[ConstantFolding] Fix nvvm_round folding on PPC

Fix a failing test for constant-folding the nvvm_round intrinsic.
The original implementation added in llvm#141233 used a native libm
call to the "round" function, but on PPC this produces +0.0 if the
input is -0.0, which caused a test failure.

This patch updates it to use APFloat functions instead of native
libm calls to ensure cross-platform consistency.

Author: LewisCrawford

llvm/lib/Analysis/ConstantFolding.cpp

@@ -2677,11 +2677,14 @@ static Constant *ConstantFoldScalarCall1(StringRef Name,
 
       case Intrinsic::nvvm_round_ftz_f:
       case Intrinsic::nvvm_round_f:
-      case Intrinsic::nvvm_round_d:
-        return ConstantFoldFP(
-            round, APF, Ty,
-            nvvm::GetNVVMDenromMode(
-                nvvm::UnaryMathIntrinsicShouldFTZ(IntrinsicID)));
+      case Intrinsic::nvvm_round_d: {
+        // Use APFloat implementation instead of native libm call, as some
+        // implementations (e.g. on PPC) do not preserve the sign of negative 0.
+        bool IsFTZ = nvvm::UnaryMathIntrinsicShouldFTZ(IntrinsicID);
+        auto V = IsFTZ ? FTZPreserveSign(APF) : APF;
+        V.roundToIntegral(APFloat::rmNearestTiesToAway);
+        return ConstantFP::get(Ty->getContext(), V);
+      }
 
       case Intrinsic::nvvm_saturate_ftz_f:
       case Intrinsic::nvvm_saturate_d: