[PATCH] D107254: [VectorUtils] Teach findScalarElement to return splat value.

[Sander de Smalen via Phabricator via llvm-commits]

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If the vector is a splat of some scalar value, findScalarElement()
can simply return the scalar value if it knows the requested lane
is in the vector.

This is only needed for scalable vectors, because the InsertElement/ShuffleVector
case is already handled explicitly for the fixed-width case.

This helps to recognize an InstCombine fold like:

  extractelt(bitcast(splat(%v))) -> bitcast(%v)


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D107254

Files:
  llvm/lib/Analysis/VectorUtils.cpp
  llvm/test/Transforms/InstCombine/vscale_extractelement.ll


Index: llvm/test/Transforms/InstCombine/vscale_extractelement.ll
===================================================================
--- llvm/test/Transforms/InstCombine/vscale_extractelement.ll
+++ llvm/test/Transforms/InstCombine/vscale_extractelement.ll
@@ -243,6 +243,21 @@
   ret i8 %1
 }
 
+define i64* @ext_lane_from_bitcast_of_splat(i32* %v) {
+; CHECK-LABEL: @ext_lane_from_bitcast_of_splat(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[R:%.*]] = bitcast i32* [[V:%.*]] to i64*
+; CHECK-NEXT:    ret i64* [[R]]
+;
+entry:
+  %in = insertelement <vscale x 4 x i32*> undef, i32* %v, i32 0
+  %splat = shufflevector <vscale x 4 x i32*> %in, <vscale x 4 x i32*> undef, <vscale x 4 x i32> zeroinitializer
+  %bc = bitcast <vscale x 4 x i32*> %splat to <vscale x 4 x i64*>
+  %r = extractelement <vscale x 4 x i64*> %bc, i32 3
+  ret i64* %r
+}
+
+
 declare <vscale x 4 x i64> @llvm.experimental.stepvector.nxv4i64()
 declare <vscale x 4 x i32> @llvm.experimental.stepvector.nxv4i32()
 declare <vscale x 512 x i8> @llvm.experimental.stepvector.nxv512i8()
Index: llvm/lib/Analysis/VectorUtils.cpp
===================================================================
--- llvm/lib/Analysis/VectorUtils.cpp
+++ llvm/lib/Analysis/VectorUtils.cpp
@@ -323,6 +323,12 @@
     return findScalarElement(SVI->getOperand(1), InEl - LHSWidth);
   }
 
+  // If the vector is a splat then we can trivially find the scalar element.
+  if (isa<ScalableVectorType>(VTy))
+    if (Value *Splat = getSplatValue(V))
+      if (EltNo < VTy->getElementCount().getKnownMinValue())
+        return Splat;
+
   // Extract a value from a vector add operation with a constant zero.
   // TODO: Use getBinOpIdentity() to generalize this.
   Value *Val; Constant *C;


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