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
5package ssa
6
7// tightenTupleSelectors ensures that tuple selectors (Select0, Select1,
8// and SelectN 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.
14func tightenTupleSelectors(f *Func) {
15	selectors := make(map[struct {
16		id    ID
17		which int
18	}]*Value)
19	for _, b := range f.Blocks {
20		for _, selector := range b.Values {
21			// Key fields for de-duplication
22			var tuple *Value
23			idx := 0
24			switch selector.Op {
25			default:
26				continue
27			case OpSelect1:
28				idx = 1
29				fallthrough
30			case OpSelect0:
31				tuple = selector.Args[0]
32				if !tuple.Type.IsTuple() {
33					f.Fatalf("arg of tuple selector %s is not a tuple: %s", selector.String(), tuple.LongString())
34				}
35			case OpSelectN:
36				tuple = selector.Args[0]
37				idx = int(selector.AuxInt)
38				if !tuple.Type.IsResults() {
39					f.Fatalf("arg of result selector %s is not a results: %s", selector.String(), tuple.LongString())
40				}
41			}
42
43			// If there is a pre-existing selector in the target block then
44			// use that. Do this even if the selector is already in the
45			// target block to avoid duplicate tuple selectors.
46			key := struct {
47				id    ID
48				which int
49			}{tuple.ID, idx}
50			if t := selectors[key]; t != nil {
51				if selector != t {
52					selector.copyOf(t)
53				}
54				continue
55			}
56
57			// If the selector is in the wrong block copy it into the target
58			// block.
59			if selector.Block != tuple.Block {
60				t := selector.copyInto(tuple.Block)
61				selector.copyOf(t)
62				selectors[key] = t
63				continue
64			}
65
66			// The selector is in the target block. Add it to the map so it
67			// cannot be duplicated.
68			selectors[key] = selector
69		}
70	}
71}
72