cmd/compile: don't update outer variables after capturevars is complete

When compiling concurrently, we walk all functions before compiling
any of them. Walking functions can cause variables to switch from
being non-addrtaken to addrtaken, e.g. to prepare for a runtime call.
Typechecking propagates addrtaken-ness of closure variables to
their outer variables, so that capturevars can decide whether to
pass the variable's value or a pointer to it.

When all functions are compiled immediately, as long as the containing
function is compiled prior to the closure, this propagation has no effect.
When compilation is deferred, though, in rare cases, this results in 
a change in the addrtaken-ness of a variable in the outer function,
which in turn changes the compiler's output.
(This is rare because in a great many cases, a temporary has been
introduced, insulating the outer variable from modification.)
But concurrent compilation must generate identical results.

To fix this, track whether capturevars has run.
If it has, there is no need to update outer variables
when closure variables change.
Capturevars always runs before any functions are walked or compiled.

The remainder of the changes in this CL are to support the test.
In particular, -d=compilelater forces the compiler to walk all
functions before compiling any of them, despite being non-concurrent.
This is useful because -live is fundamentally incompatible with
concurrent compilation, but we want -c=1 to have no behavior changes.

Fixes #20250

Change-Id: I89bcb54268a41e8588af1ac8cc37fbef856a90c2
Reviewed-on: https://go-review.googlesource.com/42853
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -243,6 +243,9 @@ func makeclosure(func_ *Node) *Node {
 	return xfunc
 }
 
+// capturevarscomplete is set to true when the capturevars phase is done.
+var capturevarscomplete bool
+
 // capturevars is called in a separate phase after all typechecking is done.
 // It decides whether each variable captured by a closure should be captured
 // by value or by reference.

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

@@ -36,6 +36,7 @@ var (
 	Debug_append       int
 	Debug_asm          bool
 	Debug_closure      int
+	Debug_compilelater int
 	debug_dclstack     int
 	Debug_panic        int
 	Debug_slice        int
@@ -56,6 +57,7 @@ var debugtab = []struct {
 }{
 	{"append", "print information about append compilation", &Debug_append},
 	{"closure", "print information about closure compilation", &Debug_closure},
+	{"compilelater", "compile functions as late as possible", &Debug_compilelater},
 	{"disablenil", "disable nil checks", &disable_checknil},
 	{"dclstack", "run internal dclstack check", &debug_dclstack},
 	{"gcprog", "print dump of GC programs", &Debug_gcprog},
@@ -493,6 +495,7 @@ func Main(archInit func(*Arch)) {
 			capturevars(n)
 		}
 	}
+	capturevarscomplete = true
 
 	Curfn = nil
 

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

@@ -226,7 +226,7 @@ func compile(fn *Node) {
 // they are enqueued in compilequeue,
 // which is drained by compileFunctions.
 func compilenow() bool {
-	return nBackendWorkers == 1
+	return nBackendWorkers == 1 && Debug_compilelater == 0
 }
 
 // compileSSA builds an SSA backend function,

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

@@ -815,7 +815,11 @@ OpSwitch:
 		var l *Node
 		for l = n.Left; l != r; l = l.Left {
 			l.SetAddrtaken(true)
-			if l.IsClosureVar() {
+			if l.IsClosureVar() && !capturevarscomplete {
+				// Mark the original variable as Addrtaken so that capturevars
+				// knows not to pass it by value.
+				// But if the capturevars phase is complete, don't touch it,
+				// in case l.Name's containing function has not yet been compiled.
 				l.Name.Defn.SetAddrtaken(true)
 			}
 		}
@@ -824,7 +828,8 @@ OpSwitch:
 			Fatalf("found non-orig name node %v", l)
 		}
 		l.SetAddrtaken(true)
-		if l.IsClosureVar() {
+		if l.IsClosureVar() && !capturevarscomplete {
+			// See comments above about closure variables.
 			l.Name.Defn.SetAddrtaken(true)
 		}
 		n.Left = defaultlit(n.Left, nil)

test/fixedbugs/issue20250.go

+// errorcheck -0 -live -d=compilelater
+
+// 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.
+
+// Issue 20250: liveness differed with concurrent compilation
+// due to propagation of addrtaken to outer variables for
+// closure variables.
+
+package p
+
+type T struct {
+	s string
+}
+
+func f(a T) { // ERROR "live at entry to f: a"
+	var e interface{}
+	func() { // ERROR "live at entry to f.func1: &e a"
+		e = a.s // ERROR "live at call to convT2Estring: &e a" "live at call to writebarrierptr: a"
+	}() // ERROR "live at call to f.func1: e$"
+	// Before the fix, both a and e were live at the previous line.
+	_ = e
+}