Simple optimizations - removal of redundant code
15.3.4 Simple optimizations - removal of redundant code
Production quality compilers often expend considerable effort in the detection of structures for which no code need be generated at all. For example, a source statement of the form WHILE TRUE DO Something does not require the generation of code like LAB IF NOT TRUE GOTO EXIT END Something GOTO LAB EXIT but can be reduced to LAB Something GOTO LAB and, to take a more extreme case, if it were ever written, source code like WHILE 15 6 DO Something could be disregarded completely. Once again, optimizations of this sort are most easily attempted after an internal representation of the source program has been created in the form of a tree or graph. A full discussion of this fascinating subject is beyond the scope of this text, and it will suffice merely to mention a few improvements that might be incorporated into a simple tree-walking code generator for expressions. For example, the remaining multiplication by 1 in the expression we have used for illustration is redundant, and is easily eliminated. Similarly, multiplications by small powers of 2 could be converted into shift operations if the machine supports these, and multiplication by zero could be recognized as a golden opportunity to load a constant of 0 rather than perform any multiplications at all. To exemplify this, consider an extract from an improved routine that generates code to load the value resulting from a binary operation onto the run-time stack of our simple machine: void BINOPNODE::emit1void load value onto stack resulting from binary operation { bool folded = false; if left right beware of corrupt trees { switch op redundant operations? { case CGEN_opadd: if right-defined right-value == 0 x + 0 = x { left-emit1; folded = true; } ... other special cases break; case CGEN_opsub: ... other special cases case CGEN_opmul: if right-defined right-value == 1 x 1 = x { left-emit1; folded = true; } else if right-defined right-value == 0 x 0 = 0 { right-emit1; folded = true; } ... other special cases break; case CGEN_opdvd: ... other special cases } } if folded still have to generate code { if left left-emit1; beware of corrupt trees if right right-emit1; CGen-emitintSTKMC_add + intop; careful - ordering used } delete left; delete right; remove debris } These sorts of optimizations can have a remarkable effect on the volume of code that is generated - assuming, of course, that the expressions are littered with constants. So far we have assumed that the structures set up as we parse expressions are all binary trees - each node has subtrees that are disjoint. Other structures are possible, although creating these calls for routines more complex than we have considered up till now. If we relax the restriction that subtrees must be disjoint, we introduce the possibility of using a so-called directed acyclic graph DAG. This finds application in optimizations in which common subexpressions are identified, so that code for them is generated as few times as possible. For example, the expression a a + b b a a - b b could be optimized so as to compute each of a a and b b only once. A binary tree structure and a DAG for this expression are depicted in Figure 15.3, but further treatment of this topic is beyond the scope of this text.15.3.5 Generation of assembler code
Parts
» compilers and compiler generators 1996
» Objectives compilers and compiler generators 1996
» Systems programs and translators
» The relationship between high-level languages and translators
» T-diagrams Classes of translator
» Phases in translation compilers and compiler generators 1996
» Multi-stage translators compilers and compiler generators 1996
» Interpreters, interpretive compilers, and emulators
» Using a high-level host language Porting a high level translator
» Bootstrapping Self-compiling compilers compilers and compiler generators 1996
» The half bootstrap compilers and compiler generators 1996
» Bootstrapping from a portable interpretive compiler A P-code assembler
» Simple machine architecture compilers and compiler generators 1996
» Addressing modes compilers and compiler generators 1996
» Machine architecture Case study 1 - A single-accumulator machine
» Instruction set Case study 1 - A single-accumulator machine
» A specimen program Case study 1 - A single-accumulator machine
» An emulator for the single-accumulator machine
» A minimal assembler for the machine
» Machine architecture Case study 2 - a stack-oriented computer
» Instruction set Case study 2 - a stack-oriented computer
» Specimen programs Case study 2 - a stack-oriented computer
» An emulator for the stack machine
» Syntax, semantics, and pragmatics
» Languages, symbols, alphabets and strings Regular expressions
» Grammars and productions compilers and compiler generators 1996
» Classic BNF notation for productions
» Simple examples compilers and compiler generators 1996
» Phrase structure and lexical structure -productions
» Semantic overtones The British Standard for EBNF Lexical and phrase structure emphasis
» One- and two-pass assemblers, and symbol tables
» Source handling Towards the construction of an assembler
» Lexical analysis Towards the construction of an assembler
» Syntax analysis Towards the construction of an assembler
» Symbol table structures The first pass - static semantic analysis
» The second pass - code generation
» Symbol table structures The first pass - analysis and code generation
» Error detection compilers and compiler generators 1996
» Simple expressions as addresses
» Improved symbol table handling - hash tables
» Macro processing facilities compilers and compiler generators 1996
» Conditional assembly compilers and compiler generators 1996
» Relocatable code compilers and compiler generators 1996
» Further projects compilers and compiler generators 1996
» Equivalent grammars Case study - equivalent grammars for describing expressions
» Useless productions and reduced grammars -free grammars
» Ambiguous grammars compilers and compiler generators 1996
» Type 1 Grammars Context-sensitive The Chomsky hierarchy
» The relationship between grammar type and language type
» EBNF description of Clang A sample program
» Context sensitivity Deterministic top-down parsing
» Terminal successors, the FOLLOW function, and LL1 conditions for non -free grammars
» Further observations Restrictions on grammars so as to allow LL1 parsing
» Alternative formulations of the LL1 conditions
» The effect of the LL1 conditions on language design
» Construction of simple recursive descent parsers
» Case studies compilers and compiler generators 1996
» Syntax error detection and recovery
» Construction of simple scanners
» LR parsing compilers and compiler generators 1996
» Automated construction of scanners and parsers
» Embedding semantic actions into syntax rules
» Attribute grammars compilers and compiler generators 1996
» Synthesized and inherited attributes
» Classes of attribute grammars
» Case study - a small student database
» Installation Input file preparation
» Execution Output from CocoR Assembling the generated system
» Case study - a simple adding machine
» Character sets Comments and ignorable characters
» Pragmas User names The scanner interface
» Syntax error recovery Parser specification
» Grammar checks Semantic errors
» Interfacing to support modules The parser interface
» Essentials of the driver program Customizing the driver frame file
» Case study - Understanding C declarations
» Case study - Generating one-address code from expressions
» Case study - Generating one-address code from an AST
» Case study - How do parser generators work?
» Project suggestions compilers and compiler generators 1996
» Overall compiler structure compilers and compiler generators 1996
» A hand-crafted source handler
» A hand-crafted scanner Lexical analysis
» A CocoR generated scanner Efficient keyword recognition
» A hand-crafted parser A CocoR generated parser
» Syntax error handling in hand-crafted parsers
» Error reporting The symbol table handler
» Other aspects of symbol table management - further types
» The code generation interface
» Code generation for a simple stack machine
» Machine tyranny Other aspects of code generation
» Abstract syntax trees as intermediate representations
» Simple optimizations - constant folding
» Simple optimizations - removal of redundant code
» Generation of assembler code
» Source handling, lexical analysis and error reporting Syntax
» The scope and extent of identifiers
» Symbol table support for the scope rules
» Parsing declarations and procedure calls
» Hypothetical machine support for the static link model
» Code generation for the static link model
» Hypothetical machine support for the display model
» Code generation for the display model
» Relative merits of the static link and display models
» Syntax and semantics compilers and compiler generators 1996
» Symbol table support for context-sensitive features
» Actual parameters and stack frames
» Hypothetical stack machine support for parameter passing
» Semantic analysis and code generation in a hand-crafted compiler
» Semantic analysis and code generation in a CocoR generated compiler
» Scope rules Language design issues
» Function return mechanisms Language design issues
» Context sensitivity and LL1 conflict resolution Fundamental concepts
» Parallel processes, exclusion and synchronization
Show more