[DAGCombine] Check the uses of negated floating constant and remove the hack

PowerPC hits an assertion due to somewhat the same reason as https://reviews.llvm.org/D70975.
Though there are already some hack, it still failed with some case, when the operand 0 is NOT
a const fp, it is another fma that with const fp. And that const fp is negated which result in multi-uses.

A better fix is to check the uses of the negated const fp. If there are already use of its negated
value, we will have benefit as no extra Node is added.

Differential revision: https://reviews.llvm.org/D75501

(cherry picked from commit 3906ae387f0775dfe4426e4336748269fafbd190)

1 files changed, 15 insertions, 11 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 24ab65171a1..368e2100031 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -5490,9 +5490,20 @@ char TargetLowering::isNegatibleForFree(SDValue Op, SelectionDAG &DAG,
   EVT VT = Op.getValueType();
   const SDNodeFlags Flags = Op->getFlags();
   const TargetOptions &Options = DAG.getTarget().Options;
-  if (!Op.hasOneUse() && !(Op.getOpcode() == ISD::FP_EXTEND &&
-                           isFPExtFree(VT, Op.getOperand(0).getValueType())))
-    return 0;
+  if (!Op.hasOneUse()) {
+    bool IsFreeExtend = Op.getOpcode() == ISD::FP_EXTEND &&
+                        isFPExtFree(VT, Op.getOperand(0).getValueType());
+
+    // If we already have the use of the negated floating constant, it is free
+    // to negate it even it has multiple uses.
+    bool IsFreeConstant =
+        Op.getOpcode() == ISD::ConstantFP &&
+        !getNegatedExpression(Op, DAG, LegalOperations, ForCodeSize)
+             .use_empty();
+
+    if (!IsFreeExtend && !IsFreeConstant)
+      return 0;
+  }
 
   // Don't recurse exponentially.
   if (Depth > SelectionDAG::MaxRecursionDepth)
@@ -5687,14 +5698,7 @@ SDValue TargetLowering::getNegatedExpression(SDValue Op, SelectionDAG &DAG,
                                  ForCodeSize, Depth + 1);
     char V1 = isNegatibleForFree(Op.getOperand(1), DAG, LegalOperations,
                                  ForCodeSize, Depth + 1);
-    // TODO: This is a hack. It is possible that costs have changed between now
-    //       and the initial calls to isNegatibleForFree(). That is because we
-    //       are rewriting the expression, and that may change the number of
-    //       uses (and therefore the cost) of values. If the negation costs are
-    //       equal, only negate this value if it is a constant. Otherwise, try
-    //       operand 1. A better fix would eliminate uses as a cost factor or
-    //       track the change in uses as we rewrite the expression.
-    if (V0 > V1 || (V0 == V1 && isa<ConstantFPSDNode>(Op.getOperand(0)))) {
+    if (V0 > V1) {
       // fold (fneg (fma X, Y, Z)) -> (fma (fneg X), Y, (fneg Z))
       SDValue Neg0 = getNegatedExpression(
           Op.getOperand(0), DAG, LegalOperations, ForCodeSize, Depth + 1);