CODEWITHSUNDEEP
c++gcccompiler-optimization
Steve Lorimer
Steve Lorimer

Reputation: 28659

Will an optimiser deduce mathematical expressions based on compile-time constants?

If I have some mathematical equations which rely on inputs which can be zero or non-zero (template argument, known at compile time), will the optimiser evaluate the equations and optimise out expressions it knows will evaluate to 0 or 1.

For example:

double x = y * Eval<type>::value;

if Eval<type>::value is 0, x will always be 0. 

double x = exp(y * Eval<type>::value);

if Eval<type>::value is 0, x will always be 1.

Can the optimiser figure this out and replace x with 0 or 1 elsewhere in the code, or will these calculations be carried out at runtime?

I am using gcc 4.7 with -O3

Upvotes: 5

Views: 249

Answers (1)

user406009
user406009

Reputation:

EDIT: I was wrong, the compiler works as expected when using a floating point number.

Well gcc 4.6.3 in -O3 certainly does seem to do this, as long as the expression is integer related.

Example code:

#include <cstdio>
inline int x(double y)
{
   if (y == 0)
      printf("Hello bob3\n");
   else
      printf("Why do I bother\n");

};

const int c = 0;

int main()
{
   int f;
   scanf("%d",&f);

   x(f * c);
}

Resulting assembly

    .file   "foo.cpp"
    .section    .rodata.str1.1,"aMS",@progbits,1
.LC0:
    .string "%d"
.LC1:
    .string "Hello bob3"
    .section    .text.startup,"ax",@progbits
    .p2align 4,,15
    .globl  main
    .type   main, @function
main:
.LFB13:
    .cfi_startproc
    subq    $24, %rsp
    .cfi_def_cfa_offset 32
    movl    $.LC0, %edi
    xorl    %eax, %eax
    leaq    12(%rsp), %rsi
    call    scanf
    movl    $.LC1, %edi
    call    puts
    xorl    %eax, %eax
    addq    $24, %rsp
    .cfi_def_cfa_offset 8
    ret
    .cfi_endproc
.LFE13:
    .size   main, .-main
    .ident  "GCC: (Debian 4.6.3-1) 4.6.3"
    .section    .note.GNU-stack,"",@progbits

Upvotes: 1

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