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// 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
// 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 {
copyelimValue(v)
}
}
// Update block control values.
for _, b := range f.Blocks {
for i, v := range b.ControlValues() {
if v.Op == OpCopy {
b.ReplaceControl(i, v.Args[0])
}
}
}
// Update named values.
for _, name := range f.Names {
values := f.NamedValues[*name]
for i, v := range values {
if v.Op == OpCopy {
values[i] = v.Args[0]
}
}
}
}
// copySource returns the (non-copy) op which is the
// ultimate source of v. v must be a copy op.
func copySource(v *Value) *Value {
w := v.Args[0]
// This loop is just:
// for w.Op == OpCopy {
// w = w.Args[0]
// }
// but we take some extra care to make sure we
// don't get stuck in an infinite loop.
// Infinite copy loops may happen in unreachable code.
// (TODO: or can they? Needs a test.)
slow := w
var advance bool
for w.Op == OpCopy {
w = w.Args[0]
if w == slow {
w.reset(OpUnknown)
break
}
if advance {
slow = slow.Args[0]
}
advance = !advance
}
// The answer is w. Update all the copies we saw
// to point directly to w. Doing this update makes
// sure that we don't end up doing O(n^2) work
// for a chain of n copies.
for v != w {
x := v.Args[0]
v.SetArg(0, w)
v = x
}
return w
}
// copyelimValue ensures that no args of v are copies.
func copyelimValue(v *Value) {
for i, a := range v.Args {
if a.Op == OpCopy {
v.SetArg(i, copySource(a))
}
}
}
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