[DAG] recognize div/rem by 0 as undef before trying constant folding

As discussed in the review thread for rL297026, this is actually 2 changes that 
would independently fix all of the test cases in the patch:

1. Return undef in FoldConstantArithmetic for div/rem by 0.
2. Move basic undef simplifications for div/rem (simplifyDivRem()) before 
   foldBinopIntoSelect() as a matter of efficiency.

I will handle the case of vectors with any zero element as a follow-up. That change
is the DAG sibling for D30665 + adding a check of vector elements to FoldConstantVectorArithmetic().

I'm deleting the test for PR30693 because it does not test for the actual bug any more
(dangers of using bugpoint).

Differential Revision:
https://reviews.llvm.org/D30741

llvm-svn: 297384

1 files changed, 11 insertions, 11 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index ba8f12e8d31..046c9e9520c 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -1717,12 +1717,12 @@ SDValue DAGCombiner::foldBinOpIntoSelect(SDNode *BO) {
   EVT VT = Sel.getValueType();
   SDLoc DL(Sel);
   SDValue NewCT = DAG.getNode(BinOpcode, DL, VT, CT, C1);
-  assert((isConstantOrConstantVector(NewCT) ||
+  assert((NewCT.isUndef() || isConstantOrConstantVector(NewCT) ||
           isConstantFPBuildVectorOrConstantFP(NewCT)) &&
          "Failed to constant fold a binop with constant operands");
 
   SDValue NewCF = DAG.getNode(BinOpcode, DL, VT, CF, C1);
-  assert((isConstantOrConstantVector(NewCF) ||
+  assert((NewCF.isUndef() || isConstantOrConstantVector(NewCF) ||
           isConstantFPBuildVectorOrConstantFP(NewCF)) &&
          "Failed to constant fold a binop with constant operands");
 
@@ -2417,6 +2417,9 @@ SDValue DAGCombiner::visitSDIV(SDNode *N) {
   if (N1C && N1C->isAllOnesValue())
     return DAG.getNode(ISD::SUB, DL, VT, DAG.getConstant(0, DL, VT), N0);
 
+  if (SDValue V = simplifyDivRem(N, DAG))
+    return V;
+
   if (SDValue NewSel = foldBinOpIntoSelect(N))
     return NewSel;
 
@@ -2482,9 +2485,6 @@ SDValue DAGCombiner::visitSDIV(SDNode *N) {
     if (SDValue DivRem = useDivRem(N))
         return DivRem;
 
-  if (SDValue V = simplifyDivRem(N, DAG))
-    return V;
-
   return SDValue();
 }
 
@@ -2508,6 +2508,9 @@ SDValue DAGCombiner::visitUDIV(SDNode *N) {
                                                     N0C, N1C))
       return Folded;
 
+  if (SDValue V = simplifyDivRem(N, DAG))
+    return V;
+
   if (SDValue NewSel = foldBinOpIntoSelect(N))
     return NewSel;
 
@@ -2553,9 +2556,6 @@ SDValue DAGCombiner::visitUDIV(SDNode *N) {
     if (SDValue DivRem = useDivRem(N))
         return DivRem;
 
-  if (SDValue V = simplifyDivRem(N, DAG))
-    return V;
-
   return SDValue();
 }
 
@@ -2575,6 +2575,9 @@ SDValue DAGCombiner::visitREM(SDNode *N) {
     if (SDValue Folded = DAG.FoldConstantArithmetic(Opcode, DL, VT, N0C, N1C))
       return Folded;
 
+  if (SDValue V = simplifyDivRem(N, DAG))
+    return V;
+
   if (SDValue NewSel = foldBinOpIntoSelect(N))
     return NewSel;
 
@@ -2629,9 +2632,6 @@ SDValue DAGCombiner::visitREM(SDNode *N) {
   if (SDValue DivRem = useDivRem(N))
     return DivRem.getValue(1);
 
-  if (SDValue V = simplifyDivRem(N, DAG))
-    return V;
-
   return SDValue();
 }