In this chapter, we presented several pre synthesis transformations that are important to our parallelizing highlevel synthesis methodology. The slam tool developed at microsoft research implements such an auto. Program verification systems typically transform a program into a logical expression which is then fed to a theorem prover. Common requirements are to minimize a programs execution time, memory requirement, and power consumption the last two being popular for portable computers. Weakest precondition synthesis for compiler optimizations np lopes, j monteiro international conference on verification, model checking, and abstract, 2014. So reorderingcombining math operations are out of scope of optimization. Substitute the right hand side of the assignment statement for a in the postcondition 2b1 0 2b2 0 2b 2 b 1. Common requirements are to minimize a programs execution time, memory requirement, and power consumption the last two being popular for portable computers compiler optimization is generally implemented using a sequence of optimizing transformations. A collection of compiler optimizations with brief descriptions and examples of code transformations. Precondition calculation for loops iterating over data. In the real world, of course, it isnt particularly likely that a developer is going to explicitly write a statement like x x 1 so it isnt particularly important that a compiler optimizes that specific statement.
Optimizations in compiler design software engineering. Compilers are a fundamental component of the toolset program. However i was trying to understand, apart from the example, if llvm makes use of logical mathematical provers to infer assertions that are used to optimize the code, or all the optimizations are based more on value propagation and alias analysis rather than logical provers. An analyzer for maintaining and analyzing source code is disclosed.
Some compilers can optimize the code so that it runs faster. Automatic partial equivalence checking, applied to compiler optimization verification. Double locking lock unlock l ock u nlock an attempt to reacquire an acquired lock or release a released lock will cause a. Mar 26, 20 compute the weakest precondition for each of the following sequences of assignment statements and their postconditions. Download citation on aug 1, 2016, juan zhai and others published precondition calculation for loops iterating over data structures find, read and cite all the research you need on researchgate. One of the neater and lesserknown optimizations a compiler can perform is a branchless conditional a conditional statement that contains no branches. Location is a pair l1,l2 of pc program counter in the. Green hills optimizing compilers green hills software. Its not that obvious of an optimization the compiler can perform that optimization, but only if it is sure that. The pinfer prototype successfully generates either the partial precondition or the weakest precondition for 164 out of 174 peephole optimizations in the alive suite. Verification, model checking, and abstract interpretation 15th international conference, vmcai 2014, san diego, ca, usa, january 1921, 2014, proceedings. Second, while constructing the weakest precondition, we only keep satis able disjuncts disjuncts appear in the weakest precondition because of aliasing. When such sequences warrant, the optimizer replaces one sequence of code with a jump to the other and eliminates the first piece of code. Verification, model checking, and abstract interpretation.
Fyi, the compiler is clever enough to optimise it away entirely if it can. Here, we give the weakest precondition semantics of qugcl programs. The synthesized preconditions are provably correct by construction, and they are guaranteed to be the weakest in the precondition language that we consider. In a tree representation this corresponds to insert exp at all leaves that correspond to h as subtrees in e x p usually, a simplification algorithm refines the resulting representation of e x p h. Since software clones behave similarly, their nonequivalent mutants tend to be the. A compiler is a computer program that translates computer code written in one programming language the source language into another language the target language. Provably correct peephole optimizations with alive acm. Precondition synthesizer for compiler optimizations. Play around with the glasgow haskell compiler optimization and code generation.
Despite being a topic with so many important applications, program equivalence checking has seen little advances over the past decades due to its inherent high complexity. Compiler optimization should not change the behavior of the program although it does happen. Optimizations in compiler design software engineering stack. In this paper, we propose, to the best of our knowledge, the first algorithm for the automatic synthesis of preconditions for compiler optimizations. Proving equivalence of programs has several important applications, including algorithm recognition, regression checking, compiler optimization verification and validation, and information flow che. Compute the weakest precondition for the following sequence of assignment statements, for the postcondition given. Apr 24, 2018 github is where people build software. Since the variable x has value v after the assignmentand nothing else changes in the state, it must be that q x holds after that assignment. Aliveinfer prototype generates the weakest precondition for 3 of them within. Weakest precondition synthesis for compiler optimizations verification to the rescue. Applications such as compiler validation, or automatic comparison of student attempts to.
The compiler optimizes for size by choosing the smallest code sequence possible. Model checking and abstract interpretation, vmcai 2014, held in san diego, ca, usa, in january. A weakest precondition is the least restrictive precondition that will guarantee the postcondition. In proceedings of the 15th international conference on verification, model checking, and abstract interpretation, vmcai, pages 203221, 2014. Weakestprecondition of unstructured programs mike barnett and k. Green hills software has led the embedded industry for the past thirty years with our optimizing compliers.
As optimizations go, its simple but it doesnt gain much it saves some space, which might be important on embedded systems, but not in most. Compiler design frank pfenning lecture 17 october 22, 20 1 introduction optimizing loops is particularly important in compilation, since loops and in particular the inner loops account for much of the executions times of many programs. As software becomes larger, programming languages become higherlevel, and. The examples in the document youre linking to are and are intended to be simplified examples of a particular type of optimization. More than 40 million people use github to discover, fork, and contribute to over 100 million projects. What is the purpose of code optimization at intermediate. Third, we remove predicates that relate variables not in the current. We present examples of preconditions synthesized by psyco, as well as the results of running psyco on a set of optimizations. Nov 03, 2002 a special optimization can be implemented in this case which isnt that rare, the call can be replaced by a straight jump. Synthesis of weakest precondition via static analysis. Weakestprecondition of unstructured programs request pdf.
For a synthesis tool to compete with modern compilers, its equivalence checker should be able to. Weakest precondition of assignment rule 14 we have argued that the assignment axiom of hoare logic is designed to give the best i. Us6029002a method and apparatus for analyzing computer. For evaluating the weakest precondition procedures, we used a selection of the binaries transpiled.
Weakest precondition synthesis for compiler optimizations npl, jm, pp. Automatic synthesis of weakest preconditions for compiler optimizations. Proving equivalence of programs has several important applications, including algorithm recognition, regression checking, compiler optimization verification and validation, and information flow checking. The following graph demonstrates the computation of weakest preconditions for the sum program. This is an interesting optimization because it has effects that ripple far from the reduced call overhead.
Weakestprecondition reasoning what do we mean by weakest. Explain what the preconditions and postconditions of a given statement mean in axiomatic semantics. However, the pec paper 20 does include a precondition for software. Automatic rootcausing for program equivalence failures in. The weakest precondition of a program is a least restrictive precondition under which it is safe to call the program.
It has been used widely in the translation validation of compilers 10, 14, 19, regression verification, crossversion verification 7, 11 and checking independent implementations 17, 21. The compiler will create the correct machine code to represent your statement. We evaluate the checker by testing it across unoptimized and optimized binaries of spec. The parentheses are there solely for your benefit not the compilers.
Because the call is executed many times in this situation it is a profitable trick. An intermediate language is designed specifically to apply optimizations. Developed a technique for discovering weakened liberal preconditions that can establish the given postconditions. The techniques described include both machineindependent and machinedependent optimizations. Your suggested optimization for x1 is not functionally equivalent in the face of multithreading if the field were volatile, for example, the compiler is definitely disallowed from performing the suggested optimization. Equivalence checking between two imperative programs has several applications in software validation. In proceedings of the theory and practice of software, 14th.
Weakestprecondition of unstructured programs microsoft. These optimzation categories are tested in the nullstone automated compiler performance analysis suite. The majority of compiler optimizations accept ir code as input and produce hopefully optimized ir code as output well, some compilers may gradually lower the ir till one optimization emits binary code. Compute the weakest precondition for the assignment statement a. Weakest precondition synthesis for compiler optimizations psyco. A static slicing method for functional programs and its incremental version. If the unknown variables 120 in the precondition are positive, the weakest precondition typically has a small number of predicates 110. Automatic synthesis of weakest preconditions for compiler. Compute the weakest precondition for each of the following sequences of assignment statements and their postconditions. Precondition inference for peephole optimizations in llvm. It is often called a predicate transformer because it takes a predicate, or assertion as a parameter and returns another predicate. As a method of software construction, this is ideal.
Blackbox equivalence checking across compiler optimizations. Branchless conditionals compiler optimization technique. The analyzer includes a software translator for converting conventional source code into an intermediate language, slicing capability based upon weakest precondition determination, dual direction flow analysis and incorporation of a computational model to facilitate iterative code. An assertion following a statement is a postcondition. On eembc benchmarksthe most widely accepted benchmarks in the embedded industry green hills compilers consistently outperform competing compilers to generate the fastest and smallest code for 32 and 64bit processors. Llvm static value analysis for optimization stack overflow. Well, the compiler may do some optimization, but there is still a lot that can be done to improve performance at designtime. Section 5 presents psyco, a tool that implements the proposed algorithm, as well as examples of preconditions generated by psyco and results of running it over a set of compiler optimizations. An assertion before a statement a precondition states the relationships and constraints among variables that are true at that point in execution. Dijkstra, a discipline of programming, prenticehall, 1976. Provably correct peephole optimizations with alive.
In your examples, this would get turned into int a 6. Branchless conditionals compiler optimization technique posted by blueraja on may 10, 2011 2. The technique uses a statebased abstract interpretation, which may use nondistributive transfer functions andor a. Weakest precondition an overview sciencedirect topics. Once complete, theres no interpretation done, except for eval calls. Our recent work has explored compiler nontermination errors with a suite of peephole optimizations 34, which typically occurs when profitability metrics are not included in the precondition. Homework 1 answers department of computer science and. Peephole optimizations are a common source of compiler bugs.
Oct 20, 2015 a compiler takes source code and converts it into byte code. In computing, an optimizing compiler is a compiler that tries to minimize or maximize some attributes of an executable computer program. Lecture notes for comp 150dao dataflow analysis and optimization. Shantanu thakoor, simoni shah, ganesh ramakrishnan, amitabha sanyal. A compiler takes source code and converts it into byte code. Proving optimizations correct using parameterized program. The logical expression represents the weakest precondition of the program relative to its specification. Precondition calculation for loops iterating over data structures. We presented common subexpression elimination, loop invariant code motion, loop unrolling, and loop index variable elimination in sections 5. Terminationchecking for llvm peephole optimizations. The recent rise in the power of proof tools revitalized the area of compiler and optimizer.
For example if you have source code that looks like this. The weakest precondition procedure is automatic only for loopfree programs, for this reason we extracted loop free fragments using an automatic. We implemented the proposed technique in a tool named psyco. Section 4 describes our algorithm for the synthesis of weakest preconditions for compiler optimizations. Other work of formally verifying the correctness of compiler optimizations was done by lerner et al. One such effective heuristic relies on compiler optimization techniques 37,152. This book constitutes the refereed proceedings of the 15th international conference on verification, model checking and abstract interpretation, vmcai 2014, held in san diego, ca, usa, in january 20. A weakest precondition model for assembly language programs january 28, 2003 6 where ns is a ghost variable not part of the program state used to reference the initial value of n within the postcondition qx,ns. The name compiler is primarily used for programs that translate source code from a highlevel programming language to a lower level language e. Automatic weakest precondition synthesis for compiler optimizations. The weakest precondition of it is established by substituting the occurrence of h in e x p.