cmd/compile: prevent constant folding of +/- when result is NaN

Missed as part of CL 221790. It isn't just * and / that can make NaNs.

Update #36400
Fixes #38359

Change-Id: I3fa562f772fe03b510793a6dc0cf6189c0c3e652
Reviewed-on: https://go-review.googlesource.com/c/go/+/227860
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Alberto Donizetti <[email protected]>
Reviewed-by: Cuong Manh Le <[email protected]>

Author: randall77

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

@@ -103,17 +103,17 @@
 (Add16  (Const16 [c])  (Const16 [d]))  -> (Const16 [int64(int16(c+d))])
 (Add32  (Const32 [c])  (Const32 [d]))  -> (Const32 [int64(int32(c+d))])
 (Add64  (Const64 [c])  (Const64 [d]))  -> (Const64 [c+d])
-(Add32F (Const32F [c]) (Const32F [d])) -> (Const32F [auxFrom32F(auxTo32F(c) + auxTo32F(d))])
-(Add64F (Const64F [c]) (Const64F [d])) -> (Const64F [auxFrom64F(auxTo64F(c) + auxTo64F(d))])
+(Add32F (Const32F [c]) (Const32F [d])) && !math.IsNaN(float64(auxTo32F(c) + auxTo32F(d))) -> (Const32F [auxFrom32F(auxTo32F(c) + auxTo32F(d))])
+(Add64F (Const64F [c]) (Const64F [d])) && !math.IsNaN(auxTo64F(c) + auxTo64F(d)) -> (Const64F [auxFrom64F(auxTo64F(c) + auxTo64F(d))])
 (AddPtr <t> x (Const64 [c])) -> (OffPtr <t> x [c])
 (AddPtr <t> x (Const32 [c])) -> (OffPtr <t> x [c])
 
 (Sub8   (Const8 [c]) (Const8 [d]))     -> (Const8 [int64(int8(c-d))])
 (Sub16  (Const16 [c]) (Const16 [d]))   -> (Const16 [int64(int16(c-d))])
 (Sub32  (Const32 [c]) (Const32 [d]))   -> (Const32 [int64(int32(c-d))])
 (Sub64  (Const64 [c]) (Const64 [d]))   -> (Const64 [c-d])
-(Sub32F (Const32F [c]) (Const32F [d])) -> (Const32F [auxFrom32F(auxTo32F(c) - auxTo32F(d))])
-(Sub64F (Const64F [c]) (Const64F [d])) -> (Const64F [auxFrom64F(auxTo64F(c) - auxTo64F(d))])
+(Sub32F (Const32F [c]) (Const32F [d])) && !math.IsNaN(float64(auxTo32F(c) - auxTo32F(d))) -> (Const32F [auxFrom32F(auxTo32F(c) - auxTo32F(d))])
+(Sub64F (Const64F [c]) (Const64F [d])) && !math.IsNaN(auxTo64F(c) - auxTo64F(d)) -> (Const64F [auxFrom64F(auxTo64F(c) - auxTo64F(d))])
 
 (Mul8   (Const8 [c])   (Const8 [d]))   -> (Const8  [int64(int8(c*d))])
 (Mul16  (Const16 [c])  (Const16 [d]))  -> (Const16 [int64(int16(c*d))])

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

@@ -0,0 +1,19 @@
+// compile
+
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Make sure NaN-NaN compiles correctly.
+
+package p
+
+func f() {
+	var st struct {
+		f    float64
+		_, _ string
+	}
+
+	f := 1e308
+	st.f = 2*f - 2*f
+}