Add the missing sub identity "A-(A-B) -> B" to DAGCombine.

This happens e.g. for code like "X - X%10" where we lower the modulo operation
to a series of multiplies and shifts that are then subtracted from X, leading to
this missed optimization.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@124532 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 186f4afc3d015bcb37fab2aeef2aaabfc2bd9805..a5b2d9594d5ccc4e153d375f5d16c2015953745e 100644 (file)
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -1542,6 +1542,9 @@ SDValue DAGCombiner::visitSUB(SDNode *N) {
   // Canonicalize (sub -1, x) -> ~x, i.e. (xor x, -1)
   if (N0C && N0C->isAllOnesValue())
     return DAG.getNode(ISD::XOR, N->getDebugLoc(), VT, N1, N0);
+  // fold A-(A-B) -> B
+  if (N1.getOpcode() == ISD::SUB && N0 == N1.getOperand(0))
+    return N1.getOperand(1);
   // fold (A+B)-A -> B
   if (N0.getOpcode() == ISD::ADD && N0.getOperand(0) == N1)
     return N0.getOperand(1);