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cmd/compile: combine phielim and copyelim into a single pass

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Change-Id: Id21145b14169d28bac2144a31f6d3d9729f4be1e
GitHub-Last-Rev: 5413f4753e
GitHub-Pull-Request: golang/go#63818
Reviewed-on: https://go-review.googlesource.com/c/go/+/538535
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
This commit is contained in:
jeffery 2024-04-22 08:02:22 +00:00 committed by Gopher Robot
parent 4bb67bc21e
commit 69aa1974f3
3 changed files with 86 additions and 98 deletions
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7
src/cmd/compile/internal/ssa/compile.go
View File

@ -455,10 +455,8 @@ commas. For example:
// list of passes for the compiler
var passes = [...]pass{
// TODO: combine phielim and copyelim into a single pass?
{name: "number lines", fn: numberLines, required: true},
{name: "early phielim", fn: phielim},
{name: "early copyelim", fn: copyelim},
{name: "early phielim and copyelim", fn: copyelim},
{name: "early deadcode", fn: deadcode}, // remove generated dead code to avoid doing pointless work during opt
{name: "short circuit", fn: shortcircuit},
{name: "decompose user", fn: decomposeUser, required: true},
@ -496,8 +494,7 @@ var passes = [...]pass{
{name: "tighten tuple selectors", fn: tightenTupleSelectors, required: true},
{name: "lowered deadcode", fn: deadcode, required: true},
{name: "checkLower", fn: checkLower, required: true},
{name: "late phielim", fn: phielim},
{name: "late copyelim", fn: copyelim},
{name: "late phielim and copyelim", fn: copyelim},
{name: "tighten", fn: tighten, required: true}, // move values closer to their uses
{name: "late deadcode", fn: deadcode},
{name: "critical", fn: critical, required: true}, // remove critical edges

102
src/cmd/compile/internal/ssa/copyelim.go
View File

@ -4,28 +4,13 @@
package ssa
// combine copyelim and phielim into a single pass.
// copyelim removes all uses of OpCopy values from f.
// A subsequent deadcode pass is needed to actually remove the copies.
func copyelim(f *Func) {
// Modify all values so no arg (including args
// of OpCopy) is a copy.
for _, b := range f.Blocks {
for _, v := range b.Values {
// This is an early place in SSA where all values are examined.
// Rewrite all 0-sized Go values to remove accessors, dereferences, loads, etc.
if t := v.Type; (t.IsStruct() || t.IsArray()) && t.Size() == 0 {
if t.IsStruct() {
v.reset(OpStructMake0)
} else {
v.reset(OpArrayMake0)
}
}
copyelimValue(v)
}
}
phielim(f)
// loop of copyelimValue(v) process has been done in phielim() pass.
// Update block control values.
for _, b := range f.Blocks {
for i, v := range b.ControlValues() {
@ -93,3 +78,84 @@ func copyelimValue(v *Value) {
}
}
}
// phielim eliminates redundant phi values from f.
// A phi is redundant if its arguments are all equal. For
// purposes of counting, ignore the phi itself. Both of
// these phis are redundant:
//
// v = phi(x,x,x)
// v = phi(x,v,x,v)
//
// We repeat this process to also catch situations like:
//
// v = phi(x, phi(x, x), phi(x, v))
//
// TODO: Can we also simplify cases like:
//
// v = phi(v, w, x)
// w = phi(v, w, x)
//
// and would that be useful?
func phielim(f *Func) {
for {
change := false
for _, b := range f.Blocks {
for _, v := range b.Values {
// This is an early place in SSA where all values are examined.
// Rewrite all 0-sized Go values to remove accessors, dereferences, loads, etc.
if t := v.Type; (t.IsStruct() || t.IsArray()) && t.Size() == 0 {
if t.IsStruct() {
v.reset(OpStructMake0)
} else {
v.reset(OpArrayMake0)
}
}
// Modify all values so no arg (including args
// of OpCopy) is a copy.
copyelimValue(v)
change = phielimValue(v) || change
}
}
if !change {
break
}
}
}
// phielimValue tries to convert the phi v to a copy.
func phielimValue(v *Value) bool {
if v.Op != OpPhi {
return false
}
// If there are two distinct args of v which
// are not v itself, then the phi must remain.
// Otherwise, we can replace it with a copy.
var w *Value
for _, x := range v.Args {
if x == v {
continue
}
if x == w {
continue
}
if w != nil {
return false
}
w = x
}
if w == nil {
// v references only itself. It must be in
// a dead code loop. Don't bother modifying it.
return false
}
v.Op = OpCopy
v.SetArgs1(w)
f := v.Block.Func
if f.pass.debug > 0 {
f.Warnl(v.Pos, "eliminated phi")
}
return true
}

75
src/cmd/compile/internal/ssa/phielim.go
View File

@ -1,75 +0,0 @@
// Copyright 2015 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
// phielim eliminates redundant phi values from f.
// A phi is redundant if its arguments are all equal. For
// purposes of counting, ignore the phi itself. Both of
// these phis are redundant:
//
// v = phi(x,x,x)
// v = phi(x,v,x,v)
//
// We repeat this process to also catch situations like:
//
// v = phi(x, phi(x, x), phi(x, v))
//
// TODO: Can we also simplify cases like:
//
// v = phi(v, w, x)
// w = phi(v, w, x)
//
// and would that be useful?
func phielim(f *Func) {
for {
change := false
for _, b := range f.Blocks {
for _, v := range b.Values {
copyelimValue(v)
change = phielimValue(v) || change
}
}
if !change {
break
}
}
}
// phielimValue tries to convert the phi v to a copy.
func phielimValue(v *Value) bool {
if v.Op != OpPhi {
return false
}
// If there are two distinct args of v which
// are not v itself, then the phi must remain.
// Otherwise, we can replace it with a copy.
var w *Value
for _, x := range v.Args {
if x == v {
continue
}
if x == w {
continue
}
if w != nil {
return false
}
w = x
}
if w == nil {
// v references only itself. It must be in
// a dead code loop. Don't bother modifying it.
return false
}
v.Op = OpCopy
v.SetArgs1(w)
f := v.Block.Func
if f.pass.debug > 0 {
f.Warnl(v.Pos, "eliminated phi")
}
return true
}