[Pixman] ARM iwmmxt patches

Matt Turner mattst88 at gmail.com
Wed Jul 27 10:03:15 PDT 2011

On Wed, Jul 27, 2011 at 12:52 PM, Soeren Sandmann <sandmann at cs.au.dk> wrote:
> Matt Turner <mattst88 at gmail.com> writes:
>> The 3 patch series adds support for compiling pixman's pixman-mmx.c
>> for ARM/iwmmxt for some performance improvements on iwmmxt-enabled ARM
>> CPUs. This is done by taking advantage of the fact that gcc provides
>> MMX-compatible _mm_*-style intrinsics for iwmmxt on ARM.
>> On my OLPC XO 1.75 (with a Marvell CPU), they pass the pixman test
>> suite (verified that test suite passes on x86/MMX as well) and improve
>> performance of most cairo-traces 7% or more. (See attached)
>> For lowlevel-blit-bench, iwmmxt paths are not always faster, at times
>> losing to ARMv6 or geneic paths (but even ARMv6 is sometimes slower
>> than generic...) but providing some massive speed-ups at times:
> A few overall comments:
> - It would make sense to rename USE_MMX to USE_X86_MMX for symmetry, and
>  also adding a comment at the top of pixman-mmx.c to indicate that it
>  is being used on both x86 and ARM.

OK, I can do that.

> - We need more details in the commit messages.

Indeed. Will do.

> Thanks for generating the detailed data. I have formatted it here:
>  low-level-blit:   http://people.freedesktop.org/~sandmann/bench-data/all-llblit.txt
>  traces:           http://people.freedesktop.org/~sandmann/bench-data/all-traces.txt
> to more clearly show the differences between the various
> implementations. As Siarhei already commented on, the most surprising
> result is that the armv6 assembly is generally slower than the generic C
> code, in some cases a lot slower.
>> gcc's current support for iwmmxt code generation is atrocious (See gcc
>> bugs 35294, 36798, 36966), so I have patched gcc to add missing shift
>> and logical iwmmxt instructions. I have seen patches posted improving
>> gcc's iwmmxt support, so I hope that gcc-4.7 will be able to use
>> pixman's iwmmxt code without trouble. (Reminds me as I write this that
>> I need to modify the configure.ac test to use instructions that cause
>> current gcc to crash.)
> Are you saying that current versions of GCC basically don't work with
> iwmmxt? If so, we should probably just check for the GCC 4.7 in
> configure.

Yes, patches have been send to gcc-patches@ but I don't think they're
in gcc-4.7 yet. gcc-4.6 and older, unless there have been some
startling regressions, certainly cannot use basic shift and logical
instruction intrinsics.

I will modify the configure.ac hunk to check for gcc-4.7 and also
modify the test code to use an intrinsic that is used in pixman-mmx.c
and known to not work with gcc-4.6.1.


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