[DAGCombiner] (add (umax X, C), -C) --> (usubsat X, C) (PR40111)

Move the (add (umax X, C), -C) --> (usubsat X, C) X86 combine into generic DAGCombiner

First of a number of saturated arithmetic folds that can be moved out of X86-specific code for PR40111.

Differential Revision: https://reviews.llvm.org/D57754

llvm-svn: 353457

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index dabb8afd104..a9904a25783 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2159,6 +2159,18 @@ SDValue DAGCombiner::visitADD(SDNode *N) {
                          DAG.getNode(ISD::ADD, SDLoc(N1), VT, N01, N11));
   }
 
+  // fold (add (umax X, C), -C) --> (usubsat X, C)
+  if (N0.getOpcode() == ISD::UMAX && hasOperation(ISD::USUBSAT, VT)) {
+    auto MatchUSUBSAT = [](ConstantSDNode *Max, ConstantSDNode *Op) {
+      return (!Max && !Op) ||
+             (Max && Op && Max->getAPIntValue() == (-Op->getAPIntValue()));
+    };
+    if (ISD::matchBinaryPredicate(N0.getOperand(1), N1, MatchUSUBSAT,
+                                  /*AllowUndefs*/ true))
+      return DAG.getNode(ISD::USUBSAT, DL, VT, N0.getOperand(0),
+                         N0.getOperand(1));
+  }
+
   if (SDValue V = foldAddSubBoolOfMaskedVal(N, DAG))
     return V;