[Bug c/70678] New: Static function compilation behaviour changes with __attribute__((optimize("O2"))) even if already compiling with -O2

[lutoma at ohai dot su]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70678

            Bug ID: 70678
           Summary: Static function compilation behaviour changes with
                    __attribute__((optimize("O2"))) even if already
                    compiling with -O2
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lutoma at ohai dot su
  Target Milestone: ---

Noticed this while tracking down another issue. I have seen this on GCC 4.6.4
and 5.3.0.

#include <stdio.h>
#include <stdlib.h>

static void test_function() {
        printf("Hello world!\n");
}

int main() {
        test_function();
        return EXIT_SUCCESS;
}

Compiled using "gcc -o test test.c -O2 -save-temps -Wall -Wextra
-fno-strict-aliasing -fwrapv", GCC will optimize the test_function() by
including it into main().

If the function definition is changed to

static void __attribute__((optimize("O2"))) test_function() {

and the file is compiled using the same command, the test_function() will stay
separate, and is not optimized into main(), even though the optimization levels
have not changed.

Intermediate with the additional optimize attribute:
https://gist.githubusercontent.com/lutoma/b36eda746d618c2e5f8b0e28e1478b34/raw/a9ce9f8d23f954a112a911621e3e3c10d3a1f307/test.i
Intermediate without the additional attribute:
https://gist.githubusercontent.com/lutoma/ac300f0349be4e4de15a4709813e17f0/raw/4b193c8d2aca363c26a0241ad6d7a821427593a5/test.i

Generated assembly for the functions:

Without extra optimize attribute:

00000000004003f0 <main>:
  4003f0:       48 83 ec 08             sub    rsp,0x8
  4003f4:       bf 94 05 40 00          mov    edi,0x400594
  4003f9:       e8 c2 ff ff ff          call   4003c0 <puts@plt>
  4003fe:       31 c0                   xor    eax,eax
  400400:       48 83 c4 08             add    rsp,0x8
  400404:       c3                      ret    
[…]

With the extra attribute:

0000000000400506 <test_function>:
  400506:       bf 94 05 40 00          mov    edi,0x400594
  40050b:       e9 b0 fe ff ff          jmp    4003c0 <puts@plt>

00000000004003f0 <main>:
  4003f0:       48 83 ec 08             sub    rsp,0x8
  4003f4:       31 c0                   xor    eax,eax
  4003f6:       e8 0b 01 00 00          call   400506 <test_function>
  4003fb:       31 c0                   xor    eax,eax
  4003fd:       48 83 c4 08             add    rsp,0x8
  400401:       c3                      ret    
[…]

Sorry if this is a duplicate or intended/undefined behaviour in some way.

Cheers,
Lukas