Constant Propagation

Constant propagation is an optimization technique used in compilers to improve execution efficiency by replacing variables with known constant values during compilation. If an expression consists entirely of constants or previously propagated values, it is evaluated at compile time and replaced with its computed result. This transformation reduces runtime computations, simplifies code, and enables further optimizations by minimizing unnecessary variable assignments. By eliminating redundant calculations, constant propagation enhances program performance while preserving its original behavior.

Constant Propagation Transformation in emmtrix Studio

emmtrix Studio implements constant propagation using #pragma directives or via the GUI. Constant propagation is a transformation that propagates all known variable values within the block it is applied to. If an expression consists of propagated values and of constants, it will itself be evaluated to and replaced with its constant value.

All arithmetic operations for integral types are supported. Interprocedural propagation, i.e. propagating into and out of function calls, is partially supported.

Typical Usage and Benefits

Constant propagation is typically used to generate a clearer code, to reduce complexity for dependency analysis and increase potential for better optimization possibilities.

Example

/* The following code tests constant propagation applied to main function. 
 * In the given example, variables a, b and c are known. 
 * They are propagated to the rest of the body of main.
 */   

#pragma EMX_TRANSFORMATION ConstPropagation
int main(void) {
    int a = 2;
    int b = a;
    
    int c, d;
    c = 2 + 2;
    d = b + c;
    
    return 0;
}

/* The following code is the generated code after the transformation has been applied.
 */

int main(void) {
    int a = 2;
    int b = a;
    int c;
    int d;
    
    c = 4;
    d = 6;

    return 0;
}

Parameters


Id	Possible Values	Default Value	Description
`global`	`true, false`	`false`	Determines whether the constant propagation should work across function boundaries
`modify_nonstatic_functions`	`true, false`	`false`	Determines whether the constant propagation may modify non-static functions. This should be used with care as it might be illegal if not all call-sites of the non-static functions are known.
`remove_dead_blocks`	`no, defensive, aggressive`	`no`	Determines whether parts of the code that are unreachable after constant propagation should be removed. The `aggressive` variant might seldomly insert additional `goto` statements in cases where the`defensive` variant rather keeps some unreachable parts of the code to avoid `goto` statements.

Constant Propagation

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