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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
// 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
}
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