// Copyright 2016 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.
|
|
package ssa
|
|
// phiopt eliminates boolean Phis based on the previous if.
|
//
|
// Main use case is to transform:
|
// x := false
|
// if b {
|
// x = true
|
// }
|
// into x = b.
|
//
|
// In SSA code this appears as
|
//
|
// b0
|
// If b -> b1 b2
|
// b1
|
// Plain -> b2
|
// b2
|
// x = (OpPhi (ConstBool [true]) (ConstBool [false]))
|
//
|
// In this case we can replace x with a copy of b.
|
func phiopt(f *Func) {
|
sdom := f.sdom()
|
for _, b := range f.Blocks {
|
if len(b.Preds) != 2 || len(b.Values) == 0 {
|
// TODO: handle more than 2 predecessors, e.g. a || b || c.
|
continue
|
}
|
|
pb0, b0 := b, b.Preds[0].b
|
for len(b0.Succs) == 1 && len(b0.Preds) == 1 {
|
pb0, b0 = b0, b0.Preds[0].b
|
}
|
if b0.Kind != BlockIf {
|
continue
|
}
|
pb1, b1 := b, b.Preds[1].b
|
for len(b1.Succs) == 1 && len(b1.Preds) == 1 {
|
pb1, b1 = b1, b1.Preds[0].b
|
}
|
if b1 != b0 {
|
continue
|
}
|
// b0 is the if block giving the boolean value.
|
|
// reverse is the predecessor from which the truth value comes.
|
var reverse int
|
if b0.Succs[0].b == pb0 && b0.Succs[1].b == pb1 {
|
reverse = 0
|
} else if b0.Succs[0].b == pb1 && b0.Succs[1].b == pb0 {
|
reverse = 1
|
} else {
|
b.Fatalf("invalid predecessors\n")
|
}
|
|
for _, v := range b.Values {
|
if v.Op != OpPhi {
|
continue
|
}
|
|
// Look for conversions from bool to 0/1.
|
if v.Type.IsInteger() {
|
phioptint(v, b0, reverse)
|
}
|
|
if !v.Type.IsBoolean() {
|
continue
|
}
|
|
// Replaces
|
// if a { x = true } else { x = false } with x = a
|
// and
|
// if a { x = false } else { x = true } with x = !a
|
if v.Args[0].Op == OpConstBool && v.Args[1].Op == OpConstBool {
|
if v.Args[reverse].AuxInt != v.Args[1-reverse].AuxInt {
|
ops := [2]Op{OpNot, OpCopy}
|
v.reset(ops[v.Args[reverse].AuxInt])
|
v.AddArg(b0.Control)
|
if f.pass.debug > 0 {
|
f.Warnl(b.Pos, "converted OpPhi to %v", v.Op)
|
}
|
continue
|
}
|
}
|
|
// Replaces
|
// if a { x = true } else { x = value } with x = a || value.
|
// Requires that value dominates x, meaning that regardless of a,
|
// value is always computed. This guarantees that the side effects
|
// of value are not seen if a is false.
|
if v.Args[reverse].Op == OpConstBool && v.Args[reverse].AuxInt == 1 {
|
if tmp := v.Args[1-reverse]; sdom.isAncestorEq(tmp.Block, b) {
|
v.reset(OpOrB)
|
v.SetArgs2(b0.Control, tmp)
|
if f.pass.debug > 0 {
|
f.Warnl(b.Pos, "converted OpPhi to %v", v.Op)
|
}
|
continue
|
}
|
}
|
|
// Replaces
|
// if a { x = value } else { x = false } with x = a && value.
|
// Requires that value dominates x, meaning that regardless of a,
|
// value is always computed. This guarantees that the side effects
|
// of value are not seen if a is false.
|
if v.Args[1-reverse].Op == OpConstBool && v.Args[1-reverse].AuxInt == 0 {
|
if tmp := v.Args[reverse]; sdom.isAncestorEq(tmp.Block, b) {
|
v.reset(OpAndB)
|
v.SetArgs2(b0.Control, tmp)
|
if f.pass.debug > 0 {
|
f.Warnl(b.Pos, "converted OpPhi to %v", v.Op)
|
}
|
continue
|
}
|
}
|
}
|
}
|
}
|
|
func phioptint(v *Value, b0 *Block, reverse int) {
|
a0 := v.Args[0]
|
a1 := v.Args[1]
|
if a0.Op != a1.Op {
|
return
|
}
|
|
switch a0.Op {
|
case OpConst8, OpConst16, OpConst32, OpConst64:
|
default:
|
return
|
}
|
|
negate := false
|
switch {
|
case a0.AuxInt == 0 && a1.AuxInt == 1:
|
negate = true
|
case a0.AuxInt == 1 && a1.AuxInt == 0:
|
default:
|
return
|
}
|
|
if reverse == 1 {
|
negate = !negate
|
}
|
|
switch v.Type.Size() {
|
case 1:
|
v.reset(OpCopy)
|
case 2:
|
v.reset(OpZeroExt8to16)
|
case 4:
|
v.reset(OpZeroExt8to32)
|
case 8:
|
v.reset(OpZeroExt8to64)
|
default:
|
v.Fatalf("bad int size %d", v.Type.Size())
|
}
|
|
a := b0.Control
|
if negate {
|
a = v.Block.NewValue1(v.Pos, OpNot, a.Type, a)
|
}
|
v.AddArg(a)
|
|
f := b0.Func
|
if f.pass.debug > 0 {
|
f.Warnl(v.Block.Pos, "converted OpPhi bool -> int%d", v.Type.Size()*8)
|
}
|
}
|
