| // 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.Config.Warnl(v.Line, "eliminated phi") |
| } |
| return true |
| } |