On Thu, Dec 1, 2011 at 12:20 AM, Ira Rosen <ira.ro...@linaro.org> wrote:
> On 30 November 2011 02:33, Michael Hope <michael.h...@linaro.org> wrote:
>
>> I then converted the vld1 and vst1 to specifiy an alignment of 64
>> bits. See:
>> http://people.linaro.org/~michaelh/incoming/set-alignment.png
>>
>> This improved the throughput in all cases and in cases for more than 50
>> words by 14 %. This graph also shows the overhead of the runtime
>> peeling check. The blue line is the vectoriser version which is
>> slower to pick up due the greater per call overhead.
>
> So, the auto-vectorized code doesn't have the alignment hints (peeling
> or not peeling), right? Is this how a hint is supposed to look like:
> vld1.i64 {d16-d17}, [r1 :"#_128"] , or am I looking for a wrong thing?
Yip. We currently use a vldmia r1!, {d16-d17} which (on the A9 at
least) only works for aligned values and takes the same time as the
unaligned-friendly vld1.i64 {d16-d17}, [r1]!
> I thought that peeling should be useful at least for the hints.
Peeling and using the vld1.i64 {d16-d17}, [r1:64]! form should be
faster for larger loops. For some reason vld1.i64 ..., [r1:128] gives
an illegal instruction trap on my board. Note that the :128 is in
bits.
>> I then went back to the vectoriser and changed the alignment of the
>> struct to cause peeling to turn on and off. See:
>> http://people.linaro.org/~michaelh/incoming/unroll.png
>>
>> At 200 words, the version without peeling is 2.9 % faster. This is
>> partly due to a fixed count loop turning into a runtime count due to
>> unknown alignment.
>>
>> This run also showed the affect of loop unrolling. The loop seems to
>> be unrolled for loops of <= 64 words and drops off in performance past
>> around 8 words. When the unrolling finally drops out, performance
>> increases by 101 %.
>
> I see register spills starting from COUNT=36.
Ah. Does the vectoriser cost model take register pressure into
account? How can I turn this on?
-- Michael
_______________________________________________
linaro-toolchain mailing list
linaro-toolchain@lists.linaro.org
http://lists.linaro.org/mailman/listinfo/linaro-toolchain
