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.
