GCC beaten by ICC in stupid trig test!

[Paolo Carlini]

Zdenek Dvorak wrote:

>doing what? I do not see any loop related optimization here.
>
Hi Zdenek. What is doing "the trick" (sorry for my informal words) on the
gcc-lno branch is -ftree-loop-optimize, *not* -funroll-loops.

When -ftree-loop-optimize is passed, the trigonometric computation (doit)
is moved outside of the loop, this is the complete result:

08048450 <main>:
 8048450:       55                      push   %ebp
 8048451:       d9 e8                   fld1
 8048453:       89 e5                   mov    %esp,%ebp
 8048455:       83 ec 18                sub    $0x18,%esp
 8048458:       83 e4 f0                and    $0xfffffff0,%esp
 804845b:       83 ec 10                sub    $0x10,%esp
 804845e:       dd 1c 24                fstpl  (%esp)
 8048461:       e8 aa ff ff ff          call   8048410 <doit>
 8048466:       d9 ee                   fldz
 8048468:       31 c0                   xor    %eax,%eax
 804846a:       8d b6 00 00 00 00       lea    0x0(%esi),%esi

 8048470:       40                      inc    %eax
 8048471:       d8 c1                   fadd   %st(1),%st
 8048473:       3d 00 e1 f5 05          cmp    $0x5f5e100,%eax
 8048478:       75 f6                   jne    8048470 <main+0x20>

 804847a:       dd d9                   fstp   %st(1)
 804847c:       dd 5c 24 04             fstpl  0x4(%esp)
 8048480:       c7 04 24 98 85 04 08    movl   $0x8048598,(%esp)
 8048487:       e8 a4 fe ff ff          call   8048330 <_init+0x48>
 804848c:       c9                      leave
 804848d:       31 c0                   xor    %eax,%eax
 804848f:       c3                      ret

Whereas, without -ftree-loop-optimize, we have:

08048450 <main>:
 8048450:       55                      push   %ebp
 8048451:       d9 ee                   fldz
 8048453:       89 e5                   mov    %esp,%ebp
 8048455:       53                      push   %ebx
 8048456:       83 ec 24                sub    $0x24,%esp
 8048459:       bb ff e0 f5 05          mov    $0x5f5e0ff,%ebx
 804845e:       dd 5d f0                fstpl  0xfffffff0(%ebp)
 8048461:       83 e4 f0                and    $0xfffffff0,%esp
 8048464:       83 ec 10                sub    $0x10,%esp
 8048467:       eb 09                   jmp    8048472 <main+0x22>
 8048469:       8d b4 26 00 00 00 00    lea    0x0(%esi),%esi

 8048470:       dd d8                   fstp   %st(0)
 8048472:       c7 04 24 00 00 00 00    movl   $0x0,(%esp)
 8048479:       b8 00 00 f0 3f          mov    $0x3ff00000,%eax
 804847e:       89 44 24 04             mov    %eax,0x4(%esp)
 8048482:       e8 89 ff ff ff          call   8048410 <doit>
 8048487:       dc 45 f0                faddl  0xfffffff0(%ebp)
 804848a:       4b                      dec    %ebx
 804848b:       dd 55 f0                fstl   0xfffffff0(%ebp)
 804848e:       79 e0                   jns    8048470 <main+0x20>

 8048490:       dd 5c 24 04             fstpl  0x4(%esp)
 8048494:       c7 04 24 b8 85 04 08    movl   $0x80485b8,(%esp)
 804849b:       e8 90 fe ff ff          call   8048330 <_init+0x48>
 80484a0:       8b 5d fc                mov    0xfffffffc(%ebp),%ebx
 80484a3:       31 c0                   xor    %eax,%eax
 80484a5:       c9                      leave
 80484a6:       c3                      ret

Perhaps it's ok moving doit even when -ffast-math is not passed, I don't
know for sure, honestly...

Thanks for your feedback,
Paolo.