DAG combine "and|or (select c, -1, 0), x" -> "select c, x, 0|-1"

Allowed folding for "and/or" binops with non-constant operand if
arguments of select are 0/-1 values.

Normally this code with "and" opcode does not get to a DAG combiner
and simplified yet in the InstCombine. However AMDGPU produces it
during lowering and InstCombine has no chance to optimize it out.

In turn the same pattern with "or" opcode can reach DAG.

Differential Revision: https://reviews.llvm.org/D48301

llvm-svn: 335250

1 files changed, 14 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 22d55a1a0ef..443c9144d31 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -1901,8 +1901,19 @@ SDValue DAGCombiner::foldBinOpIntoSelect(SDNode *BO) {
     return SDValue();
 
   // Bail out if any constants are opaque because we can't constant fold those.
+  // The exception is "and" and "or" with either 0 or -1 in which case we can
+  // propagate non constant operands into select. I.e.:
+  // and (select Cond, 0, -1), X --> select Cond, 0, X
+  // or X, (select Cond, -1, 0) --> select Cond, -1, X
+  bool CanFoldNonConst = (BinOpcode == ISD::AND || BinOpcode == ISD::OR) &&
+                         (isNullConstantOrNullSplatConstant(CT) ||
+                          isAllOnesConstantOrAllOnesSplatConstant(CT)) &&
+                         (isNullConstantOrNullSplatConstant(CF) ||
+                          isAllOnesConstantOrAllOnesSplatConstant(CF));
+
   SDValue CBO = BO->getOperand(SelOpNo ^ 1);
-  if (!isConstantOrConstantVector(CBO, true) &&
+  if (!CanFoldNonConst &&
+      !isConstantOrConstantVector(CBO, true) &&
       !isConstantFPBuildVectorOrConstantFP(CBO))
     return SDValue();
 
@@ -1923,14 +1934,14 @@ SDValue DAGCombiner::foldBinOpIntoSelect(SDNode *BO) {
   SDLoc DL(Sel);
   SDValue NewCT = SelOpNo ? DAG.getNode(BinOpcode, DL, VT, CBO, CT)
                           : DAG.getNode(BinOpcode, DL, VT, CT, CBO);
-  if (!NewCT.isUndef() &&
+  if (!CanFoldNonConst && !NewCT.isUndef() &&
       !isConstantOrConstantVector(NewCT, true) &&
       !isConstantFPBuildVectorOrConstantFP(NewCT))
     return SDValue();
 
   SDValue NewCF = SelOpNo ? DAG.getNode(BinOpcode, DL, VT, CBO, CF)
                           : DAG.getNode(BinOpcode, DL, VT, CF, CBO);
-  if (!NewCF.isUndef() &&
+  if (!CanFoldNonConst && !NewCF.isUndef() &&
       !isConstantOrConstantVector(NewCF, true) &&
       !isConstantFPBuildVectorOrConstantFP(NewCF))
     return SDValue();