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

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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/135701
Reviewed-by: Cherry Zhang <cherryyz@google.com>
This commit is contained in:
Keith Randall 2018-09-17 14:46:07 -07:00 committed by Keith Randall
parent 83dfc3b001
commit c6118af558
3 changed files with 72 additions and 111 deletions
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3
src/cmd/compile/internal/ssa/gen/generic.rules
View File

@ -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)

108
src/cmd/compile/internal/ssa/rewritegeneric.go
View File

@ -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 {

72
test/fixedbugs/issue27718.go Normal file
View File

@ -0,0 +1,72 @@
// 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()
}