------- Comment #11 from michaelni at gmx dot at 2008-03-24 00:08 -------
Subject: Re: Performance degradation when
building code that uses MMX intrinsics with gcc-4.0.0
On Sun, Mar 23, 2008 at 10:46:41AM -0000, ubizjak at gmail dot com wrote:
>
>
> ------- Comment #10 from ubizjak at gmail dot com 2008-03-23 10:46 -------
> (In reply to comment #9)
>
> > So on my duron 4.3 seems to beat 4.4 as i expected from the generated asm.
>
> Can you tell from code dumps of 4.4 vs 4.3, where you think that 4.4 code is
> worse than 4.3 for Duron? For Core2, 4.4 avoids store forwarding stall, but
> I'm
> not sure why Duron prefers moves via memory instead of keeping values in %mm
> registers.
--- freaky_mmx_code-4.3.s 2008-03-24 00:48:11.000000000 +0100
+++ freaky_mmx_code-4.4.s 2008-03-24 00:48:03.000000000 +0100
...
.L24:
movl -36(%ebp), %eax
testl %ebx, %ebx
movl (%edi,%esi,4), %edx
movl (%eax,%esi,4), %ecx
@@ -182,113 +183,102 @@
xorl %eax, %eax
movq -24(%ebp), %mm2
.p2align 4,,7
.p2align 3
.L23:
movq (%ecx,%eax,2), %mm0
psubw (%edx,%eax,2), %mm0
addl $4, %eax
cmpl %eax, %ebx
movq %mm0, %mm1
- psraw $15, %mm0
- pxor %mm0, %mm1
- psubw %mm0, %mm1
- movq %mm1, %mm0
- punpcklwd %mm1, %mm1
- punpckhwd %mm3, %mm0
- psrad $16, %mm1
- paddd %mm0, %mm1
- paddd %mm1, %mm2
+ psraw $15, %mm1
+ pxor %mm1, %mm0
+ psubw %mm1, %mm0
+ movq %mm0, %mm1
+ punpcklwd %mm0, %mm0
+ punpckhwd %mm3, %mm1
+ psrad $16, %mm0
+ paddd %mm1, %mm0
+ paddd %mm0, %mm2
movq %mm2, -24(%ebp)
jg .L23
.L22:
addl $1, %esi
- cmpl %esi, -40(%ebp)
- jg .L24
+ cmpl -40(%ebp), %esi
+ jl .L24
...
- .ident "GCC: (Debian 4.3.0-1) 4.3.1 20080309 (prerelease)"
+ .ident "GCC: (GNU) 4.4.0 20080321 (experimental)"
------------------
What i _think_ makes 4.4 slower on duron is that
psraw $15, %mm1
reads a register which has been written in the previous instruction
while 4.3 choose the other register which contains the same value.
4.4 simply has a longer dependancy chain than 4.3.
PS: both compiled with -mmmx -O2 -S
PS2: 4.3 is from debian, 4.4 is from gcc svn
[...]
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21395
