[PATCH] [DAGCombiner] Fold (vextract v, (sext (trunc i))) -> (vextract v, i)

[Michael Spencer]

Hi grosbach, nadav,

EXTRACT_VECTOR_ELT has undefined behavior for element indices greater than
the input vector width.

http://reviews.llvm.org/D3494

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/X86/vec_splat.ll

Index: lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -9691,6 +9691,16 @@
   SDValue EltNo = N->getOperand(1);
   bool ConstEltNo = isa<ConstantSDNode>(EltNo);
 
+  // (vextract v, (sext (trunc i))) -> (vextract v, i)
+  if (EltNo.getOpcode() == ISD::SIGN_EXTEND &&
+      EltNo.getOperand(0).getOpcode() == ISD::TRUNCATE) {
+    SDValue Trunc = EltNo.getOperand(0);
+    if (Trunc.getValueType().getSizeInBits() >=
+        Log2_64_Ceil(VT.getVectorNumElements()))
+      return DAG.getNode(ISD::EXTRACT_VECTOR_ELT, SDLoc(N), N->getValueType(0),
+                         InVec, Trunc.getOperand(0));
+  }
+
   // Transform: (EXTRACT_VECTOR_ELT( VECTOR_SHUFFLE )) -> EXTRACT_VECTOR_ELT.
   // We only perform this optimization before the op legalization phase because
   // we may introduce new vector instructions which are not backed by TD
Index: test/CodeGen/X86/vec_splat.ll
===================================================================
--- test/CodeGen/X86/vec_splat.ll
+++ test/CodeGen/X86/vec_splat.ll
@@ -48,5 +48,6 @@
   
 ; AVX-LABEL: load_extract_splat
 ; AVX-NOT: rsp
+; AVX-NOT: mov
 ; AVX: vbroadcastss
 }
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