Source file src/pkg/cmd/compile/internal/ssa/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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