. The scope of the invention is defined by the appended claims.