Next, compilers analyze the details of how values flow through the code. Control flow statements and boolean expressions cs youtube. Feb 15, 2012 control flow analysis control flow analysis cfa static analysis technique to discover the hierarchical flow of control within a procedure function. Dataflow analysis is a technique for gathering information about the possible set of values calculated at various points in a computer program. Basic blocks and flow graphs usually compilers represent programs as control flow graphs cfg. Control flow graphs control flow graph cfg graph representation of computation and control flow in the program framework for static analysis of program controlflow nodes are basic blocks straightline, singleentry code, no branching except at end of sequence edges represent possible flow of control from the. Controlflow analysis sample exercises 2 spring 2010 problem1. Thus, the control flow graph for a minijava or java function, or a c function without goto, will always be reducible.
There is a unique entry node and a unique exit node. To overcome this overhead of executing all control. Code generation can be considered as the final phase of compilation. The type signature of a function specifies the types of the formal parameters and the type of the return value. Represents the control structure of the procedure using control flow graphs. However, the generated code still has to execute instructions in all control paths to merge values. The information gathered is often used by compilers when optimizing a program. Control flow analysispart 2 compiler design computer. The program consists of names for procedures, identifiers etc.
The control flow is expressed as a controlflow graph cfg. In computer science, a control flow graph cfg is a representation, using graph notation, of all paths that might be traversed through a program during its execution. For both functional programming languages and objectoriented programming languages, the term cfa, and elaborations such as kcfa, refer to specific algorithms that compute control flow. This video is highly rated by computer science engineering cse students and has been viewed 405 times. The global controlflow graph optimizing an eventdriven realtime system across kernel boundaries christian dietrich, martin ho. Controlflow analysis sample exercises and solutions. The optimization community developed global dataflow analysis to answer these questions. Control flow graphs are mostly used in static analysis as well as compiler applications, as they can accurately represent the flow inside of a program unit. Code generation part ii flow graphs basic blocks fsu computer. Analysis phase known as the frontend of the compiler, the analysis phase of the compiler reads the source.
Thus, the controlflow graph for a minijava or java function, or a c function without goto, will always be reducible. Data flow graph dfg a modem communications system each box is a single function or sub systems the activity of each block in the chain depends on the input of the previous block data driven each functional block may have to wait until it receives a certain amount of information before it begins processing some place to output. Obvious for this simple example with no control flow. It must check that the type of the returned value is compatible with the type of the function.
Common control flow constructs such as ifthen, ifthenelse, whiledo, repeatuntil, for, and break even multilevel break can only generate reducible flow graphs. Common controlflow constructs such as ifthen, ifthenelse, whiledo, repeatuntil, for, and break even multilevel break can only generate reducible flow graphs. A variable v is live at a point p in the controlflow graph cfg if there is a path from p to a use of v along which v is not redefined. The history of control flow analysis i 1970, frances allen, control flow analysis cfg i turing award for pioneering contributions to the theory and practice of optimizing compiler techniques, awarded 2006. Code optimization in compiler design, basic blocks, flow graph by university academy. Data flow analysis 2 source code parsed to produce ast ast transformed to cfg data flow analysis operates on control flow graph and other intermediate representations compiler structure 3 asts asts are abstract they dont contain all information in. The code generated by the compiler is an object code of some lowerlevel programming language, for example, assembly language. System calls become more transparent for the compiler especially useful for embedded realtime systems.
I control ow graph cfg is a directed graph in which the nodes. For both functional programming languages and objectoriented programming languages, the term cfa, and elaborations such as k cfa, refer to specific. A controlflow graph is a directed graph in which executed statements or procedures are represented by the nodes, and control flow is represented by the arcs. To find program flow graph we need to find basic block basic block a basic block is a sequence of three address statements where control enters at the beginning and. Flow graph is a directed graph with flow control information added to the basic blocks. Cfg models flow of control in the program procedure. The ir uses a directed graph with labeled vertices and ordered inputs but unordered outputs. Since one of the primary reasons for doing such a global analysis in a compiler is to produce optimized programs, control flow analysis has been embedded in many compilers and has been described in several papers. Compiler design and construction semantic analysis.
Transfer function f brelates inb and outb, for same b effect of flow of control. Controlflow analysis cfa helps us to understand the. Control flow analysis program control flow control flow graph. Compiler design mcq questions answers computer engineering mcq. Basic blocks and flow graphs control flow graphs we divide the intermediate code of each procedure into basic blocks. References 6,8,9 describe compilers which use various forms of control flow analysis for optimization. Control flow analysis sample exercises 2 spring 2010 problem1. Determine the dominators of each node in the cfg 3. The result of control flow analysis is a control flow graph. There is an edge from basic block b 1 to basic block b 2 if program execution can flow from b 1. A control flow graph is used to depict that how the program control is being parsed among the blocks. The result of controlflow analysis is a controlflow graph. Csci 565 compiler design spring 2016 homework 4 solution problem 1. Analysis of all possible execution paths inside a program or procedure.
It contains the flow of control information for the set of basic block. On 14 kernels, the compiler achieves distinct speedups, including. Such a graph assists testers in the analysis of a program to understand its behavior in terms of the flow of control. Likewise, because foo will eventually returns to baz and to wherever else it mightve been called from, there will be an edge from the end of foos graph back to the statement after the call to foo in baz. Control flow statements and boolean expressions cs. If t 1 and t 2 are type expressions, then their cartesian product, t 1 x t 2, is a type expression. Data flow analysis schema build a flow graph nodes basic blocks, edges control flow set up a set of equations between inb and outb for all basic blocks b effect of code in basic block. We say that a node d in a flow graph dominates node n. Consequently, when branches in the scalar baseline are frequently taken to bypass a large number of instructions, the vector code may slow compared to the baseline. Graph nodes represents objects called basic blocks bb. Compiler must check that the type of each actual parameter is compatible with the type of the corresponding formal parameter. Transfer function f brelates inb and outb, for same b effect of flow. Some recent developments in the area are reported in 4 and in 7. Through post code generation, optimization process can be applied on the code, but that can be seen as a part of code generation phase itself.
In computer science, control flow analysis cfa is a staticcodeanalysis technique for determining the control flow of a program. For the purpose of hypothesis generation, limit your controlflow graph to statements or procedures that were actually executed. The flow of control can only enter the basic block through the first instruction in the block. Control flow graph the control flow graph g n, e of a program consists of a set of nodes n and a set of edge e. In computer science, controlflow analysis cfa is a staticcodeanalysis technique for determining the control flow of a program. The optimization community developed global data flow analysis to answer these questions. In this paper, we present a methodology for generating cdfgs from scheduled and pipelined assembly code. Introducing control flow into vectorized code jaewook shin mathematics and computer science division. Prosser used boolean connectivity matrices for flow analysis before the cfg is essential to many compiler optimizations and staticanalysis tools. There is an edge from basic block b 1 to basic block b 2 if program execution can flow from b 1 to b 2.
The global controlflow graph optimizing an eventdriven realtime system across kernel boundaries. I determining the execution order of program statements or instructions i control ow graph cfg. Compiler design detection of a loop in three address. The control flow is expressed as a control flow graph cfg. If t is a type expression and i is the type expression of an index set then array i, t denotes an array of elements of type t.
Any static, global analysis of the expression and data relationships in a program requires a knowledge of the control flow of the program. A control flow graph is a graph in which each node is a basic block there is an edge from b1 to b2 if the controlflow instruction of b1 might jump to the entry label of b2. The basic blocks within one procedure are organized as a control ow graph, or cfg. Call graph construction for objectoriented programs. I determining the execution order of program statements or instructions i control ow graph cfg speci es all possible execution paths i important control ow constructs program constructs important to control ow i basic block. Generation of control and data flow graphs from scheduled. Apr 20, 2017 control flow statements and boolean expressions cs. Srikant department of computer science and automation indian institute of science bangalore 560 012 nptel course on compiler design. Compiler design 10 a compiler can broadly be divided into two phases based on the way they compile. Pdf control flow graph visualization in compiled software. The algorithm for the cfg construction from assembly code is given by scanning. Cs412cs4 introduction to compilers tim teitelbaum lecture. Compiler design detection of a loop in three address code.
Control flow graphs i control ow analysis aims to determine the execution order of program statements or instructions i basic block. Runtime environments in compiler design geeksforgeeks. Control flow analysis 5 basic blocks definition a basic block is a maximal sequence of consecutive statements with a single entry point, a single exit point, and no internal branches basic unit in control flow analysis control flow analysis 6 basic blocks local level of code optimizations redundancy elimination registerallocation. Control flow graphs for the purposes of dataflow analysis, we use the control flow graph cfg intermediate form. Discussion some times we will also consider the statementlevel cfg, where each node is a statement rather than a basic block either kind of graph is referred to as a cfg in statementlevel cfg, we often use a node to explicitly represent merging of control control merges when two different cfg nodes point to the same node note. E which consists of a set of vertices nodesv, and a set of edges e, which indicate possible flow of control between nodes or, is a directed graph that represents a superset of all possible execution paths of a procedure. To find program flow graph we need to find basic block basic block a basic block is a sequence of three address statements where control enters at the beginning and leaves only at the end without any jumps or halts. Prosser used boolean connectivity matrices for flow analysis before. Basic blocks and flow graphs examples gate vidyalay. Controlflow graph, dominators and natural loops for the program below. For the purpose of hypothesis generation, limit your control flow graph to statements or procedures that were actually executed.
A control flow graph cfg is a directed graph with basic blocks bi as. Data flow analysis 2 source code parsed to produce ast ast transformed to cfg data flow analysis operates on control flow graph and other intermediate representations compiler structure 3 asts asts are abstract they dont contain all information in the programe. There is an edge from node n 1 to node n 2 if the control may flow from the last statement in n. I would post this to rcompilers, but its very quiet over there ive been reading about the zoo of compiler data flow representations developed in the 80s and 90s such as if1, the value state dependence graph, the gated data dependence graph, etc. For detecting loops we use control flow analysiscfa using program flow graphpfg. In computer science, a controlflow graph cfg is a representation, using graph notation, of all paths that might be traversed through a program during its execution. The underlying motivation in all the different types of control flow analysis is the need to codify the flow relationships in the program. Nov 05, 2018 a cfg captures the flow of control within a program. A cfg captures the flow of control within a program. Runtime environments in compiler design 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. In computer science, a control flow graph cfg is the graphical representation of control flow or computation during the execution of programs or applications. Compiler construction controlflow graphs page has been moved. Basic blocks and flow graphs in compiler design basic block is a set of statements that always executes in a sequence one after the other. For example if the arguments of a function are two reals followed by an integer then the type expression for the arguments is.
Intervals and reducibility t1 t2 transformations and graph reduction regions y. Control flow graph, dominators and natural loops for the program below. They use the resulting information to find opportunities for improvement and to prove the safety of transformations. Controlflow analysis an overview sciencedirect topics. Controlflow, loops and loop optimizations 40 points consider the code depicted below for a function with integer local variables i, a, b, c and d, and parameters of the enclosing procedure p. A control flow graph cfg is a graph whose nodes are basic blocks. Flow control statements can be used to take charge of the execution flow on a method or block of code and direct it to jump to a different place in the application, rather than continuing through to the text statement linearly. Basic blocks and flow graphs university of arizona. Analysis phase known as the frontend of the compiler, the analysis phase of the compiler reads the source program, divides it into core parts, and then checks for lexical, grammar, and syntax errors. A programs control flow graph cfg is used to determine those parts of a program to which a particular value assigned to a variable might propagate. Control flow graph cfg is a directed graph in which the nodes represent basic. The controlflow graph cfg is a fundamental data structure. Determine the leaders, the first statements of basic blocks the first statement in the sequence entry point is a leader any statement that is the target of a branch conditional or unconditional is a leader.
440 763 365 1143 977 75 1422 1086 457 1305 1441 959 1198 1289 734 343 273 338 1367 609 816 1450 940 1085 1422 1015 263 1329 823 456 1153