1 files changed, 71 insertions, 0 deletions
diff --git a/src/cmd/compile/internal/ssa/tuple.go b/src/cmd/compile/internal/ssa/tuple.go
new file mode 100644
index 0000000..289df40
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/tuple.go
@@ -0,0 +1,71 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssa
+
+// tightenTupleSelectors ensures that tuple selectors (Select0, Select1,
+// and SelectN ops) are in the same block as their tuple generator. The
+// function also ensures that there are no duplicate tuple selectors.
+// These properties are expected by the scheduler but may not have
+// been maintained by the optimization pipeline up to this point.
+//
+// See issues 16741 and 39472.
+func tightenTupleSelectors(f *Func) {
+	selectors := make(map[struct {
+		id    ID
+		which int
+	}]*Value)
+	for _, b := range f.Blocks {
+		for _, selector := range b.Values {
+			// Key fields for de-duplication
+			var tuple *Value
+			idx := 0
+			switch selector.Op {
+			default:
+				continue
+			case OpSelect1:
+				idx = 1
+				fallthrough
+			case OpSelect0:
+				tuple = selector.Args[0]
+				if !tuple.Type.IsTuple() {
+					f.Fatalf("arg of tuple selector %s is not a tuple: %s", selector.String(), tuple.LongString())
+				}
+			case OpSelectN:
+				tuple = selector.Args[0]
+				idx = int(selector.AuxInt)
+				if !tuple.Type.IsResults() {
+					f.Fatalf("arg of result selector %s is not a results: %s", selector.String(), tuple.LongString())
+				}
+			}
+
+			// If there is a pre-existing selector in the target block then
+			// use that. Do this even if the selector is already in the
+			// target block to avoid duplicate tuple selectors.
+			key := struct {
+				id    ID
+				which int
+			}{tuple.ID, idx}
+			if t := selectors[key]; t != nil {
+				if selector != t {
+					selector.copyOf(t)
+				}
+				continue
+			}
+
+			// If the selector is in the wrong block copy it into the target
+			// block.
+			if selector.Block != tuple.Block {
+				t := selector.copyInto(tuple.Block)
+				selector.copyOf(t)
+				selectors[key] = t
+				continue
+			}
+
+			// The selector is in the target block. Add it to the map so it
+			// cannot be duplicated.
+			selectors[key] = selector
+		}
+	}
+}