cmd/compile: On a runtime.KeepAlive call, keep whole variable alive

We generate an OpKeepAlive for the idata portion of the interface
for a runtime.KeepAlive call.  But given such an op, we need to keep
the entire containing variable alive, not just the range that was
passed to the OpKeepAlive operation.

Fixes #17710

Change-Id: I90de66ec8065e22fb09bcf9722999ddda289ae6e
Reviewed-on: https://go-review.googlesource.com/32477
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/cmd/compile/internal/gc/ssa.go

@@ -4399,13 +4399,14 @@ func KeepAlive(v *ssa.Value) {
 	if !v.Args[0].Type.IsPtrShaped() {
 		v.Fatalf("keeping non-pointer alive %v", v.Args[0])
 	}
-	n, off := AutoVar(v.Args[0])
+	n, _ := AutoVar(v.Args[0])
 	if n == nil {
 		v.Fatalf("KeepAlive with non-spilled value %s %s", v, v.Args[0])
 	}
-	if off != 0 {
-		v.Fatalf("KeepAlive with non-zero offset spill location %v:%d", n, off)
-	}
+	// Note: KeepAlive arg may be a small part of a larger variable n.  We keep the
+	// whole variable n alive at this point. (Typically, this happens when
+	// we are requested to keep the idata portion of an interface{} alive, and
+	// we end up keeping the whole interface{} alive.  That's ok.)
 	Gvarlive(n)
 }
 

test/fixedbugs/issue17710.go

+// compile
+
+// Copyright 2016 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 main
+
+import "runtime"
+
+func f(x interface{}) {
+	runtime.KeepAlive(x)
+}