c - Conversion from size_t to int -


following thread ...

for piece of code:

#include <stdio.h>  int main(void) {     int i;     size_t u;      (i = 0; < 10; i++) {         u = (size_t)i;         printf("i = %d, u = %zu\n", i, u);     }     return 0; }

the output in assembly is:

edit: compiled -o2

    .file   "demo.c"     .section    .rodata.str1.1,"ams",@progbits,1 .lc0:     .string "i = %d, u = %zu\n"     .section    .text.startup,"ax",@progbits     .p2align 4,,15     .globl  main     .type   main, @function main: .lfb3:     .cfi_startproc     pushq   %rbx     .cfi_def_cfa_offset 16     .cfi_offset 3, -16     xorl    %ebx, %ebx     .p2align 4,,10     .p2align 3 .l2:     movq    %rbx, %rdx     movl    %ebx, %esi     xorl    %eax, %eax     movl    $.lc0, %edi     addq    $1, %rbx     call    printf     cmpq    $10, %rbx     jne .l2     xorl    %eax, %eax     popq    %rbx     .cfi_def_cfa_offset 8     ret     .cfi_endproc .lfe3:     .size   main, .-main     .ident  "gcc: (debian 4.7.2-5) 4.7.2"     .section    .note.gnu-stack,"",@progbits

is conversion u = (size_t)i; consuming cycles?

yes, does, changes internal representation 32bit 64bit. specifically, 

.l3:     movl    -4(%rbp), %eax     cltq     movq    %rax, -16(%rbp)     movq    -16(%rbp), %rdx

reads i, performs sign-extension , copying %rdx. i'm unsure why value has pass through stack - mats pointed out, looks code non-noptimizing compiler run.

edit

in optimized assembly code, loop counter maintained wider data type. afair, movs between registers don't differ in run-time cycles wrt quad or dword (indeed don't: see table c-16 in intels pertinent doc, referenced this post.


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