// Copyright 2016 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package ssa
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import "cmd/compile/internal/types"
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// zcse does an initial pass of common-subexpression elimination on the
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// function for values with zero arguments to allow the more expensive cse
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// to begin with a reduced number of values. Values are just relinked,
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// nothing is deleted. A subsequent deadcode pass is required to actually
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// remove duplicate expressions.
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func zcse(f *Func) {
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vals := make(map[vkey]*Value)
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for _, b := range f.Blocks {
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for i := 0; i < len(b.Values); {
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v := b.Values[i]
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next := true
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if opcodeTable[v.Op].argLen == 0 {
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key := vkey{v.Op, keyFor(v), v.Aux, v.Type}
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if vals[key] == nil {
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vals[key] = v
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if b != f.Entry {
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// Move v to the entry block so it will dominate every block
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// where we might use it. This prevents the need for any dominator
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// calculations in this pass.
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v.Block = f.Entry
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f.Entry.Values = append(f.Entry.Values, v)
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last := len(b.Values) - 1
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b.Values[i] = b.Values[last]
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b.Values[last] = nil
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b.Values = b.Values[:last]
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// process b.Values[i] again
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next = false
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}
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}
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}
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if next {
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i++
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}
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}
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}
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for _, b := range f.Blocks {
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for _, v := range b.Values {
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for i, a := range v.Args {
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if opcodeTable[a.Op].argLen == 0 {
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key := vkey{a.Op, keyFor(a), a.Aux, a.Type}
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if rv, ok := vals[key]; ok {
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v.SetArg(i, rv)
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}
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}
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}
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}
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}
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}
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// vkey is a type used to uniquely identify a zero arg value.
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type vkey struct {
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op Op
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ai int64 // aux int
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ax interface{} // aux
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t *types.Type // type
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}
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// keyFor returns the AuxInt portion of a key structure uniquely identifying a
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// zero arg value for the supported ops.
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func keyFor(v *Value) int64 {
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switch v.Op {
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case OpConst64, OpConst64F, OpConst32F:
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return v.AuxInt
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case OpConst32:
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return int64(int32(v.AuxInt))
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case OpConst16:
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return int64(int16(v.AuxInt))
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case OpConst8, OpConstBool:
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return int64(int8(v.AuxInt))
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default:
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return v.AuxInt
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}
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}
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