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pass_branchelim.go
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/
pass_branchelim.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
import (
`fmt`
`sort`
`strings`
`github.com/oleiade/lane`
)
type _Term interface {
fmt.Stringer
term()
}
type (
_TrRel uint8
_RegTerm Reg
_ValueTerm Int65
)
func (_Stmt) term() {}
func (_RegTerm) term() {}
func (_ValueTerm) term() {}
func (self _RegTerm) String() string { return Reg(self).String() }
func (self _ValueTerm) String() string { return Int65(self).String() }
const (
_R_eq _TrRel = iota
_R_ne
_R_lt
_R_ltu
_R_ge
_R_geu
)
func (self _TrRel) String() string {
switch self {
case _R_eq : return "=="
case _R_ne : return "!="
case _R_lt : return "<"
case _R_ltu : return "<#"
case _R_ge : return ">="
case _R_geu : return ">=#"
default : panic("unreachable")
}
}
type _Edge struct {
bb *BasicBlock
to *BasicBlock
}
func (self _Edge) String() string {
return fmt.Sprintf("bb_%d => bb_%d", self.bb.Id, self.to.Id)
}
type _Stmt struct {
lhs Reg
rhs _Term
rel _TrRel
}
func (self _Stmt) String() string {
return fmt.Sprintf("%s %s %s", self.lhs, self.rel, self.rhs)
}
func (self _Stmt) negated() _Stmt {
switch self.rel {
case _R_eq : return _Stmt { self.lhs, self.rhs, _R_ne }
case _R_ne : return _Stmt { self.lhs, self.rhs, _R_eq }
case _R_lt : return _Stmt { self.lhs, self.rhs, _R_ge }
case _R_ltu : return _Stmt { self.lhs, self.rhs, _R_geu }
case _R_ge : return _Stmt { self.lhs, self.rhs, _R_lt }
case _R_geu : return _Stmt { self.lhs, self.rhs, _R_ltu }
default : panic("unreachable")
}
}
func (self _Stmt) condition(cond bool) _Stmt {
if cond {
return self
} else {
return self.negated()
}
}
type _Range struct {
rr []Int65
}
func newRange(lower Int65, upper Int65) *_Range {
return &_Range {
rr: []Int65 { lower, upper },
}
}
func (self *_Range) lower() Int65 {
if len(self.rr) == 0 {
panic("empty range")
} else {
return self.rr[0]
}
}
func (self *_Range) upper() Int65 {
if n := len(self.rr); n == 0 {
panic("empty range")
} else {
return self.rr[n - 1]
}
}
func (self *_Range) truth() (bool, bool) {
var lower Int65
var upper Int65
/* empty range */
if len(self.rr) == 0 {
return false, false
}
/* fast path: there is only one range */
if len(self.rr) == 2 {
if self.rr[0].CompareZero() == 0 && self.rr[1].CompareZero() == 0 {
return false, true
} else if self.rr[0].CompareZero() > 0 || self.rr[1].CompareZero() < 0 {
return true, true
} else {
return false, false
}
}
/* check if any range contains the zero */
for i := 0; i < len(self.rr); i += 2 {
lower = self.rr[i]
upper = self.rr[i + 1]
/* the range contains zero, the truth cannot be determained */
if lower.CompareZero() <= 0 && upper.CompareZero() >= 0 {
return false, false
}
}
/* no, the range can be interpreted as true */
return true, true
}
func (self *_Range) remove(lower Int65, upper Int65) {
for i := 0; i < len(self.rr); i += 2 {
l := self.rr[i]
u := self.rr[i + 1]
/* not intersecting */
if lower.Compare(u) > 0 { break }
if upper.Compare(l) < 0 { continue }
/* splicing */
if lower.Compare(l) > 0 && upper.Compare(u) < 0 {
next := []Int65 { l, lower.OneLess(), upper.OneMore(), u }
self.rr = append(self.rr[:i], append(next, self.rr[i + 2:]...)...)
i += 2
break
}
/* remove the upper half */
if lower.Compare(l) > 0 {
self.rr[i + 1] = lower.OneLess()
continue
}
/* remove the lower half */
if upper.Compare(u) < 0 {
self.rr[i] = upper.OneMore()
break
}
/* remove the entire range */
copy(self.rr[i:], self.rr[i + 2:])
self.rr = self.rr[:len(self.rr) - 2]
i -= 2
}
}
func (self *_Range) intersect(lower Int65, upper Int65) {
if lower != MinInt65 { self.remove(MinInt65, lower.OneLess()) }
if upper != MaxInt65 { self.remove(upper.OneMore(), MaxInt65) }
}
func (self *_Range) removeRange(r *_Range) {
for i := 0; i < len(r.rr); i += 2 {
self.remove(r.rr[i], r.rr[i + 1])
}
}
func (self *_Range) intersectRange(r *_Range) {
for i := 0; i < len(r.rr); i += 2 {
self.intersect(r.rr[i], r.rr[i + 1])
}
}
func (self *_Range) String() string {
nb := len(self.rr)
rb := make([]string, nb / 2)
/* empty ranges */
if nb == 0 {
return "{ (empty) }"
}
/* dump every range */
for i := 0; i < nb; i += 2 {
l := self.rr[i]
u := self.rr[i + 1]
s := new(strings.Builder)
/* lower bounds */
if s.WriteRune('['); l == MinInt65 {
s.WriteString("-∞")
} else {
s.WriteString(fmt.Sprint(l))
}
/* upper bounds */
if s.WriteString(", "); u == MaxInt65 {
s.WriteString("+∞")
} else {
s.WriteString(fmt.Sprint(u))
}
/* build the range */
s.WriteRune(']')
rb[i / 2] = s.String()
}
/* join them together */
return fmt.Sprintf(
"{ %s }",
strings.Join(rb, " ∪ "),
)
}
type _Ranges struct {
rr map[Reg]*_Range
}
func newRanges(nb int) (r _Ranges) {
r.rr = make(map[Reg]*_Range, nb)
r.rr[Rz] = newRange(Int65{}, Int65{})
return
}
func (self _Ranges) of(reg Reg) (r *_Range) {
var ok bool
var rr *_Range
/* check for existing range */
if rr, ok = self.rr[reg]; ok {
return rr
}
/* create a new one if needed */
rr = newRange(MinInt65, MaxInt65)
self.rr[reg] = rr
return rr
}
func (self _Ranges) at(reg Reg) (r *_Range, ok bool) {
r, ok = self.rr[reg]
return
}
type _Proof struct {
cp []int
st []_Stmt
}
func (self *_Proof) define(lhs Reg, rhs _Term, rel _TrRel) {
self.st = append(self.st, _Stmt { lhs, rhs, rel })
}
func (self *_Proof) assume(ref Reg, lhs Reg, rhs _Term, rel _TrRel) {
self.st = append(self.st, _Stmt {
lhs: ref,
rel: _R_eq,
rhs: _Stmt { lhs, rhs, rel },
})
}
func (self *_Proof) restore() {
p := len(self.cp) - 1
self.st, self.cp = self.st[:self.cp[p]], self.cp[:p]
}
func (self *_Proof) checkpoint() {
self.cp = append(self.cp, len(self.st))
}
func (self *_Proof) isContradiction(st _Stmt) (ret bool) {
self.checkpoint()
self.st = append(self.st, st)
ret = !self.verifyCorrectness()
self.restore()
return
}
func (self *_Proof) verifyCorrectness() bool {
rt := true
rr := newRanges(len(self.st))
sp := make([]_Stmt, 0, len(self.st))
st := append([]_Stmt(nil), self.st...)
/* calculate ranges for every variable */
for rt {
rt = false
sp, st = st, sp[:0]
/* update all the ranges */
for _, v := range sp {
var f bool
var p _ValueTerm
/* must be a value term */
if p, f = v.rhs.(_ValueTerm); !f {
continue
}
/* evaluate the range */
switch x := Int65(p); v.rel {
default: {
panic("unreachable")
}
/* simple ranges */
case _R_ne: rr.of(v.lhs).remove(x, x)
case _R_eq: rr.of(v.lhs).intersect(x, x)
case _R_ge: rr.of(v.lhs).intersect(x, MaxInt65)
/* signed less-than */
case _R_lt: {
if x == MinInt65 {
return false
} else {
rr.of(v.lhs).intersect(MinInt65, x.OneLess())
}
}
/* unsigned greater-than-or-equal-to */
case _R_geu: {
if x.CompareZero() < 0 {
panic(fmt.Sprintf("unsigned comparison to a negative value %s", x))
} else {
rr.of(v.lhs).intersect(x, MaxInt65)
}
}
/* unsigned less-than */
case _R_ltu: {
if x.CompareZero() <= 0 {
panic(fmt.Sprintf("unsigned comparison to a non-positive value %s", x))
} else {
rr.of(v.lhs).intersect(Int65{}, x.OneLess())
}
}
}
}
/* expand all the definations */
for _, v := range sp {
if p, ok := v.rhs.(_Stmt); ok {
if r, rk := rr.at(v.lhs); rk {
if t, tk := r.truth(); tk {
rt = true
st = append(st, p.condition(t))
}
}
}
}
/* evaluate all the registers */
for _, v := range sp {
var f bool
var x Int65
var r *_Range
var t _RegTerm
/* must be a register term with a valid range */
if t, f = v.rhs.(_RegTerm) ; !f { continue }
if r, f = rr.at(Reg(t)) ; !f { continue }
/* empty range, already found contradictions */
if len(r.rr) == 0 {
return false
}
/* update the ranges */
switch v.rel {
default: {
panic("unreachable")
}
/* equality and inequality */
case _R_ne: rr.of(v.lhs).removeRange(r)
case _R_eq: rr.of(v.lhs).intersectRange(r)
/* signed less-than */
case _R_lt: {
rt = true
st = append(st, _Stmt { v.lhs, _ValueTerm(r.upper()), _R_lt })
}
/* signed greater-than */
case _R_ge: {
rt = true
st = append(st, _Stmt { v.lhs, _ValueTerm(r.lower()), _R_ge })
}
/* unsigned less-than */
case _R_ltu: {
if x, rt = r.upper(), true; x.CompareZero() > 0 {
st = append(st, _Stmt { v.lhs, _ValueTerm(x), _R_ltu })
} else {
return false
}
}
/* unsigned greater-than-or-equal-to */
case _R_geu: {
if x, rt = r.lower(), true; x.CompareZero() >= 0 {
st = append(st, _Stmt { v.lhs, _ValueTerm(x), _R_geu })
} else {
st = append(st, _Stmt { v.lhs, _ValueTerm(Int65{}), _R_geu })
}
}
}
}
}
/* the statements are valid iff there are no empty ranges */
for _, r := range rr.rr {
if len(r.rr) == 0 {
return false
}
}
/* all checked fine */
return true
}
// BranchElim removes branches that can be proved unreachable.
type BranchElim struct{}
func (self BranchElim) dfs(cfg *CFG, bb *BasicBlock, ps *_Proof) {
var ok bool
var sw *IrSwitch
/* add facts for this basic block */
for _, v := range bb.Ins {
switch p := v.(type) {
default: {
break
}
/* integer constant */
case *IrConstInt: {
ps.define(p.R, _ValueTerm(Int65i(p.V)), _R_eq)
}
/* binary operators */
case *IrBinaryExpr: {
switch p.Op {
case IrCmpEq : ps.assume(p.R, p.X, _RegTerm(p.Y), _R_eq)
case IrCmpNe : ps.assume(p.R, p.X, _RegTerm(p.Y), _R_ne)
case IrCmpLt : ps.assume(p.R, p.X, _RegTerm(p.Y), _R_lt)
case IrCmpLtu : ps.assume(p.R, p.X, _RegTerm(p.Y), _R_ltu)
case IrCmpGeu : ps.assume(p.R, p.X, _RegTerm(p.Y), _R_geu)
}
}
}
}
/* only care about switches */
if sw, ok = bb.Term.(*IrSwitch); !ok {
return
}
/* edges to be removed */
rem := lane.NewQueue()
del := make(map[_Edge]bool)
val := make([]int32, 0, len(sw.Br))
/* prove every branch */
for v, p := range sw.Br {
if val = append(val, v); ps.isContradiction(_Stmt { sw.V, _ValueTerm(Int65i(int64(v))), _R_eq }) {
delete(sw.Br, v)
rem.Enqueue(_Edge { bb, p.To })
}
}
/* create a save-point */
ps.checkpoint()
sort.Slice(val, func(i int, j int) bool { return val[i] < val[j] })
/* add all the negated conditions */
for _, i := range val {
ps.define(sw.V, _ValueTerm(Int65i(int64(i))), _R_ne)
}
/* prove the default branch */
reachable := ps.verifyCorrectness()
ps.restore()
/* check for reachability */
if !reachable {
if rem.Enqueue(_Edge { bb, sw.Ln.To }); len(sw.Br) != 1 {
sw.Ln = IrUnlikely(cfg.CreateUnreachable(bb))
} else {
sw.Ln, sw.Br = sw.Br[val[0]], make(map[int32]*IrBranch)
}
}
/* clear register reference if needed */
if len(sw.Br) == 0 {
sw.V = Rz
}
/* adjust all the edges */
for !rem.Empty() {
e := rem.Pop().(_Edge)
del[e] = true
/* adjust Phi nodes in the target block */
for _, v := range e.to.Phi {
delete(v.V, e.bb)
}
/* remove predecessors from the target block */
for i, p := range e.to.Pred {
if p == e.bb {
e.to.Pred = append(e.to.Pred[:i], e.to.Pred[i + 1:]...)
break
}
}
/* remove the entire block if no more entry edges left */
if len(e.to.Pred) == 0 {
for it := e.to.Term.Successors(); it.Next(); {
rem.Enqueue(_Edge {
bb: e.to,
to: it.Block(),
})
}
}
}
/* DFS the dominator tree */
for _, p := range cfg.DominatorOf[bb.Id] {
var f bool
var v _ValueTerm
/* no need to recurse into unreachable branches */
if del[_Edge { bb, p }] {
continue
}
/* find the branch value */
for i, b := range sw.Br {
if b.To == p {
f, v = true, _ValueTerm(Int65i(int64(i)))
break
}
}
/* it is not a direct successor, just pass all the facts down */
if !f {
self.dfs(cfg, p, ps)
continue
}
/* add the fact and recurse into the node */
ps.checkpoint()
ps.define(sw.V, v, _R_eq)
self.dfs(cfg, p, ps)
ps.restore()
}
}
func (self BranchElim) Apply(cfg *CFG) {
self.dfs(cfg, cfg.Root, new(_Proof))
cfg.Rebuild()
}