gcc(1)

CLI

Preprocessing

While debugging can be helpful to just pre-process files.

gcc -E [-dM] ...
  • -E run only preprocessor
  • -dM list only #define statements

Builtins

__builtin_expect(expr, cond)

Give the compiler a hint which branch is hot, so it can lay out the code accordingly to reduce number of jump instructions. See on compiler explorer.

echo "
extern void foo();
extern void bar();
void run0(int x) {
  if (__builtin_expect(x,0)) { foo(); }
  else { bar(); }
}
void run1(int x) {
  if (__builtin_expect(x,1)) { foo(); }
  else { bar(); }
}
" | gcc -O2 -S -masm=intel -o /dev/stdout -xc -

Will generate something similar to the following.

  • run0: bar is on the path without branch
  • run1: foo is on the path without branch
run0:
        test    edi, edi
        jne     .L4
        xor     eax, eax
        jmp     bar
.L4:
        xor     eax, eax
        jmp     foo
run1:
        test    edi, edi
        je      .L6
        xor     eax, eax
        jmp     foo
.L6:
        xor     eax, eax
        jmp     bar

ABI (Linux)