From 05bbec7357a22e2ddd238c2f8741f0f4c779eb80 Mon Sep 17 00:00:00 2001
From: Keith Randall <khr@google.com>
Date: Tue, 11 Dec 2018 16:12:57 -0800
Subject: [PATCH] cmd/compile: don't combine load+op if the op has SymAddr
 arguments

By combining the load+op, we may force the op to happen earlier in
the store chain. That might force the SymAddr operation earlier, and
in particular earlier than its corresponding VarDef. That leads to
an invalid schedule, so avoid that.

This is kind of a hack to work around the issue presented. I think
the underlying problem, that LEAQ is not directly ordered with respect
to its vardef, is the real problem. The benefit of this CL is that
it fixes the immediate issue, is small, and obviously won't break
anything. A real fix for this issue is much more invasive.

The go binary is unchanged in size.
This situation just doesn't occur very often.

Fixes #28445

Change-Id: I13a765e13f075d5b6808a355ef3c43cdd7cd47b6
Reviewed-on: https://go-review.googlesource.com/c/153641
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
---
 src/cmd/compile/internal/gc/plive.go    |  2 ++
 src/cmd/compile/internal/ssa/rewrite.go | 14 ++++++++++++++
 test/fixedbugs/issue28445.go            | 16 ++++++++++++++++
 3 files changed, 32 insertions(+)
 create mode 100644 test/fixedbugs/issue28445.go

diff --git a/src/cmd/compile/internal/gc/plive.go b/src/cmd/compile/internal/gc/plive.go
index 61a749ba0d..b48a9ea87e 100644
--- a/src/cmd/compile/internal/gc/plive.go
+++ b/src/cmd/compile/internal/gc/plive.go
@@ -306,6 +306,8 @@ func (lv *Liveness) valueEffects(v *ssa.Value) (int32, liveEffect) {
 	//
 	// Addr is a read also, as any subseqent holder of the pointer must be able
 	// to see all the values (including initialization) written so far.
+	// This also prevents a variable from "coming back from the dead" and presenting
+	// stale pointers to the garbage collector. See issue 28445.
 	if e&(ssa.SymRead|ssa.SymAddr) != 0 {
 		effect |= uevar
 	}
diff --git a/src/cmd/compile/internal/ssa/rewrite.go b/src/cmd/compile/internal/ssa/rewrite.go
index 6ea46e7327..1fd335b3e7 100644
--- a/src/cmd/compile/internal/ssa/rewrite.go
+++ b/src/cmd/compile/internal/ssa/rewrite.go
@@ -264,6 +264,20 @@ func canMergeLoad(target, load *Value) bool {
 			// to be very rare.
 			return false
 		}
+		if v.Op.SymEffect()&SymAddr != 0 {
+			// This case prevents an operation that calculates the
+			// address of a local variable from being forced to schedule
+			// before its corresponding VarDef.
+			// See issue 28445.
+			//   v1 = LOAD ...
+			//   v2 = VARDEF
+			//   v3 = LEAQ
+			//   v4 = CMPQ v1 v3
+			// We don't want to combine the CMPQ with the load, because
+			// that would force the CMPQ to schedule before the VARDEF, which
+			// in turn requires the LEAQ to schedule before the VARDEF.
+			return false
+		}
 		if v.Type.IsMemory() {
 			if memPreds == nil {
 				// Initialise a map containing memory states
diff --git a/test/fixedbugs/issue28445.go b/test/fixedbugs/issue28445.go
new file mode 100644
index 0000000000..572614051e
--- /dev/null
+++ b/test/fixedbugs/issue28445.go
@@ -0,0 +1,16 @@
+// compile
+
+// Copyright 2018 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 p
+
+var fp = (**float64)(nil)
+
+func f() {
+	switch fp {
+	case new(*float64):
+		println()
+	}
+}