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      1 // Copyright 2015 The Go Authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style
      3 // license that can be found in the LICENSE file.
      4 
      5 package ssa
      6 
      7 // phielim eliminates redundant phi values from f.
      8 // A phi is redundant if its arguments are all equal. For
      9 // purposes of counting, ignore the phi itself. Both of
     10 // these phis are redundant:
     11 //   v = phi(x,x,x)
     12 //   v = phi(x,v,x,v)
     13 // We repeat this process to also catch situations like:
     14 //   v = phi(x, phi(x, x), phi(x, v))
     15 // TODO: Can we also simplify cases like:
     16 //   v = phi(v, w, x)
     17 //   w = phi(v, w, x)
     18 // and would that be useful?
     19 func phielim(f *Func) {
     20 	for {
     21 		change := false
     22 		for _, b := range f.Blocks {
     23 			for _, v := range b.Values {
     24 				copyelimValue(v)
     25 				change = phielimValue(v) || change
     26 			}
     27 		}
     28 		if !change {
     29 			break
     30 		}
     31 	}
     32 }
     33 
     34 // phielimValue tries to convert the phi v to a copy.
     35 func phielimValue(v *Value) bool {
     36 	if v.Op != OpPhi {
     37 		return false
     38 	}
     39 
     40 	// If there are two distinct args of v which
     41 	// are not v itself, then the phi must remain.
     42 	// Otherwise, we can replace it with a copy.
     43 	var w *Value
     44 	for _, x := range v.Args {
     45 		if x == v {
     46 			continue
     47 		}
     48 		if x == w {
     49 			continue
     50 		}
     51 		if w != nil {
     52 			return false
     53 		}
     54 		w = x
     55 	}
     56 
     57 	if w == nil {
     58 		// v references only itself. It must be in
     59 		// a dead code loop. Don't bother modifying it.
     60 		return false
     61 	}
     62 	v.Op = OpCopy
     63 	v.SetArgs1(w)
     64 	f := v.Block.Func
     65 	if f.pass.debug > 0 {
     66 		f.Config.Warnl(v.Line, "eliminated phi")
     67 	}
     68 	return true
     69 }
     70