[Bug rtl-optimization/21182] [4.6/4.7/4.8 Regression] gcc can use registers but uses stack instead

[vda.linux at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21182

--- Comment #11 from Denis Vlasenko <vda.linux at googlemail dot com> 2013-01-20 14:39:42 UTC ---
(In reply to comment #10)
> 4.4.7 and 4.5.4 generate the same code (no stack use) for -D/-UNAIL_REGS.
> With 4.6.3, the -DNAIL_REGS code regresses very much (IRA ...), the
> -UNAIL_REGS code is nearly perfect but less good than 4.4/4.5 (if you
> only consider grep esp serpent.s | wc -l).  Same behavior with 4.7.2.
> 
> Trunk got somewhat worse with -UNAIL_REGS but better with -DNAIL_REGS (at -O2):
> 
>      -UNAIL_REGS  -DNAIL_REGS
> 4.5.4     3             3
> 4.6.3    15           101

This matches what I see with 4.6.3 - 15 insns with %esp (and no %ebp):

$ grep '%esp' serpent-4.6.3-O2.asm
   4:   83 ec 04                sub    $0x4,%esp
   7:   8b 4c 24 20             mov    0x20(%esp),%ecx
   b:   8b 44 24 18             mov    0x18(%esp),%eax
 22e:   89 0c 24                mov    %ecx,(%esp)
 239:   23 3c 24                and    (%esp),%edi
 588:   89 0c 24                mov    %ecx,(%esp)
 58f:   23 3c 24                and    (%esp),%edi
 8f4:   89 0c 24                mov    %ecx,(%esp)
 8fd:   23 3c 24                and    (%esp),%edi
 c60:   89 0c 24                mov    %ecx,(%esp)
 c6b:   23 3c 24                and    (%esp),%edi
 d37:   89 14 24                mov    %edx,(%esp)
 d5a:   8b 44 24 1c             mov    0x1c(%esp),%eax
 d5e:   33 14 24                xor    (%esp),%edx
 d70:   83 c4 04                add    $0x4,%esp

> The most important thing to fix is the -UNAIL_REGS case of course.

Sure. NAIL_REGS is only a hack meant to demonstrate that regs *can* be
allocated optimally.