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
  • -### dry-run, outputting exact compiler/linker invocations
  • -print-multi-lib print available multilib configurations

Target options

# List all target options with their description.
gcc --help=target

# Configure for current cpu arch and query (-Q) value of options.
gcc -march=native -Q --help=target

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.

The semantics of this hint are as follows, the compiler prioritises expr == cond. So __builtin_expect(expr, 0) means that we expect the expr to be 0 most of the time.

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)