reflect: when Converting between float32s, don't lose signal NaNs

When converting from float32->float64->float32, any signal NaNs
get converted to quiet NaNs. Avoid that so using reflect.Value.Convert
between two float32 types keeps the signal bit of NaNs.

Update #36400

Change-Id: Ic4dd04c4be7189d2171d12b7e4e8f7cf2fb22bb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/213497
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index 00c18104eb7dfa5c0cf3900d64ddaeca5eac9dfc..496d0e9241a1693d405e42393c7c7edcdaa561b0 100644 (file)
--- a/src/reflect/all_test.go
+++ b/src/reflect/all_test.go
@@ -4163,6 +4163,17 @@ func TestConvert(t *testing.T) {
        }
 }
 
+func TestConvertNaNs(t *testing.T) {
+       const snan uint32 = 0x7f800001
+       type myFloat32 float32
+       x := V(myFloat32(math.Float32frombits(snan)))
+       y := x.Convert(TypeOf(float32(0)))
+       z := y.Interface().(float32)
+       if got := math.Float32bits(z); got != snan {
+               t.Errorf("signaling nan conversion got %x, want %x", got, snan)
+       }
+}
+
 type ComparableStruct struct {
        X int
 }

diff --git a/src/reflect/value.go b/src/reflect/value.go
index 0f5e0836634633999221d71943ceeab8f3ec014a..d8b5d11cbdcf67fd7c827d2d21bdf7c0bed0e6b7 100644 (file)
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -2541,6 +2541,14 @@ func makeFloat(f flag, v float64, t Type) Value {
        return Value{typ, ptr, f | flagIndir | flag(typ.Kind())}
 }
 
+// makeFloat returns a Value of type t equal to v, where t is a float32 type.
+func makeFloat32(f flag, v float32, t Type) Value {
+       typ := t.common()
+       ptr := unsafe_New(typ)
+       *(*float32)(ptr) = v
+       return Value{typ, ptr, f | flagIndir | flag(typ.Kind())}
+}
+
 // makeComplex returns a Value of type t equal to v (possibly truncated to complex64),
 // where t is a complex64 or complex128 type.
 func makeComplex(f flag, v complex128, t Type) Value {
@@ -2613,6 +2621,12 @@ func cvtUintFloat(v Value, t Type) Value {
 
 // convertOp: floatXX -> floatXX
 func cvtFloat(v Value, t Type) Value {
+       if v.Type().Kind() == Float32 && t.Kind() == Float32 {
+               // Don't do any conversion if both types have underlying type float32.
+               // This avoids converting to float64 and back, which will
+               // convert a signaling NaN to a quiet NaN. See issue 36400.
+               return makeFloat32(v.flag.ro(), *(*float32)(v.ptr), t)
+       }
        return makeFloat(v.flag.ro(), v.Float(), t)
 }