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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
import "cmd/compile/internal/types"
// zcse does an initial pass of common-subexpression elimination on the
// function for values with zero arguments to allow the more expensive cse
// to begin with a reduced number of values. Values are just relinked,
// nothing is deleted. A subsequent deadcode pass is required to actually
// remove duplicate expressions.
func zcse(f *Func) {
vals := make(map[vkey]*Value)
for _, b := range f.Blocks {
for i := 0; i < len(b.Values); i++ {
v := b.Values[i]
if opcodeTable[v.Op].argLen == 0 {
key := vkey{v.Op, keyFor(v), v.Aux, v.Type}
if vals[key] == nil {
vals[key] = v
if b != f.Entry {
// Move v to the entry block so it will dominate every block
// where we might use it. This prevents the need for any dominator
// calculations in this pass.
v.Block = f.Entry
f.Entry.Values = append(f.Entry.Values, v)
last := len(b.Values) - 1
b.Values[i] = b.Values[last]
b.Values[last] = nil
b.Values = b.Values[:last]
i-- // process b.Values[i] again
}
}
}
}
}
for _, b := range f.Blocks {
for _, v := range b.Values {
for i, a := range v.Args {
if opcodeTable[a.Op].argLen == 0 {
key := vkey{a.Op, keyFor(a), a.Aux, a.Type}
if rv, ok := vals[key]; ok {
v.SetArg(i, rv)
}
}
}
}
}
}
// vkey is a type used to uniquely identify a zero arg value.
type vkey struct {
op Op
ai int64 // aux int
ax Aux // aux
t *types.Type // type
}
// keyFor returns the AuxInt portion of a key structure uniquely identifying a
// zero arg value for the supported ops.
func keyFor(v *Value) int64 {
switch v.Op {
case OpConst64, OpConst64F, OpConst32F:
return v.AuxInt
case OpConst32:
return int64(int32(v.AuxInt))
case OpConst16:
return int64(int16(v.AuxInt))
case OpConst8, OpConstBool:
return int64(int8(v.AuxInt))
default:
return v.AuxInt
}
}
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