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// 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) {
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 || !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.Config.Warnl(b.Line, "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]; f.sdom.isAncestorEq(tmp.Block, b) {
v.reset(OpOrB)
v.SetArgs2(b0.Control, tmp)
if f.pass.debug > 0 {
f.Config.Warnl(b.Line, "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]; f.sdom.isAncestorEq(tmp.Block, b) {
v.reset(OpAndB)
v.SetArgs2(b0.Control, tmp)
if f.pass.debug > 0 {
f.Config.Warnl(b.Line, "converted OpPhi to %v", v.Op)
}
continue
}
}
}
}
}
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