[llvm-dev] Invoke loop vectorizer

[Daniel Berlin via llvm-dev]

Errr, so you are using clang-cl but not on x86 or x86-64?
That's probably "not well tested"

On Fri, Aug 12, 2016 at 12:20 PM, Xiaochu Liu <xiaochu1122 at gmail.com> wrote:

> I'm not compiling it to x86. Should loop optimizer something independent
> of the target? If so, should the vectorized code on IR level?
>
> On Aug 12, 2016 11:39 AM, "Daniel Berlin" <dberlin at dberlin.org> wrote:
>
>> 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
>>>>>>>
>>>>>>>
>>>>>>
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>>
>>>>
>>
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