[llvm] 30463bc - [SLP]Do not count perfect diamond matches for gathers several times.

Alexander Kornienko via llvm-commits llvm-commits at lists.llvm.org
Thu May 20 09:03:06 PDT 2021


We see performance regressions after this patch. A number of benchmarks
regressed for more than 10%. One example is the flops-6.c from the LLVM
test-suite. An isolated test based on that benchmark:

$ cat flops-6.c
extern int printf (const char *__restrict __format, ...);
double T[36];
double sa,sb,sc,sd,one,two;
double four,piref;
double scale;
double A1 = -0.1666666666671334;
double A2 = 0.833333333809067E-2;
double A3 = 0.198412715551283E-3;
double A4 = 0.27557589750762E-5;
double A5 = 0.2507059876207E-7;
double A6 = 0.164105986683E-9;
double B1 = -0.4999999999982;
double B2 = 0.4166666664651E-1;
double B3 = -0.1388888805755E-2;
double B4 = 0.24801428034E-4;
double B5 = -0.2754213324E-6;
double B6 = 0.20189405E-8;
int main()
{
   double s,u,v,w,x;
   long loops;
   register long i, m, n;
   printf("\n");
   printf("   FLOPS C Program (Double Precision), V2.0 18 Dec 1992\n\n");
   loops = 15625;
   piref = 3.14159265358979324;
   one = 1.0;
   two = 2.0;
   four = 4.0;
   scale = one;
   printf("   Module     Error        RunTime      MFLOPS\n");
   printf("                            (usec)\n");
   m = loops*10000;
   x = piref / ( four * (double)m );
   s = 0.0;
   v = 0.0;
   for( i = 1 ; i <= m-1 ; i++ )
   {
   u = (double)i * x;
   w = u * u;
   v = u * ((((((A6*w+A5)*w+A4)*w+A3)*w+A2)*w+A1)*w+one);
   s = s + v*(w*(w*(w*(w*(w*(B6*w+B5)+B4)+B3)+B2)+B1)+one);
   }
   u = piref / four;
   w = u * u;
   sa = u*((((((A6*w+A5)*w+A4)*w+A3)*w+A2)*w+A1)*w+one);
   sb = w*(w*(w*(w*(w*(B6*w+B5)+B4)+B3)+B2)+B1)+one;
   sa = sa * sb;
   sa = x * ( sa + two * s ) / two;
   sb = 0.25;
   sc = sa - sb;
   printf("     6   %13.4lf  %10.4lf  %10.4lf\n",
          sc* 1e-30,
          0* 1e-30 ,
          0* 1e-30);
   return 0;
}
$ clang-base -O3 -maes -m64 -mcx16 -msse4.2 -mpclmul
'-mprefer-vector-width=128' flops-6.c -o flops-6-base
$ clang-new -O3 -maes -m64 -mcx16 -msse4.2 -mpclmul
'-mprefer-vector-width=128' flops-6.c -o flops-6-new
$ for i in $(seq 5) ; do time ./flops-6-base ; done
     6          0.0000      0.0000      0.0000

real    0m0.705s
user    0m0.700s
sys     0m0.004s
     6          0.0000      0.0000      0.0000

real    0m0.706s
user    0m0.704s
sys     0m0.001s
     6          0.0000      0.0000      0.0000

real    0m0.706s
user    0m0.705s
sys     0m0.001s
     6          0.0000      0.0000      0.0000

real    0m0.706s
user    0m0.704s
sys     0m0.001s
     6          0.0000      0.0000      0.0000

real    0m0.707s
user    0m0.705s
sys     0m0.001s
$ for i in $(seq 5) ; do time ./flops-6-new ; done
     6          0.0000      0.0000      0.0000

real    0m0.899s
user    0m0.898s
sys     0m0.000s
     6          0.0000      0.0000      0.0000

real    0m0.899s
user    0m0.898s
sys     0m0.000s
     6          0.0000      0.0000      0.0000

real    0m0.900s
user    0m0.899s
sys     0m0.000s
     6          0.0000      0.0000      0.0000

real    0m0.899s
user    0m0.898s
sys     0m0.000s
     6          0.0000      0.0000      0.0000

real    0m0.899s
user    0m0.898s
sys     0m0.000s

Can you take a look at this and maybe revert in the meantime?

Thanks!

-- Alex

On Mon, May 10, 2021 at 4:10 PM Alexey Bataev via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

>
> Author: Alexey Bataev
> Date: 2021-05-10T07:08:07-07:00
> New Revision: 30463bc3f1839e8a238be4c137e2356f3cca2771
>
> URL:
> https://github.com/llvm/llvm-project/commit/30463bc3f1839e8a238be4c137e2356f3cca2771
> DIFF:
> https://github.com/llvm/llvm-project/commit/30463bc3f1839e8a238be4c137e2356f3cca2771.diff
>
> LOG: [SLP]Do not count perfect diamond matches for gathers several times.
>
> Need to remove the old code for avoiding double counting of the gather
> nodes with perfect diamond matches within the tree after we started
> detecting perfect/shuffled matching in the previous patch D100495. We
> may skip the cost for such nodes completely.
>
> Differential Revision: https://reviews.llvm.org/D102023
>
> Added:
>
>
> Modified:
>     llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
>     llvm/test/Transforms/SLPVectorizer/AArch64/gather-cost.ll
>
> Removed:
>
>
>
>
> ################################################################################
> diff  --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
> b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
> index 22e090fd1d7c..e656b189c779 100644
> --- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
> +++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
> @@ -4233,27 +4233,6 @@ InstructionCost BoUpSLP::getTreeCost() {
>    for (unsigned I = 0, E = VectorizableTree.size(); I < E; ++I) {
>      TreeEntry &TE = *VectorizableTree[I].get();
>
> -    // We create duplicate tree entries for gather sequences that have
> multiple
> -    // uses. However, we should not compute the cost of duplicate
> sequences.
> -    // For example, if we have a build vector (i.e., insertelement
> sequence)
> -    // that is used by more than one vector instruction, we only need to
> -    // compute the cost of the insertelement instructions once. The
> redundant
> -    // instructions will be eliminated by CSE.
> -    //
> -    // We should consider not creating duplicate tree entries for gather
> -    // sequences, and instead add additional edges to the tree
> representing
> -    // their uses. Since such an approach results in fewer total entries,
> -    // existing heuristics based on tree size may yield
> diff erent results.
> -    //
> -    if (TE.State == TreeEntry::NeedToGather &&
> -        std::any_of(std::next(VectorizableTree.begin(), I + 1),
> -                    VectorizableTree.end(),
> -                    [TE](const std::unique_ptr<TreeEntry> &EntryPtr) {
> -                      return EntryPtr->State == TreeEntry::NeedToGather &&
> -                             EntryPtr->isSame(TE.Scalars);
> -                    }))
> -      continue;
> -
>      InstructionCost C = getEntryCost(&TE);
>      Cost += C;
>      LLVM_DEBUG(dbgs() << "SLP: Adding cost " << C
>
> diff  --git a/llvm/test/Transforms/SLPVectorizer/AArch64/gather-cost.ll
> b/llvm/test/Transforms/SLPVectorizer/AArch64/gather-cost.ll
> index 31c63d31f4df..57db62ace206 100644
> --- a/llvm/test/Transforms/SLPVectorizer/AArch64/gather-cost.ll
> +++ b/llvm/test/Transforms/SLPVectorizer/AArch64/gather-cost.ll
> @@ -10,7 +10,7 @@ target triple = "aarch64--linux-gnu"
>  ; REMARK-LABEL: Function: gather_multiple_use
>  ; REMARK:       Args:
>  ; REMARK-NEXT:    - String: 'Vectorized horizontal reduction with cost '
> -; REMARK-NEXT:    - Cost: '-16'
> +; REMARK-NEXT:    - Cost: '-7'
>  ;
>  ; REMARK-NOT: Function: gather_load
>
>
>
>
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