Loop Bodies Research Articles

Declarative peephole optimization using string pattern matching

Peephole optimisation as a last step of a compilation sequence capitalises on significant opportunities for removing low level code slackness left over by the code generation process. We have designed a peephole optimiser that operates using a declarative specification of the optimisations to be performed. The optimiser's implementation is based on string pattern matching using regular expressions. We used this approach to prototype an optimiser to convert target machine instruction sequences containing conditional execution of instructions inside loop bodies into code that adaptively executes the optimum branch instructions according to the program's branch behaviour.

Optimizing alpha executables on Windows NT with spike

Vol. 9 No. 4 1997 3 Spike is a performance tool developed by DIGITAL to optimize Alpha executables on the Windows NT operating system. This optimization system has two main components: the Spike Optimizer and the Spike Optimization Environment. The Spike Optimizer reads in an executable, optimizes the code, and writes out the optimized version. The Optimizer uses profile feedback from previous runs of an application to guide its optimizations. Profile feedback is not commonly used in practice because it is difficult to collect, manage, and apply profile information. The Spike Optimization Environment provides a user-transparent profile feedback system that solves most of these problems, allowing a user to easily optimize large applications composed of many executables and dynamic link libraries (DLLs). Optimizing an executable image after it has been compiled and linked has several advantages. The Spike Optimizer can see the entire image and perform interprocedural optimizations, particularly with regard to code layout. The Optimizer can use profile feedback easily, because the executable that is profiled is the same executable that is optimized; no awkward mapping of profile data back to the source language takes place. Also, Spike can be used when the sources to an application are not available, which is beneficial when DIGITAL is working with independent software vendors (ISVs) to tune applications. Applications can be loosely classified into two categories: loop-intensive programs and call-intensive programs. Conventional compiler technology is well suited to loop-intensive programs. The important loops in a program in this category are within a single procedure, which is typically the unit of compilation. The control flow is predictable, and the compiler can use simple heuristics to determine the frequently executed parts of the procedure. Spike is designed for large, call-intensive programs; it uses interprocedural optimization and profile feedback. In call-intensive programs, the important loops span multiple procedures, and the loop bodies contain procedure calls. Consequently, optimizations on the loops must be interprocedural. The control flow is Optimizing Alpha Executables on Windows NT with Spike Robert S. Cohn David W. Goodwin P. Geoffrey Lowney

Improving superscalar instruction dispatch and issue by exploiting dynamic code sequences

Superscalar processors currently have the potential to fetch multiple basic blocks per cycle by employing one of several recently proposed instruction fetch mechanisms. However, this increased fetch bandwidth cannot be exploited unless pipeline stages further downstream correspondingly improve. In particular, register renaming a large number of instructions per cycle is difficult. A large instruction window, needed to receive multiple basic blocks per cycle, will slow down dependence resolution and instruction issue. This paper addresses these and related issues by proposing (i) partitioning of the instruction window into multiple blocks, each holding a dynamic code sequence; (ii) logical partitioning of the register file into a global file and several local files, the latter holding registers local to a dynamic code sequence; (iii) the dynamic recording and reuse of register renaming information for registers local to a dynamic code sequence. Performance studies show these mechanisms improve performance over traditional superscalar processors by factors ranging from 1.5 to a little over 3 for the SPEC Integer programs. Next, it is observed that several of the loops in the benchmarks display vector-like behavior during execution, even if the static loop bodies are likely complex for compile-time vectorization. A dynamic loop vectorization mechanism that builds on top of the above mechanisms is briefly outlined. The mechanism vectorizes up to 60% of the dynamic instructions for some programs, albeit the average number of iterations per loop is quite small.

The co-invariant generator: An aid in deriving loop bodies

Abstract Given a loop invariant, I , and an assignment, α , which decreases the variant, we define a constructive function, cg, called the co-invariant generator, which has the property that I Λ cg ( α, I ) ⇒ wp ( α, I ), where wp ( α, I ) is the weakest precondition for α to establish I. Several results about the co-invariant generator are proved, important special cases are considered, and a non-trivial example of its use in deriving the body of a loop is given. We also define a function which performs a related constructive action on terms formed from binary operations. The coinvariant generator makes a useful contribution to formalising and automating a key step in program derivation.

Scalar replacement in the presence of conditional control flow

AbstractMost conventional compilers fail to allocate array elements to registers because standard data‐flow analysis treats arrays like scalars, making it impossible to analyze the definitions and uses of individual array elements. This deficiency is particularly troublesome for floating‐point registers, which are most often used as temporary repositories for subscripted variables.This paper presents a source‐to‐source transformation, called scalar replacement, that finds opportunities for reuse of subscripted variables and replaces the references involved by references to temporary scalar variables. The scalar replaced variables are more likely to be assigned to registers by the coloring‐based register allocators found in most compilers than are their unreplaced counterparts. The algorithm presented here extends previous techniques for scalar replacement by allowing the presence of forward conditional control flow within loop bodies through the mapping of partial redundancy elimination to scalar replacement. Finally, experimental results show that scalar replacement is extremely effective. On kernels, integer‐factor improvements over code generated by a good optimizing compiler of conventional design are possible.

Data parallel computers and the Forall statement

The array constructs of Fortran 90 (formerly called Fortran 8x) map naturally onto SIMD architectures, such as that of the Connection Machine computer system, that support a data parallel programming style. The Forall statement, an extension to Fortran 90 allowing for the expression of simultaneous execution of certain do loop bodies, enhances this natural fit. A Fortran 90 compiler for data parallel machines is naturally extended to handle the Forall statement. We describe the data structures and algorithms used to effect this extension and present some examples of source code fragments and the target operations generated for them.

On the use of diagnostic dependence-analysis tools in parallel programming: Experiences using PTOOL

Although considerable technology has been developed for debugging and developing sequential programs, producing verifiably correct parallel code is a much harder task. In view of the large number of possible scheduling sequences, exhaustive testing is not a feasible method for determining whether a given parallel program is correct; nor have there been sufficient theoretical developments to allow the automatic verification of parallel programs. PTOOL, a tool being developed at Rice University in collaboration with users at Los Alamos National Laboratory, provides an alternative mechanism for producing correct parallel code. PTOOL is a semi-automatic tool for detecting implicit parallelism in sequential Fortran code. It uses vectorizing compiler techniques to identify dependences preventing the parallelization of sequential regions. According to the model supported by PTOOL, a programmer should first implement and test his program using traditional sequential debugging techniques. Then, using PTOOL, he can select loop bodies that can be safely executed in parallel. At Los Alamos, we have been interested in examining the role of dependence-analysis tools in the parallel programming process. Therefore, we have used PTOOL as a static debugging tool to analyze parallel Fortran programs. Our experiences using PTOOL lead us to conclude that dependence-analysis tools are useful to today's parallel programmers. Dependence-analysis is particularly useful in the development of asynchronous parallel code. With a tool like PTOOL, a programmer can guarantee that processor scheduling cannot affect the results of his parallel program. If a programmer wishes to implement a partially parallelized region through the use of synchronization primitives, however, he will find that dependence analysis is less useful. While a dependence-analysis tool can greatly simplify the task of writing synchronization code, the ultimate responsibility of correctness is left to the programmer.

URPR—An extension of URCR for software pipelining

The software pipeline technique is an effective approach to optimizing loops in array processor programs, but existing methods are of high complexity and the results may not be satisfactory. This paper introduces the URPR algorithm, an extension of the microcode loop compaction algorithm URCR. Firstly, unroll the loop(the number of unrolled loop bodies relies on the inter-body data dependency); secondly, pipeline the unrolled loop bodies one by one; and finally, a new optimized loop body is obtained after rerolling. Preliminary tests indicate that URPR produces favorable results with lower complexity.