[llvm] [InstCombine] Fold vector.reduce.op(vector.reverse(X)) -> vector.reduce.op(X) (PR #91743)

[via llvm-commits]

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@@ -3394,8 +3436,31 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
                                 : 0;
     Value *Arg = II->getArgOperand(ArgIdx);
     Value *V;
+
+    if (!CanBeReassociated)
+      break;
+
+    if (match(Arg, m_VecReverse(m_Value(V)))) {
+      Value *Res;
+      switch (IID) {
+      case Intrinsic::vector_reduce_fadd:
+        Res = Builder.CreateFAddReduce(II->getArgOperand(0), V);
+        break;
+      case Intrinsic::vector_reduce_fmul:
+        Res = Builder.CreateFMulReduce(II->getArgOperand(0), V);
+        break;
+      case Intrinsic::vector_reduce_fmin:
+        Res = Builder.CreateFPMinReduce(V);
+        break;
+      case Intrinsic::vector_reduce_fmax:
+        Res = Builder.CreateFPMaxReduce(V);
+        break;
+      }
+      return replaceInstUsesWith(CI, Res);
+    }
+
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goldsteinn wrote:

I think a general easier way to implement this would be create a helper in `InstuctionCombiner`. The code for all them can simply be `replaceOperand` of the reduce with the pre-reversed vec. That will also make it easier to expand (for example any shuffle that preserves all elements).

https://github.com/llvm/llvm-project/pull/91743