------- Comment #7 from mikpe at it dot uu dot se 2010-09-09 10:21 -------
It's not a stage2/stage3 debug difference as far as I can tell. I've
recompiled every differing .o file with the stage 1/2/3 xgccs -fcompare-debug
without complaints.
The test case showing the different code generation is trivial (this is in the
objdir for a native bootstrap of r163951):
> cat sreal.c
void normalize(unsigned long long *sreal_sig)
{
*sreal_sig <<= 1;
}
> stage1-gcc/xgcc -Bstage1-gcc -O2 -c sreal.c; objdump -d sreal.o
sreal.o: file format elf32-littlearm
Disassembly of section .text:
00000000 <normalize>:
0: e1c020d0 ldrd r2, [r0]
4: e0922002 adds r2, r2, r2
8: e0a33003 adc r3, r3, r3
c: e1c020f0 strd r2, [r0]
10: e12fff1e bx lr
> stage2-gcc/xgcc -Bstage2-gcc -O2 -c sreal.c ; objdump -d sreal.o
sreal.o: file format elf32-littlearm
Disassembly of section .text:
00000000 <normalize>:
0: e1c020d0 ldrd r2, [r0]
4: e1b02082 lsls r2, r2, #1
8: e0a33003 adc r3, r3, r3
c: e1c020f0 strd r2, [r0]
10: e12fff1e bx lr
> stage3-gcc/xgcc -Bstage3-gcc -O2 -c sreal.c ; objdump -d sreal.o
sreal.o: file format elf32-littlearm
Disassembly of section .text:
00000000 <normalize>:
0: e1c020d0 ldrd r2, [r0]
4: e1b02082 lsls r2, r2, #1
8: e0a33003 adc r3, r3, r3
c: e1c020f0 strd r2, [r0]
10: e12fff1e bx lr
So stage1 chooses adds but stage2 and stage3 choose lsls for << of the lower
half of a long long. Since the behaviour of a stageN xgcc depends on both the
gcc source code and the compiler used to build it, I have to suspect a source
code ambiguity (e.g. evaluation order dependence) that the bootstrap compiler
(gcc-4.4.4 in my case) resolves differently from post-r162417 4.6.
I've so far not been able to reproduce this difference in a cross from i686;
those cross compilers always seem to choose the adds form regardless of the
version of gcc I'm building with.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45445
