基本の話: 「変数や関数を節約すればパフォーマンスが上がる」わけではない ref: http://qiita.com/kitsuyui/items/1275ba79c423f2d85530

a.c

#include <stdio.h>

int main(void)
{
    printf("%d\n", 24);
    return 0;
}

a.s

    .section    __TEXT,__text,regular,pure_instructions
    .globl  _main
    .align  4, 0x90
_main:                                  ## @main
    .cfi_startproc
## BB#0:
    pushq   %rbp
Ltmp2:
    .cfi_def_cfa_offset 16
Ltmp3:
    .cfi_offset %rbp, -16
    movq    %rsp, %rbp
Ltmp4:
    .cfi_def_cfa_register %rbp
    subq    $16, %rsp
    leaq    L_.str(%rip), %rdi
    movl    $24, %esi
    movl    $0, -4(%rbp)
    movb    $0, %al
    callq   _printf
    movl    $0, %esi
    movl    %eax, -8(%rbp)          ## 4-byte Spill
    movl    %esi, %eax
    addq    $16, %rsp
    popq    %rbp
    retq
    .cfi_endproc

    .section    __TEXT,__cstring,cstring_literals
L_.str:                                 ## @.str
    .asciz  "%d\n"


.subsections_via_symbols

b.c

#include <stdio.h>

int main(void)
{
    int i = 1;
    int j = 2;
    int k = 3;
    int l = 4;
    int m = i * j * k * l;
    printf("%d\n", m);
    return 0;
}

c.c

#include <stdio.h>

static int x2(int x)
{
    return x * 2;
}

static int x3(int x)
{
    return x * 3;
}

static int x4(int x)
{
    return x2(x2(x));
}

int main(void)
{
    int x = x4(x3(x2(1)));
    printf("%d\n", x);
    return 0;
}

file0.txt

$ cc --version
Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin14.1.0
Thread model: posix

file4.txt

$ cc -c -O3 a.c b.c c.c

file6.txt

$ cmp a.o b.o
$ cmp b.o c.o
$ cmp c.o a.o

file7.txt

$ cc -O3 -S a.c

file9.txt

$ cc -c -O3 -S a.c

コマンドの実行例

$ cmp a.o a.o
$ cmp a.o d.o
a.o d.o differ: char 1153, line 1