[llvm] expandFMINIMUMNUM_FMAXIMUMNUM: Improve compare between zeros (PR #140193)

[Nikita Popov via llvm-commits]

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@@ -8704,13 +8705,27 @@ SDValue TargetLowering::expandFMINIMUMNUM_FMAXIMUMNUM(SDNode *Node,
       DAG.getTargetConstant(IsMax ? fcPosZero : fcNegZero, DL, MVT::i32);
   SDValue IsZero = DAG.getSetCC(DL, CCVT, MinMax,
                                 DAG.getConstantFP(0.0, DL, VT), ISD::SETEQ);
-  SDValue LCmp = DAG.getSelect(
-      DL, VT, DAG.getNode(ISD::IS_FPCLASS, DL, CCVT, LHS, TestZero), LHS,
+  unsigned BitSize = VT.getScalarSizeInBits();
+  EVT IntVT = EVT::getIntegerVT(*DAG.getContext(), BitSize);
+  EVT FloatVT = EVT::getFloatingPointVT(32);
+  if (VT.isVector()) {
+    IntVT =
+        EVT::getVectorVT(*DAG.getContext(), IntVT, VT.getVectorElementCount());
+    FloatVT = EVT::getVectorVT(*DAG.getContext(), FloatVT,
+                               VT.getVectorElementCount());
+  }
+  SDValue LHSTrunc = LHS;
+  if (!isOperationLegal(ISD::BITCAST, IntVT) &&
+      !isOperationLegal(ISD::IS_FPCLASS, VT)) {
+    LHSTrunc = DAG.getNode(ISD::FP_ROUND, DL, FloatVT, LHS,
+                           DAG.getIntPtrConstant(0, DL, /*isTarget=*/true));
+  }
+  // It's OK to select from LHS and MinMax, with only one ISD::IS_FPCLASS, as
+  // we preferred RHS when generate MinMax, if the operands are equal.
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nikic wrote:

I think this is correct for how these opcodes are legalized, but wouldn't transfer to the maximum/minimum case with the current implementation. I guess the idea is that for the legalizations through other min/max opcodes we shouldn't need the signed zero handling at all in the future, because they should now be handling it themselves?

https://github.com/llvm/llvm-project/pull/140193