The static singleassignment ssa form of a program provides data flow information in a form which makes some compiler optimizations easy to perform. But for a shorter, readable, entertaining piece, see appels ssa is functional programming. Using software metrics to evaluate static single assignment form in gcc. In static single assignment formssa each assignment to a variable should be specified with distinct names.
Methods to implement a registerbased ir in static single. Simple generation of static singleassignment form researchgate. The static single assignment form construction and application to program. The static single assignment ssa form of a program provides data flow information in a form which makes some compiler optimizations easy to perform. Static singleassignment form arranges for every value computed by a program to have a unique assignment aka, definition a procedure is in ssa form if every variable has statically exactly one definition ssa form simplifies several important optimizations, including various forms of redundancy elimination. Runtime environments in compiler design geeksforgeeks. In functional programming, assignment is discouraged in favor of single assignment, also called initialization. According to the wikipedia page on static single assignment ssa, ssa is used by large and wellknown projects such as llvm, gcc, msvc, mono, dalvik, spidermonkey, and v8 while the page on projects using continuationpassing style cps is a bit lacking in comparison. The staticsingle assignment formconstruction and applicationto program optimizations tutorial of compiler design course by prof y. How are variables stored in a language compiler or.
Singlepass generation of static singleassignment form. This property facilitates program analysis and optimization in the compiler. That is why function ends up with a slice z containing the same values. According to the wikipedia page on staticsingle assignment ssa, ssa is used by large and wellknown projects such as llvm, gcc, msvc, mono, dalvik, spidermonkey, and v8 while the page on projects using continuationpassing style cps is a bit lacking in comparison. This property facilitates program analysis and optimization in compilers. In compiler design, static single assignment form often abbreviated as ssa form or ssa is an intermediate representation ir in which every variable is assigned exactly once. Static single assignment ssa 4, 5 is a compiler transformation that converts a cdfg. In ssa form, usedef chains are explicit and each contains a single element. Aug 08, 2017 this video explains the concept of static single assignment. The staticsingle assignment formconstruction and applicationto program optimizations. Over the last few years, static single assignment ssa form 5 has been. Efficiently computing static single assignment form and the control. A basic block is a sequence of instructions with one entry point and one exit point. 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.
Static single assignment form and dominators, postdominators, dominance frontiers cs252r spring 2011 almost all slides shamelessly stolen from. In proceedings of the 10th international workshop on languages and compilers for parallel computing, volume 66 of lecture notes in computer science, pages 114. A correspondence between continuation passing style and. The static single assignment form construction and. Over the past 20 years, static single assignment form ssa has risen to become the compiler intermediate representation of choice. For a detailed background on compiler irs and the cdfg model in particular, please refer to. The successful compiler writer must be familiar with current bestpractice techniques in optimization, such as the construction of static single assignment form, and in code generation, such as software pipelining. It is decidedly nontrivial to take a given program and produce a sanely efficient set of phi nodes. Static single assignment form represents data dependences elegantly and provides a basis for powerful optimizations. Pdf over the past 20 years, static single assignment form ssa has risen to become the compiler intermediate representation of choice. Static single assignment ssa form is a program representation that is becoming increasingly popular for compilerbased code optimization. Jul 12, 2011 for more on ssa and dominators, see the very readable 1991 paper by cytron, ferrante, rosen, wegman, and zadeck, efficiently computing static single assignment form and the control dependence graph.
In static single assignment form ssa each assignment to a variable should be specified with distinct names. Mar 21, 2016 everyone seems quite aware of the pros, so let me try to cover the cons. Typical implementations of ssa embed in each basic block pointers to the predecessors and successors of the blocks, as well as the blocks. A very detailed and practical book about many aspects of the ssa intermediate representation. Source code is an essential part of all software development. I believe clang or the swift compiler uses something like what you are talking about. Citeseerx document details isaac councill, lee giles, pradeep teregowda. Im writing a compiler for a registerbased bytecode to an ir with static single assignment ssa form specifically, from dalvik vm bytecode to llvm ir, but im hoping the question can be taken for the general topic and am wondering the best, or theoretically cleanest way to go about this. In the 1999 toplas ssapre paper, a particular realization of pre, known as ssapre, is described, which is more efficient than traditional pre implementations because it relies on useful properties of static single assignment ssa form to perform dataflow analysis in a much more sparse manner than the traditional bitvectorbased approach. Socalled minimal ssa is not minimal in any meaningful sense, and so pr. A program would have to perform reaching definition analysis to determine this. The static single assignment form ssa form is becoming popular as the intermediate representation of compilers because of its simplicity in the data.
Using software metrics to evaluate static single assignment. The goal was to find out how much effort it was to augment the java hotspot. No list of todos, please compile with tikz to get the todolist for this work. Ssa for trees gnu project free software foundation fsf. Comparison and evaluation of back translation algorithms.
The static singleassignment ssa form of a program pro vides data flow information in a form which makes some compiler opti mizations easy to perform. An efficient method of computing static single assignment form. Designing the mccat compiler based on a family of structured intermediate representations. Translate abstract syntax trees to intermediate representations and static single assignment form. The goal of this project is to build an optimization framework for trees based on the static single assignment ssa form 1. For more, see richard kelseys classic paper, a correspondence between continuationpassing style and static single assignment form, or my earlier article about landin, steele, letrec, and labels.
A translation needs to relate the static source text of a program to the dynamic actions that must occur at runtime to implement the program. The seminal paper that describes the ssa intermediate program representation has over 2800 citations almost every compiler text book talks about ssa form. The authors present strong arguments for the practicality and utility of the static single assignment ssa form and the control dependence graph cdg in optimizing compilers. Static single assignment form for machine code acm. Compiler this depends very much on the language and compiler and is extremely simplified, so its just meant to give some idea. Pdf using software metrics to evaluate static single. Client compiler with static single assignment ssa form and with a graph coloring register allocator. Optimizations in static single assignment form stack overflow. To convert a program to ssa form, merely create new variables for every assignment beyond the first. However in this study, we present clear quantitative benefits of ssa, by applying several standard software. Single assignment is an example of name binding and differs from assignment as described in this article in that it can only be done once, usually when the variable is created. Static single assignment is used for intermediate code in compiler design. Static single assignment form an intermediate form widely used by modern optimizing compilers.
Everyone seems quite aware of the pros, so let me try to cover the cons. Most of the tree optimizers rely on the data flow information provided by the static single assignment ssa form. There are compilers that dont use ssa or at least dont use it for some parts of the compiler. Modern compilers use an intermediate representation in static single assignment ssa form, which simplifies many optimizations. Pdf singlepass generation of static singleassignment form for. Engineering a compiler 2, cooper, keith, torczon, linda. Slice which is appending to the z slice is created against the same memory area s.
An ssa representation for whole program is possible i. Methods using this model utilize an equation to estimate the desired value such as cost, time, and effort and so on. Simple and efficient construction of static single assignment form. Static single assignment ssa is an effective intermediate representation in optimizing compilers. The way how the program is represented in the compiler has large influence on the efficiency and effectiveness of the compiler. Alternatives to phi nodes in static single assignment.
Practical improvements to the construction and destruction. What are the pros and cons of using single static assignment. Using static single assignment form in a code optimizer. Static single variable and static multivariable model. Compiler developers cite many qualitative reasons for choosing ssa.
Rusts newly introduced mir midlevel ir is also not ssabased. Ssa form not only simplifies register allocation but also helps performing optimizations such as common subexpression elimination, loopinvariant code motion or. In compiler design, static single assignment form is a property of an intermediate representation, which requires that each variable is assigned exactly once, and every variable is defined before it is used. Static single assignment ssa form is a program representation that is becoming increasingly popular for compiler based code optimization. Single static assignment optimization mikroelektronika. The static single assignment formconstruction and application to program. We use subscripts to distinguish each definition of variables. They must also have the background and insight to understand new techniques as they appear during the coming years. Introduction many modern compilers employ intermediate representations in static single assignment ssa form for their optimization phases. In the given code segment, there are two assignments of the variable x. An optimizing compiler may or may not have access to the bodies of all procedures called directly or indirectly or to summaries of their effects. This video explains the concept of static single assignment. Verified construction of static single assignment form abstract.
The program consists of names for procedures, identifiers etc. Static single assignment ssa cfgs but with immutable variables plus a slight hack to make graphs work out now widely used e. However, it is not easy to make a compiler that generates efficient object codes. Compiler design gate 2015 solved question part 2 address code, fields. Existing variables in the original ir are split into versions, new variables typically indicated by the original name with a subscript in textbooks, so that every definition gets its own version.
Software engineering assignment help, static single variable and static multivariable model, question compare the static single variable model and static multivariable model answer static single variable model. In particular, from nowhere in the program do we jump into the middle of the basic block, nor do we exit the block from the middle. It is capable of creating code for a platform other than the one on which the compiler is running. The paper includes a brief introductory explanation of the ssa form, an explanation of the algorithm for generating the form for programs written without a goto, and a demonstration of how to build. Tracedriven, justintime compilation with a new application. Static single assignment form has been used in optimizations targeted towards imperative languages, for exam ple eliminating induction variables and par tim redundancies 2. But if the program is in ssa form, both of these are immediate. However in this study, we present clear quantitative benefits of ssa, by applying several standard software metrics to compiler intermediate code. In this paper, we address three problems that have arisen in our use of ssa form. They use the concept of a dominator tree to formalize the concept of dominance frontiers and use this to derive efficient algorithms for computing the ssa form and the cdg.
Efficiently computing static single assignment form and the. Compiler design gate 2017 subject wise complete solution duration. Static single assignment form for messagepassing programs. This paper presents a novel scheme for maintaining accurate information about distributed data in messagepassing programs. Static single assignment enables the e cient implementation of many important deompilerc ompconents, including expression propagation, preservation analysis, type analysis, and the analysis of indircte jumps and alcls. Feedbackdriven optimization of synthesized hardware using. Issues related to generation of the static single assignment ssa form of programs written using structured programming languages are discussed. The compiler modifies the program representation so that every time a variable is. Concurrent static single assignment form and constant propagation for explicitly parallel programs. The ssa form is based on the premise that program variables are assigned in exactly one location in the program. Efficiently computing static single assignment form and. Acm transactions on programming languages and systems, 4. Ssa elimination after register allocation springerlink.
To reduce the efforts needed to develop high quality compilers, two aspects have been studied. Static single assignment ssa form is a program representation that is becoming increasingly popular in language processors. Partial redundancy elimination the static single assignment form automatic parallelization instruction scheduling software pipelining energyaware software systems justin. An efficient method of computing static single assignment form ron cytron jeanne ferrante barry k. Compiler allocated separate local variable for keeping local state of the next item from the iterator on each iteration of forrange loop. We describe static single assignment ssa based algorithms to build up an intermediate representation of a sequential program while targeting code generation for distributed memory machines employing the single program multiple data spmd model of programming. A global variable exists only once in the program, so the compiler reserves one memory area of 4 bytes int size in a data area. Kenneth zadeckt 1 introduction in optimizing compilers, data structure choices directly in. However, using such machineindependent software on resourceconstrained devices such as mobile phones and pdas remains a challenge, as both interpretation and justintime compilation of the intermediate vm language run into. In ssa form, each use of a variable has a single definition point. A decade after java arrived, there have been improvements in the runtime performance of platformindependent virtualmachine based software. In terms of research compiler infrastructures, suif 17, microsofts bartok 11 and phoenix, ibms jikes rvm 6, and llvm all use ssabased interme. Originally written and maintained by michael van acken and erick nikitin classictoolsooc. How to read golang static singleassignment ssa form.
Abstract static single assignment ssa form is a program representation that is becoming increasingly popular in language processors. Static single assignment form and the control dependence graph. Optimizing oberon2 compiler ooc based on static single assignment ssa representation also known as ooc2. For example gccs lowlevel ir called rtl is not ssabased, gos backend uses ssa only since a few releases. Two are variations to the ssa construction algorithms presented by cytron et al. For instance, if we have the registerbased instructions. Using static single assignment form in a code optimizer acm. All of these optimizations increase code execution speed and reduce code size. Verified construction of static single assignment form. Static single assignment form in the coins compiler.
In compiler design, static single assignment form often abbreviated as ssa form or simply ssa is a property of an intermediate representation ir, which requires that each variable is assigned exactly once, and every variable is defined before it is used. However, the high implementation complexity of efficient ssa construction algorithms poses a challenge to verified compilers. However, traditional ssa form and optimizations are not applicable to programs represented as native machine instructions because the use of dedicated registers imposed by calling conventions, the runtime system, and target architecture must be made explicit. Compilers such as gcc use static single assignment ssa form as an intermediate representation and usually perform ssa elimination before register allocation. Optimizations in compilers are becoming crucial for e. An ssabased compiler modifies the program representation so that every time a variable is assigned in the original program, a new version of the variable is created. Compiler is a software which converts a program written in high level language source language to low level language objecttargetmachine language cross compiler that runs on a machine a and produces a code for another machine b. The static single assignment form construction and application to program tutorial of compiler design course by prof y. In compiler design, static single assignment form is a property of an intermediate representation. Static single assignment ssa form is widely used in modern compilers even at the code generation level as it allows for. Partial redundancy elimination the static single assignment form automatic parallelization instruction scheduling software pipelining energyaware software systems justintime.
Practical improvements to the construction and destruction of. Efficiently computing static single assignment form and the control dependence graph. Tabledriven techniques for peephole optimization and code generation are straightforward and effective. However, using such machineindependent software on resourceconstrained devices such as mobile phones and pdas remains a challenge, as both interpretation and justintime compilation of the intermediate vm language run into technological limitations. For example gccs lowlevel ir called rtl is not ssa.