[PATCH] Fold EXTRACT_VECTOR_ELT(BUILD_VECTOR(Elt[0], ...), CstX ) -> Elt[CstX]

[Mehdi AMINI]

Thanks for the review. I'll wait for you to have time to fix the ARM64 failure ;)


================
Comment at: lib/CodeGen/SelectionDAG/DAGCombiner.cpp:11004
@@ +11003,3 @@
+  // Fold EXTRACT_VECTOR_ELT(BUILD_VECTOR(Elt[0], ...), CstX ) -> Elt[CstX]
+  if (InVec.getOpcode() == ISD::BUILD_VECTOR && ConstEltNo) {
+    auto Elt = InVec.getOperand(N->getConstantOperandVal(1));
----------------
ab wrote:
> Would it make sense to check that the BUILD_VECTOR .hasOneUse()?  
> 
Good question, I can imagine cases where I don't want to this check but I'm not sure I see a lot of cases where it would be beneficial?

================
Comment at: lib/CodeGen/SelectionDAG/DAGCombiner.cpp:11010
@@ +11009,3 @@
+    // when folding this case.
+    if(Elt.getValueType() == NVT)
+      return Elt;
----------------
ab wrote:
> space before (
grahh clang-format :(

================
Comment at: test/CodeGen/R600/ds_read2.ll:220-221
@@ -223,1 +219,4 @@
+; SI: ds_read2_b32
+; SI-NOT: ds_read_b32
+; SI-NOT: ds_read_b32
 ; SI: s_endpgm
----------------
ab wrote:
> A single -NOT is enough
Of course! Thanks.

http://reviews.llvm.org/D9094

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