github.com/miolini/go@v0.0.0-20160405192216-fca68c8cb408/src/cmd/compile/internal/ssa/phielim.go (about)

     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  func phielimValue(v *Value) bool {
    35  	if v.Op != OpPhi {
    36  		return false
    37  	}
    38  
    39  	// If there are two distinct args of v which
    40  	// are not v itself, then the phi must remain.
    41  	// Otherwise, we can replace it with a copy.
    42  	var w *Value
    43  	for i, x := range v.Args {
    44  		if b := v.Block.Preds[i]; b.Kind == BlockFirst && b.Succs[1] == v.Block {
    45  			// This branch is never taken so we can just eliminate it.
    46  			continue
    47  		}
    48  		if x == v {
    49  			continue
    50  		}
    51  		if x == w {
    52  			continue
    53  		}
    54  		if w != nil {
    55  			return false
    56  		}
    57  		w = x
    58  	}
    59  
    60  	if w == nil {
    61  		// v references only itself. It must be in
    62  		// a dead code loop. Don't bother modifying it.
    63  		return false
    64  	}
    65  	v.Op = OpCopy
    66  	v.SetArgs1(w)
    67  	f := v.Block.Func
    68  	if f.pass.debug > 0 {
    69  		f.Config.Warnl(v.Line, "eliminated phi")
    70  	}
    71  	return true
    72  }