cmd/compile: don't do floating point optimization x+0 -> x

That optimization is not valid if x == -0.

The test is a bit tricky because 0 == -0. We distinguish
0 from -0 with 1/0 == inf, 1/-0 == -inf.

This has been a bug since CL 24790 in Go 1.8. Probably doesn't
warrant a backport.

Fixes #27718

Note: the optimization x-0 -> x is actually valid.
But it's probably best to take it out, so as to not confuse readers.

Change-Id: I99f16a93b45f7406ec8053c2dc759a13eba035fa
Reviewed-on: https://go-review.googlesource.com/135701Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -1329,9 +1329,6 @@
 (Mul8  (Const8  <t> [c]) (Mul8  (Const8  <t> [d]) x)) -> (Mul8  (Const8  <t> [int64(int8(c*d))]) x)
 
 // floating point optimizations
-(Add(32|64)F x (Const(32|64)F [0])) -> x
-(Sub(32|64)F x (Const(32|64)F [0])) -> x
-
 (Mul(32|64)F x (Const(32|64)F [auxFrom64F(1)])) -> x
 (Mul32F x (Const32F [auxFrom32F(-1)])) -> (Neg32F x)
 (Mul64F x (Const64F [auxFrom64F(-1)])) -> (Neg64F x)

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

@@ -2445,42 +2445,6 @@ func rewriteValuegeneric_OpAdd32F_0(v *Value) bool {
 		v.AuxInt = auxFrom32F(auxTo32F(c) + auxTo32F(d))
 		return true
 	}
-	// match: (Add32F x (Const32F [0]))
-	// cond:
-	// result: x
-	for {
-		_ = v.Args[1]
-		x := v.Args[0]
-		v_1 := v.Args[1]
-		if v_1.Op != OpConst32F {
-			break
-		}
-		if v_1.AuxInt != 0 {
-			break
-		}
-		v.reset(OpCopy)
-		v.Type = x.Type
-		v.AddArg(x)
-		return true
-	}
-	// match: (Add32F (Const32F [0]) x)
-	// cond:
-	// result: x
-	for {
-		_ = v.Args[1]
-		v_0 := v.Args[0]
-		if v_0.Op != OpConst32F {
-			break
-		}
-		if v_0.AuxInt != 0 {
-			break
-		}
-		x := v.Args[1]
-		v.reset(OpCopy)
-		v.Type = x.Type
-		v.AddArg(x)
-		return true
-	}
 	return false
 }
 func rewriteValuegeneric_OpAdd64_0(v *Value) bool {
@@ -3490,42 +3454,6 @@ func rewriteValuegeneric_OpAdd64F_0(v *Value) bool {
 		v.AuxInt = auxFrom64F(auxTo64F(c) + auxTo64F(d))
 		return true
 	}
-	// match: (Add64F x (Const64F [0]))
-	// cond:
-	// result: x
-	for {
-		_ = v.Args[1]
-		x := v.Args[0]
-		v_1 := v.Args[1]
-		if v_1.Op != OpConst64F {
-			break
-		}
-		if v_1.AuxInt != 0 {
-			break
-		}
-		v.reset(OpCopy)
-		v.Type = x.Type
-		v.AddArg(x)
-		return true
-	}
-	// match: (Add64F (Const64F [0]) x)
-	// cond:
-	// result: x
-	for {
-		_ = v.Args[1]
-		v_0 := v.Args[0]
-		if v_0.Op != OpConst64F {
-			break
-		}
-		if v_0.AuxInt != 0 {
-			break
-		}
-		x := v.Args[1]
-		v.reset(OpCopy)
-		v.Type = x.Type
-		v.AddArg(x)
-		return true
-	}
 	return false
 }
 func rewriteValuegeneric_OpAdd8_0(v *Value) bool {
@@ -29841,24 +29769,6 @@ func rewriteValuegeneric_OpSub32F_0(v *Value) bool {
 		v.AuxInt = auxFrom32F(auxTo32F(c) - auxTo32F(d))
 		return true
 	}
-	// match: (Sub32F x (Const32F [0]))
-	// cond:
-	// result: x
-	for {
-		_ = v.Args[1]
-		x := v.Args[0]
-		v_1 := v.Args[1]
-		if v_1.Op != OpConst32F {
-			break
-		}
-		if v_1.AuxInt != 0 {
-			break
-		}
-		v.reset(OpCopy)
-		v.Type = x.Type
-		v.AddArg(x)
-		return true
-	}
 	return false
 }
 func rewriteValuegeneric_OpSub64_0(v *Value) bool {
@@ -30265,24 +30175,6 @@ func rewriteValuegeneric_OpSub64F_0(v *Value) bool {
 		v.AuxInt = auxFrom64F(auxTo64F(c) - auxTo64F(d))
 		return true
 	}
-	// match: (Sub64F x (Const64F [0]))
-	// cond:
-	// result: x
-	for {
-		_ = v.Args[1]
-		x := v.Args[0]
-		v_1 := v.Args[1]
-		if v_1.Op != OpConst64F {
-			break
-		}
-		if v_1.AuxInt != 0 {
-			break
-		}
-		v.reset(OpCopy)
-		v.Type = x.Type
-		v.AddArg(x)
-		return true
-	}
 	return false
 }
 func rewriteValuegeneric_OpSub8_0(v *Value) bool {

test/fixedbugs/issue27718.go

+// run
+
+// Copyright 2018 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.
+
+// (-0)+0 should be 0, not -0.
+
+package main
+
+//go:noinline
+func add64(x float64) float64 {
+	return x + 0
+}
+
+func testAdd64() {
+	var zero float64
+	inf := 1.0 / zero
+	negZero := -1 / inf
+	if 1/add64(negZero) != inf {
+		panic("negZero+0 != posZero (64 bit)")
+	}
+}
+
+//go:noinline
+func sub64(x float64) float64 {
+	return x - 0
+}
+
+func testSub64() {
+	var zero float64
+	inf := 1.0 / zero
+	negZero := -1 / inf
+	if 1/sub64(negZero) != -inf {
+		panic("negZero-0 != negZero (64 bit)")
+	}
+}
+
+//go:noinline
+func add32(x float32) float32 {
+	return x + 0
+}
+
+func testAdd32() {
+	var zero float32
+	inf := 1.0 / zero
+	negZero := -1 / inf
+	if 1/add32(negZero) != inf {
+		panic("negZero+0 != posZero (32 bit)")
+	}
+}
+
+//go:noinline
+func sub32(x float32) float32 {
+	return x - 0
+}
+
+func testSub32() {
+	var zero float32
+	inf := 1.0 / zero
+	negZero := -1 / inf
+	if 1/sub32(negZero) != -inf {
+		panic("negZero-0 != negZero (32 bit)")
+	}
+}
+
+func main() {
+	testAdd64()
+	testSub64()
+	testAdd32()
+	testSub32()
+}