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

Trying this CL again, with a test that skips 387.

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.

Skip the test on 387. I don't see any sane way of ensuring that a
float load + float store is faithful on that platform.

Fixes #36400

Change-Id: Ic316c74ddc155632e40424e207375b5d50dcd853
Reviewed-on: https://go-review.googlesource.com/c/go/+/221792
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index 00c18104eb7dfa5c0cf3900d64ddaeca5eac9dfc..66d9661aeb51ac5a4ffb010378860f047bb70613 100644 (file)
--- a/src/reflect/all_test.go
+++ b/src/reflect/all_test.go
@@ -4163,6 +4163,37 @@ func TestConvert(t *testing.T) {
        }
 }
 
+var gFloat32 float32
+
+func TestConvertNaNs(t *testing.T) {
+       const snan uint32 = 0x7f800001
+
+       // Test to see if a store followed by a load of a signaling NaN
+       // maintains the signaling bit. The only platform known to fail
+       // this test is 386,GO386=387. The real test below will always fail
+       // if the platform can't even store+load a float without mucking
+       // with the bits.
+       gFloat32 = math.Float32frombits(snan)
+       runtime.Gosched() // make sure we don't optimize the store/load away
+       r := math.Float32bits(gFloat32)
+       if r != snan {
+               // This should only happen on 386,GO386=387. We have no way to
+               // test for 387, so we just make sure we're at least on 386.
+               if runtime.GOARCH != "386" {
+                       t.Errorf("store/load of sNaN not faithful")
+               }
+               t.Skip("skipping test, float store+load not faithful")
+       }
+
+       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 51e7d195fe0b8d0f92f968c2ecdc4fc3d686bd1c..08f0d259de777770f6ecb6cdb3227d2f0f99eca1 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)
 }