cmd/compile: combine phielim and copyelim into a single pass
Change-Id: Id21145b14169d28bac2144a31f6d3d9729f4be1e
GitHub-Last-Rev: 5413f4753e5acb60db6a93cb3409047bddc8df6d
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>
diff --git a/src/cmd/compile/internal/ssa/compile.go b/src/cmd/compile/internal/ssa/compile.go
index d125891..80ef53d 100644
--- a/src/cmd/compile/internal/ssa/compile.go
+++ b/src/cmd/compile/internal/ssa/compile.go
@@ -455,10 +455,8 @@
// 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 @@
{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
diff --git a/src/cmd/compile/internal/ssa/copyelim.go b/src/cmd/compile/internal/ssa/copyelim.go
index 17471e3..ea888f4 100644
--- a/src/cmd/compile/internal/ssa/copyelim.go
+++ b/src/cmd/compile/internal/ssa/copyelim.go
@@ -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 {
+ phielim(f)
- // 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)
- }
- }
-
+ // 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 @@
}
}
}
+
+// 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
+}
diff --git a/src/cmd/compile/internal/ssa/phielim.go b/src/cmd/compile/internal/ssa/phielim.go
deleted file mode 100644
index 4fc9423..0000000
--- a/src/cmd/compile/internal/ssa/phielim.go
+++ /dev/null
@@ -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
-}
