[Instcombine] Combine extractelement from a vector_extract at index 0 (#151491)

Extracting any element from a subvector starting at index 0 is
equivalent to extracting from the original vector, i.e.
  extract_elt(vector_extract(x, 0), y) -> extract_elt(x, y)

Author: kmclaughlin-arm

llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp

@@ -462,6 +462,13 @@ Instruction *InstCombinerImpl::visitExtractElementInst(ExtractElementInst &EI) {
         return ScalarPHI;
   }
 
+  // If SrcVec is a subvector starting at index 0, extract from the
+  // wider source vector
+  Value *V;
+  if (match(SrcVec,
+            m_Intrinsic<Intrinsic::vector_extract>(m_Value(V), m_Zero())))
+    return ExtractElementInst::Create(V, Index);
+
   // TODO come up with a n-ary matcher that subsumes both unary and
   // binary matchers.
   UnaryOperator *UO;

llvm/test/Transforms/InstCombine/scalable-extract-subvec-elt.ll

@@ -0,0 +1,36 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -passes=instcombine < %s | FileCheck %s
+
+define i1 @extract_const_idx(<vscale x 4 x i1> %a) {
+; CHECK-LABEL: define i1 @extract_const_idx(
+; CHECK-SAME: <vscale x 4 x i1> [[A:%.*]]) {
+; CHECK-NEXT:    [[ELT:%.*]] = extractelement <vscale x 4 x i1> [[A]], i64 1
+; CHECK-NEXT:    ret i1 [[ELT]]
+;
+  %subvec = call <vscale x 2 x i1> @llvm.vector.extract.nxv2i1.nxv4i1.i64(<vscale x 4 x i1> %a, i64 0)
+  %elt = extractelement <vscale x 2 x i1> %subvec, i32 1
+  ret i1 %elt
+}
+
+define float @extract_variable_idx(<vscale x 4 x float> %a, i32 %idx) {
+; CHECK-LABEL: define float @extract_variable_idx(
+; CHECK-SAME: <vscale x 4 x float> [[A:%.*]], i32 [[IDX:%.*]]) {
+; CHECK-NEXT:    [[ELT:%.*]] = extractelement <vscale x 4 x float> [[A]], i32 [[IDX]]
+; CHECK-NEXT:    ret float [[ELT]]
+;
+  %subvec = call <vscale x 2 x float> @llvm.vector.extract.nxv2f32.nxv4f32.i64(<vscale x 4 x float> %a, i64 0)
+  %elt = extractelement <vscale x 2 x float> %subvec, i32 %idx
+  ret float %elt
+}
+
+define float @negative_test(<vscale x 4 x float> %a) {
+; CHECK-LABEL: define float @negative_test(
+; CHECK-SAME: <vscale x 4 x float> [[A:%.*]]) {
+; CHECK-NEXT:    [[SUBVEC:%.*]] = call <vscale x 2 x float> @llvm.vector.extract.nxv2f32.nxv4f32(<vscale x 4 x float> [[A]], i64 2)
+; CHECK-NEXT:    [[ELT:%.*]] = extractelement <vscale x 2 x float> [[SUBVEC]], i64 1
+; CHECK-NEXT:    ret float [[ELT]]
+;
+  %subvec = call <vscale x 2 x float> @llvm.vector.extract.nxv2f32.nxv4f32.i64(<vscale x 4 x float> %a, i64 2)
+  %elt = extractelement <vscale x 2 x float> %subvec, i32 1
+  ret float %elt
+}