[llvm-dev] Invoke loop vectorizer

[Daniel Berlin via llvm-dev]

cat > test.c

#define SIZE 128

void bar(int *restrict A, int* restrict B,int K) {

  #pragma clang loop vectorize(enable) vectorize_width(2) unroll_count(8)

  for (int i = 0; i < SIZE; ++i)

    A[i] += B[i] + K;

}

[dannyb at dannyb-macbookpro3 11:37:20] ~ :) $ clang -O3  test.c -c -save-temps
[dannyb at dannyb-macbookpro3 11:38:28] ~ :) $ pcregrep -i "^\s*p" test.s|less
        pushq   %rbp
        pshufd  $68, %xmm0, %xmm0       ## xmm0 = xmm0[0,1,0,1]
        pslldq  $8, %xmm1               ## xmm1 =
zero,zero,zero,zero,zero,zero,zero,zero,xmm1[0,1,2,3,4,5,6,7]
        pshufd  $68, %xmm3, %xmm3       ## xmm3 = xmm3[0,1,0,1]
        paddq   %xmm1, %xmm3
        pshufd  $78, %xmm3, %xmm4       ## xmm4 = xmm3[2,3,0,1]
        punpckldq       %xmm5, %xmm4    ## xmm4 =
xmm4[0],xmm5[0],xmm4[1],xmm5[1]
        pshufd  $212, %xmm4, %xmm4      ## xmm4 = xmm4[0,1,1,3]



Note:
It also vectorizes at SIZE=8.

Not sure what the exact translation of options from clang-cl to clang is.
Maybe try adding /O3?




On Fri, Aug 12, 2016 at 11:23 AM, Xiaochu Liu <xiaochu1122 at gmail.com> wrote:

> Hi Daniel,
>
> I increased the size of your test to be 128 but -stats still shows no loop
> optimized...
>
> Xiaochu
>
> On Aug 12, 2016 11:11 AM, "Daniel Berlin" <dberlin at dberlin.org> wrote:
>
>> It's not possible to know that A and B don't alias in this example.  It's
>> almost certainly not profitable to add a runtime check given the size of
>> the loop.
>>
>>
>> try
>>
>> #define SIZE 8
>>
>> void bar(int *restrict A, int* restrict B,int K) {
>>
>>   #pragma clang loop vectorize(enable) vectorize_width(2) unroll_count(8)
>>
>>   for (int i = 0; i < SIZE; ++i)
>>
>>     A[i] += B[i] + K;
>>
>> }
>>
>> (i don't remember if llvm also does runtime alias checks, but if it does,
>> you'd probably need to increase size to get it to vectorize)
>>
>> On Fri, Aug 12, 2016 at 11:08 AM, Xiaochu Liu via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Hi Andrey,
>>>
>>> Thanks. I found even when loop vectorizer and SLP vectorizer are
>>> enabled, my simple test still not get optimized. I also tried clang pragma
>>> in my test to force vectorization. What do you think is the problem?
>>>
>>> Test:
>>>
>>> #define SIZE 8
>>>
>>> void bar(int *A, int* B,int K) {
>>>
>>>   #pragma clang loop vectorize(enable) vectorize_width(2) unroll_count(8)
>>>
>>>   for (int i = 0; i < SIZE; ++i)
>>>
>>>     A[i] += B[i] + K;
>>>
>>> }
>>>
>>> Thanks,
>>> Xiaochu
>>>
>>> On Aug 12, 2016 4:06 AM, "Andrey Bokhanko" <andreybokhanko at gmail.com>
>>> wrote:
>>>
>>>> Hi Xiaochu,
>>>>
>>>> Clang uses -O0 by default, that doesn't run any optimizations. Try
>>>> supplying -O1 or higher.
>>>>
>>>> Yours,
>>>> Andrey
>>>>
>>>>
>>>> On Fri, Aug 12, 2016 at 1:04 AM, Xiaochu Liu via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> Hi there ,
>>>>>
>>>>> I use clang-cl /Qvec test.c to compile the code. But the pass
>>>>> LoopVectorizer is never invoked.
>>>>>
>>>>> I was wondering if this is sufficient to enable auto vectorizer?
>>>>>
>>>>> Thanks,
>>>>> Xiaochu
>>>>>
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>>
>>>>
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>>
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