add a minor dag combine noticed when looking at PR945
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@30915 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 2c15f39..70ff837 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -1664,6 +1664,13 @@
return DAG.getNode(ISD::ANY_EXTEND, VT, SmallShift);
}
+ // fold (srl (sra X, Y), 31) -> (srl X, 31). This srl only looks at the sign
+ // bit, which is unmodified by sra.
+ if (N1C && N1C->getValue()+1 == MVT::getSizeInBits(VT)) {
+ if (N0.getOpcode() == ISD::SRA)
+ return DAG.getNode(ISD::SRL, VT, N0.getOperand(0), N1);
+ }
+
// fold (srl (ctlz x), "5") -> x iff x has one bit set (the low bit).
if (N1C && N0.getOpcode() == ISD::CTLZ &&
N1C->getValue() == Log2_32(MVT::getSizeInBits(VT))) {