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pass_copyelim.go
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/
pass_copyelim.go
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/*
* Copyright 2022 ByteDance Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package ssa
// CopyElim removes unnessecery register copies.
type CopyElim struct{}
func (CopyElim) Apply(cfg *CFG) {
regs := make(map[Reg]Reg)
consts := make(map[Reg]_ConstData)
/* constant zero registers */
consts[Rz] = constint(0)
consts[Pn] = constptr(nil, Const)
/* register replacement func */
replacereg := func(rr *Reg) {
var rv Reg
var ok bool
var cc _ConstData
/* substitute registers */
for {
if rv, ok = regs[*rr]; ok {
*rr = rv
} else {
break
}
}
/* substitute zero registers */
if cc, ok = consts[*rr]; ok {
if cc.i && cc.v == 0 {
*rr = Rz
} else if !cc.i && cc.p == nil {
*rr = Pn
}
}
}
/* Phase 1: Find all the constants */
cfg.PostOrder().ForEach(func(bb *BasicBlock) {
for _, v := range bb.Ins {
switch p := v.(type) {
case *IrConstInt: consts[p.R] = constint(p.V)
case *IrConstPtr: consts[p.R] = constptr(p.P, p.M)
}
}
})
/* Phase 2: Identify all the identity operations */
for _, bb := range cfg.PostOrder().Reversed() {
for _, v := range bb.Ins {
switch p := v.(type) {
default: {
continue
}
/* pointer arithmetic */
case *IrLEA: {
if cc, ok := consts[p.Off]; ok && cc.i && cc.v == 0 {
regs[p.R] = p.Mem
}
}
/* integer arithmetic */
case *IrBinaryExpr: {
var c bool
var i int64
var x _ConstData
var y _ConstData
/* calculate mathematical identities */
switch p.Op {
case IrOpAdd : i = 0
case IrOpSub : i = 0
case IrOpMul : i = 1
case IrOpAnd : i = -1
case IrOpOr : i = 0
case IrOpXor : i = 0
case IrOpShr : i = 0
default : continue
}
/* check for identity operations */
if x, c = consts[p.X]; c && x.i && x.v == i {
regs[p.R] = p.Y
} else if y, c = consts[p.Y]; c && y.i && y.v == i {
regs[p.R] = p.X
}
}
}
}
}
/* Phase 3: Replace all the register references */
cfg.PostOrder().ForEach(func(bb *BasicBlock) {
var ok bool
var use IrUsages
/* replace in Phi nodes */
for _, v := range bb.Phi {
for _, u := range v.Usages() {
replacereg(u)
}
}
/* replace in instructions */
for _, v := range bb.Ins {
if use, ok = v.(IrUsages); ok {
for _, u := range use.Usages() {
replacereg(u)
}
}
}
/* replace in terminators */
if use, ok = bb.Term.(IrUsages); ok {
for _, u := range use.Usages() {
replacereg(u)
}
}
})
}