[release-branch.go1.8] cmd/compile: repaired loop-finder to handle trickier nesting

The loop-A-encloses-loop-C code did not properly handle the
case where really C was already known to be enclosed by B,
and A was nearest-outer to B, not C.

Fixes #19217.

Change-Id: I755dd768e823cb707abdc5302fed39c11cdb34d4
Reviewed-on: https://go-review.googlesource.com/39596
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

2 files changed, 56 insertions, 6 deletions

diff --git a/src/cmd/compile/internal/ssa/likelyadjust.go b/src/cmd/compile/internal/ssa/likelyadjust.go
index 38a5e81f91..a4e5534cf8 100644
--- a/src/cmd/compile/internal/ssa/likelyadjust.go
+++ b/src/cmd/compile/internal/ssa/likelyadjust.go
@@ -33,13 +33,24 @@ type loop struct {
 
 // outerinner records that outer contains inner
 func (sdom SparseTree) outerinner(outer, inner *loop) {
+	// There could be other outer loops found in some random order,
+	// locate the new outer loop appropriately among them.
 	oldouter := inner.outer
-	if oldouter == nil || sdom.isAncestorEq(oldouter.header, outer.header) {
-		inner.outer = outer
-		outer.isInner = false
-		if inner.containsCall {
-			outer.setContainsCall()
-		}
+	for oldouter != nil && sdom.isAncestor(outer.header, oldouter.header) {
+		inner = oldouter
+		oldouter = inner.outer
+	}
+	if outer == oldouter {
+		return
+	}
+	if oldouter != nil {
+		outer.outer = oldouter
+	}
+
+	inner.outer = outer
+	outer.isInner = false
+	if inner.containsCall {
+		outer.setContainsCall()
 	}
 }
 
diff --git a/test/fixedbugs/issue19217.go b/test/fixedbugs/issue19217.go
new file mode 100644
index 0000000000..96794064a4
--- /dev/null
+++ b/test/fixedbugs/issue19217.go
@@ -0,0 +1,39 @@
+// compile
+
+// Copyright 2017 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 foo
+
+import (
+	"encoding/binary"
+)
+
+type DbBuilder struct {
+	arr []int
+}
+
+func (bld *DbBuilder) Finish() error {
+	defer bld.Finish()
+
+	var hash []byte
+	for _, ixw := range bld.arr {
+		for {
+			if ixw != 0 {
+				panic("ixw != 0")
+			}
+			ixw--
+		insertOne:
+			for {
+				for i := 0; i < 1; i++ {
+					if binary.LittleEndian.Uint16(hash[i:]) == 0 {
+						break insertOne
+					}
+				}
+			}
+		}
+	}
+
+	return nil
+}