[SelectionDAG] Add fcmp UNDEF handling to SelectionDAG::FoldSetCC

Second half of PR40800, this patch adds DAG undef handling to fcmp instructions to match the behavior in llvm::ConstantFoldCompareInstruction, this permits constant folding of vector comparisons where some elements had been reduced to UNDEF (by SimplifyDemandedVectorElts etc.).

This involves a lot of tweaking to reduced tests as bugpoint loves to reduce fcmp arguments to undef........

Differential Revision: https://reviews.llvm.org/D60006

llvm-svn: 357765

1 files changed, 8 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 148a3805b3b..caa3f14b8a1 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2085,14 +2085,19 @@ SDValue SelectionDAG::FoldSetCC(EVT VT, SDValue N1, SDValue N2,
     case ISD::SETUGE: return getBoolConstant(R!=APFloat::cmpLessThan, dl, VT,
                                              OpVT);
     }
-  } else if (N1CFP && OpVT.isSimple()) {
+  } else if (N1CFP && OpVT.isSimple() && !N2.isUndef()) {
     // Ensure that the constant occurs on the RHS.
     ISD::CondCode SwappedCond = ISD::getSetCCSwappedOperands(Cond);
     if (!TLI->isCondCodeLegal(SwappedCond, OpVT.getSimpleVT()))
       return SDValue();
     return getSetCC(dl, VT, N2, N1, SwappedCond);
-  } else if (N2CFP && N2CFP->getValueAPF().isNaN()) {
-    // If an operand is known to be a nan, we can fold it.
+  } else if ((N2CFP && N2CFP->getValueAPF().isNaN()) ||
+             (OpVT.isFloatingPoint() && (N1.isUndef() || N2.isUndef()))) {
+    // If an operand is known to be a nan (or undef that could be a nan), we can
+    // fold it.
+    // Choosing NaN for the undef will always make unordered comparison succeed
+    // and ordered comparison fails.
+    // Matches behavior in llvm::ConstantFoldCompareInstruction.
     switch (ISD::getUnorderedFlavor(Cond)) {
     default:
       llvm_unreachable("Unknown flavor!");