[SelectionDAG] Swap commutative binops before constant-based folding

In searching for a fix for the underlying code-quality bug highlighted by
r246937 (that SDAG simplification can lead to us generating an ISD::OR node
with a constant zero LHS), I ran across this:

We generically canonicalize commutative binary-operation nodes in SDAG getNode
so that, if only one operand is a constant, it will be on the RHS.  However, we
were doing this only after a bunch of constant-based simplification checks that
all assume this canonical form (that any constant will be on the RHS). Moving
the operand-swapping canonicalization prior to these checks seems like the
right thing to do (and, as it turns out, causes SDAG to completely fold away the
computation in test/CodeGen/ARM/2012-11-14-subs_carry.ll, just like InstCombine
would do).

llvm-svn: 246938

1 files changed, 7 insertions, 6 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 7ccaaf72c37..3acf749bf3e 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -3348,6 +3348,13 @@ SDValue SelectionDAG::getNode(unsigned Opcode, SDLoc DL, EVT VT, SDValue N1,
                               SDValue N2, const SDNodeFlags *Flags) {
   ConstantSDNode *N1C = dyn_cast<ConstantSDNode>(N1);
   ConstantSDNode *N2C = dyn_cast<ConstantSDNode>(N2);
+
+  // Canonicalize constant to RHS if commutative.
+  if (N1C && !N2C && isCommutativeBinOp(Opcode)) {
+    std::swap(N1C, N2C);
+    std::swap(N1, N2);
+  }
+
   switch (Opcode) {
   default: break;
   case ISD::TokenFactor:
@@ -3692,12 +3699,6 @@ SDValue SelectionDAG::getNode(unsigned Opcode, SDLoc DL, EVT VT, SDValue N1,
           FoldConstantArithmetic(Opcode, DL, VT, N1.getNode(), N2.getNode()))
     return SV;
 
-  // Canonicalize constant to RHS if commutative.
-  if (N1C && !N2C && isCommutativeBinOp(Opcode)) {
-    std::swap(N1C, N2C);
-    std::swap(N1, N2);
-  }
-
   // Constant fold FP operations.
   bool HasFPExceptions = TLI->hasFloatingPointExceptions();
   ConstantFPSDNode *N1CFP = dyn_cast<ConstantFPSDNode>(N1);