cmd/compile: don't allow NaNs in floating-point constant ops

We store 32-bit floating point constants in a 64-bit field, by
converting that 32-bit float to 64-bit float to store it, and convert
it back to use it.

That works for *almost* all floating-point constants. The exception is
signaling NaNs. The round trip described above means we can't represent
a 32-bit signaling NaN, because conversions strip the signaling bit.

To fix this issue, just forbid NaNs as floating-point constants in SSA
form. This shouldn't affect any real-world code, as people seldom
constant-propagate NaNs (except in test code).

Additionally, NaNs are somewhat underspecified (which of the many NaNs
do you get when dividing 0/0?), so when cross-compiling there's a
danger of using the compiler machine's NaN regime for some math, and
the target machine's NaN regime for other math. Better to use the
target machine's NaN regime always.

This has been a bug since 1.10, and there's an easy workaround
(declare a global varaible containing the signaling NaN pattern, and
use that as the argument to math.Float32frombits) so we'll fix it in
1.15.

Fixes #36400
Update #36399

Change-Id: Icf155e743281560eda2eed953d19a829552ccfda
Reviewed-on: https://go-review.googlesource.com/c/go/+/213477
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Josh Bleecher Snyder <[email protected]>

Author: randall77

src/cmd/compile/internal/gc/float_test.go

@@ -483,6 +483,66 @@ func TestFloat32StoreToLoadConstantFold(t *testing.T) {
 	}
 }
 
+// Signaling NaN values as constants.
+const (
+	snan32bits uint32 = 0x7f800001
+	snan64bits uint64 = 0x7ff0000000000001
+)
+
+// Signaling NaNs as variables.
+var snan32bitsVar uint32 = snan32bits
+var snan64bitsVar uint64 = snan64bits
+
+func TestFloatSignalingNaN(t *testing.T) {
+	// Make sure we generate a signaling NaN from a constant properly.
+	// See issue 36400.
+	f32 := math.Float32frombits(snan32bits)
+	g32 := math.Float32frombits(snan32bitsVar)
+	x32 := math.Float32bits(f32)
+	y32 := math.Float32bits(g32)
+	if x32 != y32 {
+		t.Errorf("got %x, want %x (diff=%x)", x32, y32, x32^y32)
+	}
+
+	f64 := math.Float64frombits(snan64bits)
+	g64 := math.Float64frombits(snan64bitsVar)
+	x64 := math.Float64bits(f64)
+	y64 := math.Float64bits(g64)
+	if x64 != y64 {
+		t.Errorf("got %x, want %x (diff=%x)", x64, y64, x64^y64)
+	}
+}
+
+func TestFloatSignalingNaNConversion(t *testing.T) {
+	// Test to make sure when we convert a signaling NaN, it converts to a quiet NaN.
+	// See issue 36399.
+	s32 := math.Float32frombits(snan32bitsVar)
+	q64 := float64(s32)
+	if math.Float64bits(q64)>>52&1 == 0 {
+		t.Errorf("got signaling NaN, want quiet NaN")
+	}
+	s64 := math.Float64frombits(snan64bitsVar)
+	q32 := float32(s64)
+	if math.Float32bits(q32)>>22&1 == 0 {
+		t.Errorf("got signaling NaN, want quiet NaN")
+	}
+}
+
+func TestFloatSignalingNaNConversionConst(t *testing.T) {
+	// Test to make sure when we convert a signaling NaN, it converts to a quiet NaN.
+	// See issue 36399 and 36400.
+	s32 := math.Float32frombits(snan32bits)
+	q64 := float64(s32)
+	if math.Float64bits(q64)>>52&1 == 0 {
+		t.Errorf("got signaling NaN, want quiet NaN")
+	}
+	s64 := math.Float64frombits(snan64bits)
+	q32 := float32(s64)
+	if math.Float32bits(q32)>>22&1 == 0 {
+		t.Errorf("got signaling NaN, want quiet NaN")
+	}
+}
+
 var sinkFloat float64
 
 func BenchmarkMul2(b *testing.B) {

src/cmd/compile/internal/ssa/check.go

@@ -141,15 +141,23 @@ func checkFunc(f *Func) {
 					f.Fatalf("bad int32 AuxInt value for %v", v)
 				}
 				canHaveAuxInt = true
-			case auxInt64, auxFloat64:
+			case auxInt64:
 				canHaveAuxInt = true
 			case auxInt128:
 				// AuxInt must be zero, so leave canHaveAuxInt set to false.
 			case auxFloat32:
 				canHaveAuxInt = true
+				if math.IsNaN(v.AuxFloat()) {
+					f.Fatalf("value %v has an AuxInt that encodes a NaN", v)
+				}
 				if !isExactFloat32(v.AuxFloat()) {
 					f.Fatalf("value %v has an AuxInt value that is not an exact float32", v)
 				}
+			case auxFloat64:
+				canHaveAuxInt = true
+				if math.IsNaN(v.AuxFloat()) {
+					f.Fatalf("value %v has an AuxInt that encodes a NaN", v)
+				}
 			case auxString, auxSym, auxTyp, auxArchSpecific:
 				canHaveAux = true
 			case auxSymOff, auxSymValAndOff, auxTypSize:

src/cmd/compile/internal/ssa/gen/PPC64.rules

@@ -78,7 +78,7 @@
 
 // Constant folding
 (FABS (FMOVDconst [x])) -> (FMOVDconst [auxFrom64F(math.Abs(auxTo64F(x)))])
-(FSQRT (FMOVDconst [x])) -> (FMOVDconst [auxFrom64F(math.Sqrt(auxTo64F(x)))])
+(FSQRT (FMOVDconst [x])) && auxTo64F(x) >= 0 -> (FMOVDconst [auxFrom64F(math.Sqrt(auxTo64F(x)))])
 (FFLOOR (FMOVDconst [x])) -> (FMOVDconst [auxFrom64F(math.Floor(auxTo64F(x)))])
 (FCEIL (FMOVDconst [x])) -> (FMOVDconst [auxFrom64F(math.Ceil(auxTo64F(x)))])
 (FTRUNC (FMOVDconst [x])) -> (FMOVDconst [auxFrom64F(math.Trunc(auxTo64F(x)))])

src/cmd/compile/internal/ssa/gen/Wasm.rules

@@ -372,7 +372,7 @@
 (I64Or  (I64Const [x]) (I64Const [y])) -> (I64Const [x | y])
 (I64Xor (I64Const [x]) (I64Const [y])) -> (I64Const [x ^ y])
 (F64Add (F64Const [x]) (F64Const [y])) -> (F64Const [auxFrom64F(auxTo64F(x) + auxTo64F(y))])
-(F64Mul (F64Const [x]) (F64Const [y])) -> (F64Const [auxFrom64F(auxTo64F(x) * auxTo64F(y))])
+(F64Mul (F64Const [x]) (F64Const [y])) && !math.IsNaN(auxTo64F(x) * auxTo64F(y)) -> (F64Const [auxFrom64F(auxTo64F(x) * auxTo64F(y))])
 (I64Eq  (I64Const [x]) (I64Const [y])) && x == y -> (I64Const [1])
 (I64Eq  (I64Const [x]) (I64Const [y])) && x != y -> (I64Const [0])
 (I64Ne  (I64Const [x]) (I64Const [y])) && x == y -> (I64Const [0])
@@ -382,15 +382,16 @@
 (I64ShrU (I64Const [x]) (I64Const [y])) -> (I64Const [int64(uint64(x) >> uint64(y))])
 (I64ShrS (I64Const [x]) (I64Const [y])) -> (I64Const [x >> uint64(y)])
 
-(I64Add (I64Const [x]) y) -> (I64Add y (I64Const [x]))
-(I64Mul (I64Const [x]) y) -> (I64Mul y (I64Const [x]))
-(I64And (I64Const [x]) y) -> (I64And y (I64Const [x]))
-(I64Or  (I64Const [x]) y) -> (I64Or  y (I64Const [x]))
-(I64Xor (I64Const [x]) y) -> (I64Xor y (I64Const [x]))
-(F64Add (F64Const [x]) y) -> (F64Add y (F64Const [x]))
-(F64Mul (F64Const [x]) y) -> (F64Mul y (F64Const [x]))
-(I64Eq  (I64Const [x]) y) -> (I64Eq y  (I64Const [x]))
-(I64Ne  (I64Const [x]) y) -> (I64Ne y  (I64Const [x]))
+// TODO: declare these operations as commutative and get rid of these rules?
+(I64Add (I64Const [x]) y) && y.Op != OpWasmI64Const -> (I64Add y (I64Const [x]))
+(I64Mul (I64Const [x]) y) && y.Op != OpWasmI64Const -> (I64Mul y (I64Const [x]))
+(I64And (I64Const [x]) y) && y.Op != OpWasmI64Const -> (I64And y (I64Const [x]))
+(I64Or  (I64Const [x]) y) && y.Op != OpWasmI64Const -> (I64Or  y (I64Const [x]))
+(I64Xor (I64Const [x]) y) && y.Op != OpWasmI64Const -> (I64Xor y (I64Const [x]))
+(F64Add (F64Const [x]) y) && y.Op != OpWasmF64Const -> (F64Add y (F64Const [x]))
+(F64Mul (F64Const [x]) y) && y.Op != OpWasmF64Const -> (F64Mul y (F64Const [x]))
+(I64Eq  (I64Const [x]) y) && y.Op != OpWasmI64Const -> (I64Eq y  (I64Const [x]))
+(I64Ne  (I64Const [x]) y) && y.Op != OpWasmI64Const -> (I64Ne y  (I64Const [x]))
 
 (I64Eq x (I64Const [0])) -> (I64Eqz x)
 (I64Ne x (I64Const [0])) -> (I64Eqz (I64Eqz x))

src/cmd/compile/internal/ssa/gen/generic.rules

@@ -118,8 +118,8 @@
 (Mul16  (Const16 [c])  (Const16 [d]))  -> (Const16 [int64(int16(c*d))])
 (Mul32  (Const32 [c])  (Const32 [d]))  -> (Const32 [int64(int32(c*d))])
 (Mul64  (Const64 [c])  (Const64 [d]))  -> (Const64 [c*d])
-(Mul32F (Const32F [c]) (Const32F [d])) -> (Const32F [auxFrom32F(auxTo32F(c) * auxTo32F(d))])
-(Mul64F (Const64F [c]) (Const64F [d])) -> (Const64F [auxFrom64F(auxTo64F(c) * auxTo64F(d))])
+(Mul32F (Const32F [c]) (Const32F [d])) && !math.IsNaN(float64(auxTo32F(c) * auxTo32F(d))) -> (Const32F [auxFrom32F(auxTo32F(c) * auxTo32F(d))])
+(Mul64F (Const64F [c]) (Const64F [d])) && !math.IsNaN(auxTo64F(c) * auxTo64F(d)) -> (Const64F [auxFrom64F(auxTo64F(c) * auxTo64F(d))])
 
 (And8   (Const8 [c])   (Const8 [d]))   -> (Const8  [int64(int8(c&d))])
 (And16  (Const16 [c])  (Const16 [d]))  -> (Const16 [int64(int16(c&d))])
@@ -144,8 +144,8 @@
 (Div16u (Const16 [c])  (Const16 [d])) && d != 0 -> (Const16 [int64(int16(uint16(c)/uint16(d)))])
 (Div32u (Const32 [c])  (Const32 [d])) && d != 0 -> (Const32 [int64(int32(uint32(c)/uint32(d)))])
 (Div64u (Const64 [c])  (Const64 [d])) && d != 0 -> (Const64 [int64(uint64(c)/uint64(d))])
-(Div32F (Const32F [c]) (Const32F [d])) -> (Const32F [auxFrom32F(auxTo32F(c) / auxTo32F(d))])
-(Div64F (Const64F [c]) (Const64F [d])) -> (Const64F [auxFrom64F(auxTo64F(c) / auxTo64F(d))])
+(Div32F (Const32F [c]) (Const32F [d])) && !math.IsNaN(float64(auxTo32F(c) / auxTo32F(d))) -> (Const32F [auxFrom32F(auxTo32F(c) / auxTo32F(d))])
+(Div64F (Const64F [c]) (Const64F [d])) && !math.IsNaN(auxTo64F(c) / auxTo64F(d)) -> (Const64F [auxFrom64F(auxTo64F(c) / auxTo64F(d))])
 (Select0 (Div128u (Const64 [0]) lo y)) -> (Div64u lo y)
 (Select1 (Div128u (Const64 [0]) lo y)) -> (Mod64u lo y)
 
@@ -588,8 +588,8 @@
 	-> x
 
 // Pass constants through math.Float{32,64}bits and math.Float{32,64}frombits
-(Load <t1> p1 (Store {t2} p2 (Const64  [x]) _)) && isSamePtr(p1,p2) && sizeof(t2) == 8 && is64BitFloat(t1) -> (Const64F [x])
-(Load <t1> p1 (Store {t2} p2 (Const32  [x]) _)) && isSamePtr(p1,p2) && sizeof(t2) == 4 && is32BitFloat(t1) -> (Const32F [auxFrom32F(math.Float32frombits(uint32(x)))])
+        (Load <t1> p1 (Store {t2} p2 (Const64  [x]) _)) && isSamePtr(p1,p2) && sizeof(t2) == 8 && is64BitFloat(t1) && !math.IsNaN(math.Float64frombits(uint64(x))) -> (Const64F [x])
+        (Load <t1> p1 (Store {t2} p2 (Const32  [x]) _)) && isSamePtr(p1,p2) && sizeof(t2) == 4 && is32BitFloat(t1) && !math.IsNaN(float64(math.Float32frombits(uint32(x)))) -> (Const32F [auxFrom32F(math.Float32frombits(uint32(x)))])
 (Load <t1> p1 (Store {t2} p2 (Const64F [x]) _)) && isSamePtr(p1,p2) && sizeof(t2) == 8 && is64BitInt(t1)   -> (Const64  [x])
 (Load <t1> p1 (Store {t2} p2 (Const32F [x]) _)) && isSamePtr(p1,p2) && sizeof(t2) == 4 && is32BitInt(t1)   -> (Const32  [int64(int32(math.Float32bits(auxTo32F(x))))])
 
@@ -1858,7 +1858,7 @@
 (Div32F x (Const32F <t> [c])) && reciprocalExact32(auxTo32F(c)) -> (Mul32F x (Const32F <t> [auxFrom32F(1/auxTo32F(c))]))
 (Div64F x (Const64F <t> [c])) && reciprocalExact64(auxTo64F(c)) -> (Mul64F x (Const64F <t> [auxFrom64F(1/auxTo64F(c))]))
 
-(Sqrt (Const64F [c])) -> (Const64F [auxFrom64F(math.Sqrt(auxTo64F(c)))])
+(Sqrt (Const64F [c])) && !math.IsNaN(math.Sqrt(auxTo64F(c))) -> (Const64F [auxFrom64F(math.Sqrt(auxTo64F(c)))])
 
 // recognize runtime.newobject and don't Zero/Nilcheck it
 (Zero (Load (OffPtr [c] (SP)) mem) mem)

src/cmd/compile/internal/ssa/gen/genericOps.go

@@ -339,7 +339,12 @@ var genericOps = []opData{
 	{name: "Const32", aux: "Int32"},      // auxint is sign-extended 32 bits
 	// Note: ConstX are sign-extended even when the type of the value is unsigned.
 	// For instance, uint8(0xaa) is stored as auxint=0xffffffffffffffaa.
-	{name: "Const64", aux: "Int64"},    // value is auxint
+	{name: "Const64", aux: "Int64"}, // value is auxint
+	// Note: for both Const32F and Const64F, we disallow encoding NaNs.
+	// Signaling NaNs are tricky because if you do anything with them, they become quiet.
+	// Particularly, converting a 32 bit sNaN to 64 bit and back converts it to a qNaN.
+	// See issue 36399 and 36400.
+	// Encodings of +inf, -inf, and -0 are fine.
 	{name: "Const32F", aux: "Float32"}, // value is math.Float64frombits(uint64(auxint)) and is exactly representable as float 32
 	{name: "Const64F", aux: "Float64"}, // value is math.Float64frombits(uint64(auxint))
 	{name: "ConstInterface"},           // nil interface

src/cmd/compile/internal/ssa/rewrite.go

@@ -487,11 +487,17 @@ func DivisionNeedsFixUp(v *Value) bool {
 
 // auxFrom64F encodes a float64 value so it can be stored in an AuxInt.
 func auxFrom64F(f float64) int64 {
+	if f != f {
+		panic("can't encode a NaN in AuxInt field")
+	}
 	return int64(math.Float64bits(f))
 }
 
 // auxFrom32F encodes a float32 value so it can be stored in an AuxInt.
 func auxFrom32F(f float32) int64 {
+	if f != f {
+		panic("can't encode a NaN in AuxInt field")
+	}
 	return int64(math.Float64bits(extend32Fto64F(f)))
 }