[llvm] [InstCombine] Transform `vector.reduce.add` and `splat` into multiplication (PR #161020)

[Hongyu Chen via llvm-commits]

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@@ -3761,6 +3762,26 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
             return replaceInstUsesWith(CI, Res);
           }
       }
+
+      // Handle the case where a splat is summarized. In that case we have a
+      // multpilication. For example: %2 = insertelement <4 x i32> poison, i32
+      // %0, i64 0 %3 = shufflevector <4 x i32> %2, poison, <4 x i32>
+      // zeroinitializer %4 = tail call i32 @llvm.vector.reduce.add.v4i32(%3)
+      // =>
+      // %2 = mul i32 %0, 4
+      if (Value *Splat = getSplatValue(Arg)) {
+        // It is only a multiplication if we add the same element over and over.
+        assert(Arg->getType()->isVectorTy() &&
+               "The vector.reduce.add intrinsic's argument must be a vector!");
----------------
XChy wrote:

```suggestion
```
Remove the assertion. `cast<VectorType>` will assert itself.

https://github.com/llvm/llvm-project/pull/161020