DAGCombiner: make sure or/shl/srl really has zero high bits before forming bswap

We want to convert code like (or (srl N, 8), (shl N, 8)) into (srl (bswap N),
const), but this is only valid if the bits above 16 on the source pattern are
0, the checks we were doing on this were slightly wrong before.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189348 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 24 insertions, 6 deletions

diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index bd127b5..47c332a 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2861,6 +2861,14 @@ SDValue DAGCombiner::visitAND(SDNode *N) {
     }
   }
 
+  // fold (and (or (srl N, 8), (shl N, 8)), 0xffff) -> (srl (bswap N), const)
+  if (N1C && N1C->getAPIntValue() == 0xffff && N0.getOpcode() == ISD::OR) {
+    SDValue BSwap = MatchBSwapHWordLow(N0.getNode(), N0.getOperand(0),
+                                       N0.getOperand(1), false);
+    if (BSwap.getNode())
+      return BSwap;
+  }
+
   return SDValue();
 }
 
@@ -2945,13 +2953,23 @@ SDValue DAGCombiner::MatchBSwapHWordLow(SDNode *N, SDValue N0, SDValue N1,
   if (N00 != N10)
     return SDValue();
 
-  // Make sure everything beyond the low halfword is zero since the SRL 16
-  // will clear the top bits.
+  // Make sure everything beyond the low halfword gets set to zero since the SRL
+  // 16 will clear the top bits.
   unsigned OpSizeInBits = VT.getSizeInBits();
-  if (DemandHighBits && OpSizeInBits > 16 &&
-      (!LookPassAnd0 || !LookPassAnd1) &&
-      !DAG.MaskedValueIsZero(N10, APInt::getHighBitsSet(OpSizeInBits, 16)))
-    return SDValue();
+  if (DemandHighBits && OpSizeInBits > 16) {
+    // If the left-shift isn't masked out then the only way this is a bswap is
+    // if all bits beyond the low 8 are 0. In that case the entire pattern
+    // reduces to a left shift anyway: leave it for other parts of the combiner.
+    if (!LookPassAnd0)
+      return SDValue();
+
+    // However, if the right shift isn't masked out then it might be because
+    // it's not needed. See if we can spot that too.
+    if (!LookPassAnd1 &&
+        !DAG.MaskedValueIsZero(
+            N10, APInt::getHighBitsSet(OpSizeInBits, OpSizeInBits - 16)))
+      return SDValue();
+  }
 
   SDValue Res = DAG.getNode(ISD::BSWAP, SDLoc(N), VT, N00);
   if (OpSizeInBits > 16)