[PATCH] D98714: [SLP] Add insertelement instructions to vectorizable tree

[Douglas Yung via Phabricator via llvm-commits]

dyung added a comment.

In D98714#2764828 <https://reviews.llvm.org/D98714#2764828>, @ABataev wrote:

> In D98714#2764805 <https://reviews.llvm.org/D98714#2764805>, @dyung wrote:
>
>> Hi, this change has caused a regression in the codegen for one of our internal tests. Consider the following code:
>>
>>   __attribute__((noinline))
>>   __m256d add_pd_002(__m256d a, __m256d b) {
>>     __m256d r = (__m256d){ a[0] + a[1], a[2] + a[3], b[0] + b[1], b[2] + b[3] };
>>     return __builtin_shufflevector(r, a, 0, -1, 2, 3);
>>   }
>>
>> If you compile this with "-g0 -O3 -march=btver2", prior to your commit the compiler would generate the following code for the function:
>>
>>   # %bb.0:                                # %entry
>>           vinsertf128     $1, %xmm1, %ymm0, %ymm0
>>           vhaddpd %ymm1, %ymm0, %ymm0
>>
>> But after your change it is now generating the following code:
>>
>>   # %bb.0:                                # %entry
>>           vextractf128    $1, %ymm1, %xmm2
>>           vhaddpd %xmm0, %xmm0, %xmm0
>>           vhaddpd %xmm2, %xmm1, %xmm1
>>           vperm2f128      $2, %ymm0, %ymm1, %ymm0 # ymm0 = ymm0[0,1],ymm1[0,1]
>>
>> From your commit description, it sounds like this is expected and will be fixed in a follow-up commit. Is my understanding of this correct?
>
> Hi, could you check if D101555 <https://reviews.llvm.org/D101555> fixes this issue?

Hi, I applied the patch locally and built the compiler, but the generated assembly actually seems it might be worse:

  # %bb.0:                                # %entry
          vinsertf128     $1, %xmm0, %ymm1, %ymm2
          vinsertf128     $1, %xmm1, %ymm0, %ymm0
          vextractf128    $1, %ymm1, %xmm1
          vhaddpd %ymm2, %ymm0, %ymm0
          vhaddpd %xmm1, %xmm1, %xmm1
          vextractf128    $1, %ymm0, %xmm2
          vunpcklpd       %xmm1, %xmm2, %xmm1     # xmm1 = xmm2[0],xmm1[0]
          vinsertf128     $1, %xmm1, %ymm0, %ymm0
          retq


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