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