http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57534
Bug ID: 57534
Summary: Performance regression versus 4.7.3, 4.8.1 is ~15%
slower
Product: gcc
Version: 4.8.1
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ncahill_alt at yahoo dot com
Created attachment 30261
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30261&action=edit
Reduced source code - timing functions
With x86 GCC 4.8.1, slower code is produced (than with 4.7.3) for a particular
benchmark I ran, about 15% slower.
Whatever is wrong must be happening here:
80486e5: d9 ee fldz
80486e7: d9 c0 fld %st(0)
80486e9: 8d b4 26 00 00 00 00 lea 0x0(%esi,%eiz,1),%esi
80486f0: 8d 04 f5 00 00 00 00 lea 0x0(,%esi,8),%eax
80486f7: dd 04 f3 fldl (%ebx,%esi,8)
80486fa: dc 44 03 08 faddl 0x8(%ebx,%eax,1)
80486fe: dc 44 03 10 faddl 0x10(%ebx,%eax,1)
8048702: dc 44 03 18 faddl 0x18(%ebx,%eax,1)
8048706: de c2 faddp %st,%st(2)
8048708: dd 44 03 20 fldl 0x20(%ebx,%eax,1)
804870c: dc 44 03 28 faddl 0x28(%ebx,%eax,1)
8048710: dc 44 03 30 faddl 0x30(%ebx,%eax,1)
8048714: dc 44 03 38 faddl 0x38(%ebx,%eax,1)
8048718: 8d 46 08 lea 0x8(%esi),%eax
804871b: 39 c7 cmp %eax,%edi
804871d: de c1 faddp %st,%st(1)
804871f: 7f 0e jg 804872f
8048721: a1 34 91 04 08 mov 0x8049134,%eax
8048726: 85 c0 test %eax,%eax
8048728: 74 0e je 8048738
804872a: 83 c5 01 add $0x1,%ebp
804872d: 31 c0 xor %eax,%eax
804872f: 89 c6 mov %eax,%esi
8048731: eb bd jmp 80486f0
8048733: 90 nop
8048734: 8d 74 26 00 lea 0x0(%esi,%eiz,1),%esi
8048738: dd 5c 24 10 fstpl 0x10(%esp)
804873c: 83 c6 10 add $0x10,%esi
804873f: dd 5c 24 08 fstpl 0x8(%esp)
This is the commandline: gcc -O2 reduceme.c timer.o -o cachebench
This is from a benchmark (llcbench, GPL software) and uses timers which may be
a problem, if I preprocess them, they may not work. I'll attach the main code
(reduced) for now, and I'll work on getting the timing code included very soon.
I'll also test with 4.8.0 to see whether that version is also affected.
Attached is the reduced code minus the timing functions. Uncommenting the
commented line in the source code removes the bug.
Thanks.
Neil.
