[InstSimplify] Eliminate vector reverse of a splat vector

[mnadeem]

Summary

experimental.vector.reverse(splat(X)) -> splat(X)

[mnadeem]

llvm/lib/Analysis/InstructionSimplify.cpp

5477

I'm planning to add ptrue(31) to this function in another patch.

[david-arm]

LGTM!

llvm/lib/Analysis/InstructionSimplify.cpp

if (match(Op0, m_SIToFP(m_Value())) || match(Op0, m_UIToFP(m_Value())))
       return Op0;
     break;
   }
   case Intrinsic::experimental_vector_reverse:
     // experimental.vector.reverse(experimental.vector.reverse(x)) -> x
     if (match(Op0,
               m_Intrinsic<Intrinsic::experimental_vector_reverse>(m_Value(X))))
       return X;
+    // experimental.vector.reverse(splat(X)) -> splat(X)
+    if (isSplatValue(Op0))
+      return Op0;
     break;
   default:
     break;
   }
 
   return nullptr;
 }
 

llvm/test/Transforms/InstSimplify/named-vector-shuffle-reverse.ll

 ; RUN: opt  -instsimplify -S < %s | FileCheck %s
 
 ; Test back to back reverse shuffles are eliminated.
 define <vscale x 4 x i32> @shuffle_b2b_reverse(<vscale x 4 x i32> %a) {
 ; CHECK-LABEL: @shuffle_b2b_reverse(
 ; CHECK: ret <vscale x 4 x i32> %a
   %rev = tail call <vscale x 4 x i32> @llvm.experimental.vector.reverse.nxv4i32(<vscale x 4 x i32> %a)
   %rev.rev = tail call <vscale x 4 x i32> @llvm.experimental.vector.reverse.nxv4i32(<vscale x 4 x i32> %rev)
   ret <vscale x 4 x i32> %rev.rev
 }
 
+; Test reverse of a splat is eliminated.
+define <vscale x 4 x i32> @splat_reverse(i32 %a) {
+; CHECK-LABEL: @splat_reverse(
+; CHECK-NEXT:    [[SPLAT_INSERT:%.*]] = insertelement <vscale x 4 x i32> poison, i32 [[A:%.*]], i32 0
+; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <vscale x 4 x i32> [[SPLAT_INSERT]], <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
+; CHECK-NEXT:    ret <vscale x 4 x i32> [[SPLAT]]
+;
+  %splat_insert = insertelement <vscale x 4 x i32> poison, i32 %a, i32 0
+  %splat = shufflevector <vscale x 4 x i32> %splat_insert, <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
+  %rev = tail call <vscale x 4 x i32> @llvm.experimental.vector.reverse.nxv4i32(<vscale x 4 x i32> %splat)
+  ret <vscale x 4 x i32> %rev
+}
+
 declare <vscale x 4 x i32> @llvm.experimental.vector.reverse.nxv4i32(<vscale x 4 x i32>)