[ConstantFolding] Fix incorrect nvvm_round folding (#151563)

The `nvvm_round` intrinsic should round to the nearest even number in
the case of ties. It lowers to PTX `cvt.rni`, which will "round to
nearest integer, choosing even integer if source is equidistant between
two integers", so it matches the semantics of `rint` (and not `round` as
the name suggests).

Author: LewisCrawford

llvm/lib/Analysis/ConstantFolding.cpp

@@ -2679,11 +2679,12 @@ static Constant *ConstantFoldScalarCall1(StringRef Name,
       case Intrinsic::nvvm_round_ftz_f:
       case Intrinsic::nvvm_round_f:
       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.
+        // nvvm_round is lowered to PTX cvt.rni, which will round to nearest
+        // integer, choosing even integer if source is equidistant between two
+        // integers, so the semantics are closer to "rint" rather than "round".
         bool IsFTZ = nvvm::UnaryMathIntrinsicShouldFTZ(IntrinsicID);
         auto V = IsFTZ ? FTZPreserveSign(APF) : APF;
-        V.roundToIntegral(APFloat::rmNearestTiesToAway);
+        V.roundToIntegral(APFloat::rmNearestTiesToEven);
         return ConstantFP::get(Ty->getContext(), V);
       }
 

llvm/test/Transforms/InstSimplify/const-fold-nvvm-unary-arithmetic.ll

@@ -416,6 +416,54 @@ define float @test_round_ftz_f_neg_1_5() {
   ret float %res
 }
 
+define double @test_round_d_2_5() {
+; CHECK-LABEL: define double @test_round_d_2_5() {
+; CHECK-NEXT:    ret double 2.000000e+00
+;
+  %res = call double @llvm.nvvm.round.d(double 2.5)
+  ret double %res
+}
+
+define float @test_round_f_2_5() {
+; CHECK-LABEL: define float @test_round_f_2_5() {
+; CHECK-NEXT:    ret float 2.000000e+00
+;
+  %res = call float @llvm.nvvm.round.f(float 2.5)
+  ret float %res
+}
+
+define float @test_round_ftz_f_2_5() {
+; CHECK-LABEL: define float @test_round_ftz_f_2_5() {
+; CHECK-NEXT:    ret float 2.000000e+00
+;
+  %res = call float @llvm.nvvm.round.ftz.f(float 2.5)
+  ret float %res
+}
+
+define double @test_round_d_neg_2_5() {
+; CHECK-LABEL: define double @test_round_d_neg_2_5() {
+; CHECK-NEXT:    ret double -2.000000e+00
+;
+  %res = call double @llvm.nvvm.round.d(double -2.5)
+  ret double %res
+}
+
+define float @test_round_f_neg_2_5() {
+; CHECK-LABEL: define float @test_round_f_neg_2_5() {
+; CHECK-NEXT:    ret float -2.000000e+00
+;
+  %res = call float @llvm.nvvm.round.f(float -2.5)
+  ret float %res
+}
+
+define float @test_round_ftz_f_neg_2_5() {
+; CHECK-LABEL: define float @test_round_ftz_f_neg_2_5() {
+; CHECK-NEXT:    ret float -2.000000e+00
+;
+  %res = call float @llvm.nvvm.round.ftz.f(float -2.5)
+  ret float %res
+}
+
 define double @test_round_d_neg_subnorm() {
 ; CHECK-LABEL: define double @test_round_d_neg_subnorm() {
 ; CHECK-NEXT:    ret double -0.000000e+00