http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51182
Bug #: 51182
Summary: [ipa-iterations] running multiple passes of early IPA
on a file produces difference code when it shouldn't
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: m...@use.net
Created attachment 25841
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25841
pre-procecessed source that produces the above code differances
As requested by Richard (http://gcc.gnu.org/ml/gcc-cvs/2011-11/msg00669.html),
I am testing the outstanding multiple iterations patch and reporting when
multiple early IPA passes produce differences in code generation that should
probably be gotten in one pass (or not at all).
The attached file is from the open source pmccabe project. When compiling with
-O1, there are register scheduling differences and the elimination of a nop
instruction when doing a second early IPA pass.
with -O1 --param eipa-iterations=1:
2b3: 8d 6d 01 lea 0x1(%rbp),%ebp
2b6: 48 98 cltq
2b8: 48 8d 5c c3 f8 lea -0x8(%rbx,%rax,8),%rbx
2bd: 83 3d 00 00 00 00 00 cmpl $0x0,0x0(%rip) # 2c4 <main+0x180>
2c4: 74 12 je 2d8 <main+0x194>
2c6: 48 89 de mov %rbx,%rsi
2c9: 89 ef mov %ebp,%edi
[...]
429: 80 78 50 01 cmpb $0x1,0x50(%rax)
42d: 0f 1f 00 nopl (%rax)
430: 76 2e jbe 460 <stats_accumulate+0x4c>
with -O1 --param eipa-iterations=2:
2b3: 44 8d 65 01 lea 0x1(%rbp),%r12d
2b7: 48 98 cltq
2b9: 48 8d 5c c3 f8 lea -0x8(%rbx,%rax,8),%rbx
2be: 83 3d 00 00 00 00 00 cmpl $0x0,0x0(%rip) # 2c5 <main+0x181>
2c5: 74 13 je 2da <main+0x196>
2c7: 48 89 de mov %rbx,%rsi
2ca: 44 89 e7 mov %r12d,%edi
[...]
42f: 80 78 50 01 cmpb $0x1,0x50(%rax)
433: 76 2e jbe 463 <stats_accumulate+0x49>
There are additional/different differences at -O2, but I'll file those in
another bug once I get feedback on this one.
