[DAGCombiner] Masked merge: enhance handling of 'andn' with immediates

Summary:
Split off from D46031.

The previous patch, D46493, completely disabled unfolding in case of immediates.
But we can do better:
{F6120274} {F6120277}

https://rise4fun.com/Alive/xJS

Reviewers: spatel, craig.topper

Reviewed By: spatel

Subscribers: andreadb, llvm-commits

Differential Revision: https://reviews.llvm.org/D46494

llvm-svn: 331685

1 files changed, 14 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index b809bbc1c6c..863fcf1be0b 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -5372,6 +5372,9 @@ SDValue DAGCombiner::MatchLoadCombine(SDNode *N) {
 //    |  D  |
 // Into:
 //   (x & m) | (y & ~m)
+// If y is a constant, and the 'andn' does not work with immediates,
+// we unfold into a different pattern:
+//   ~(~x & m) & (m | y)
 // NOTE: we don't unfold the pattern if 'xor' is actually a 'not', because at
 //       the very least that breaks andnpd / andnps patterns, and because those
 //       patterns are simplified in IR and shouldn't be created in the DAG
@@ -5428,12 +5431,19 @@ SDValue DAGCombiner::unfoldMaskedMerge(SDNode *N) {
   if (!TLI.hasAndNot(M))
     return SDValue();
 
-  // If Y is a constant, check that 'andn' works with immediates.
-  if (!TLI.hasAndNot(Y))
-    return SDValue();
-
   SDLoc DL(N);
 
+  // If Y is a constant, check that 'andn' works with immediates.
+  if (!TLI.hasAndNot(Y)) {
+    assert(TLI.hasAndNot(X) && "Only mask is a variable? Unreachable.");
+    // If not, we need to do a bit more work to make sure andn is still used.
+    SDValue NotX = DAG.getNOT(DL, X, VT);
+    SDValue LHS = DAG.getNode(ISD::AND, DL, VT, NotX, M);
+    SDValue NotLHS = DAG.getNOT(DL, LHS, VT);
+    SDValue RHS = DAG.getNode(ISD::OR, DL, VT, M, Y);
+    return DAG.getNode(ISD::AND, DL, VT, NotLHS, RHS);
+  }
+
   SDValue LHS = DAG.getNode(ISD::AND, DL, VT, X, M);
   SDValue NotM = DAG.getNOT(DL, M, VT);
   SDValue RHS = DAG.getNode(ISD::AND, DL, VT, Y, NotM);