tuple.go

Documentation: cmd/compile/internal/ssa

     1  // Copyright 2020 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  // tightenTupleSelectors ensures that tuple selectors (Select0 and
     8  // Select1 ops) are in the same block as their tuple generator. The
     9  // function also ensures that there are no duplicate tuple selectors.
    10  // These properties are expected by the scheduler but may not have
    11  // been maintained by the optimization pipeline up to this point.
    12  //
    13  // See issues 16741 and 39472.
    14  func tightenTupleSelectors(f *Func) {
    15  	selectors := make(map[struct {
    16  		id ID
    17  		op Op
    18  	}]*Value)
    19  	for _, b := range f.Blocks {
    20  		for _, selector := range b.Values {
    21  			if selector.Op != OpSelect0 && selector.Op != OpSelect1 {
    22  				continue
    23  			}
    24  
    25  			// Get the tuple generator to use as a key for de-duplication.
    26  			tuple := selector.Args[0]
    27  			if !tuple.Type.IsTuple() {
    28  				f.Fatalf("arg of tuple selector %s is not a tuple: %s", selector.String(), tuple.LongString())
    29  			}
    30  
    31  			// If there is a pre-existing selector in the target block then
    32  			// use that. Do this even if the selector is already in the
    33  			// target block to avoid duplicate tuple selectors.
    34  			key := struct {
    35  				id ID
    36  				op Op
    37  			}{tuple.ID, selector.Op}
    38  			if t := selectors[key]; t != nil {
    39  				if selector != t {
    40  					selector.copyOf(t)
    41  				}
    42  				continue
    43  			}
    44  
    45  			// If the selector is in the wrong block copy it into the target
    46  			// block.
    47  			if selector.Block != tuple.Block {
    48  				t := selector.copyInto(tuple.Block)
    49  				selector.copyOf(t)
    50  				selectors[key] = t
    51  				continue
    52  			}
    53  
    54  			// The selector is in the target block. Add it to the map so it
    55  			// cannot be duplicated.
    56  			selectors[key] = selector
    57  		}
    58  	}
    59  }
    60
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