phielim.go

     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.Warnl(v.Pos, "eliminated phi")
    67  	}
    68  	return true
    69  }
    70
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