Continuation from 42586 and 42585 who handle other missed optimizations for this test case.
>From one of the examples of http://embed.cs.utah.edu/embarrassing/dec_09/harvest/gcc-head_llvm-gcc-head/ struct _fat_ptr { unsigned char *curr; unsigned char *base; unsigned char *last_plus_one; }; int Cyc_string_ungetc (int ignore, struct _fat_ptr *sptr); int Cyc_string_ungetc (int ignore, struct _fat_ptr *sptr) { struct _fat_ptr *_T0; struct _fat_ptr *_T1; struct _fat_ptr _T2; int _T3; struct _fat_ptr _ans; int _change; { _T0 = sptr; _T1 = sptr; _T2 = *sptr; _T3 = -1; _ans = _T2; _change = -1; _ans.curr += 4294967295U; *sptr = _ans; return (0); } } generates hen compiled with -O2 -m32 on 4.5.0 20091219 generates Cyc_string_ungetc: subl $32, %esp movl 40(%esp), %eax movl (%eax), %edx subl $1, %edx movl %edx, (%eax) xorl %eax, %eax addl $32, %esp ret The stack frame manipulation is completely unnecessary. A.Pinski commented on the other bug on this: ell one thing is SRA does not do its job which is why there is useless stack frame changes. ------- Comment #2 From Andrew Pinski 2010-01-03 06:27 [reply] ------- Oh and -Os is very bad looking :). ------- Comment #3 From Andrew Pinski 2010-01-03 06:30 [reply] ------- Oh in fact I think the reason is the SRA issue. In that: int Cyc_string_ungetc (int ignore, struct _fat_ptr *sptr) { sptr->curr += 4294967295U; } Works. ------- Comment #4 From Andrew Pinski 2010-01-03 06:33 [reply] ------- Hmm, 4.5 is worse off than 4.4 with respect of the stack space usage. ------- Comment #5 From Andrew Pinski 2010-01-03 06:35 [reply] ------- 4.5 has: _T2 = *sptr_1(D); _T2$curr_14 = sptr_1(D)->curr; _ans = _T2; D.2697_7 = _T2$curr_14 + -1; *sptr_1(D) = _ans; sptr_1(D)->curr = D.2697_7; While 4.4 does: _T2$base = sptr->base; D.1587 = sptr->curr + -1; sptr->last_plus_one = sptr->last_plus_one; sptr->base = _T2$base; sptr->curr = D.1587; ------- Comment #6 From Andrew Pinski 2010-01-03 06:38 [reply] ------- I think if we get the old SRA behavior back for this code, we will get this optimization in 4.5 since we remove the sptr->base and the other unnecessary store during PRE. -- Summary: unnecessary stack frame set up Product: gcc Version: unknown Status: UNCONFIRMED Severity: enhancement Priority: P3 Component: middle-end AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: andi-gcc at firstfloor dot org GCC host triplet: x86_64-linux GCC target triplet: x86_64-linux http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42590
