runtime: add GODEBUG=gccheckmark=0/1

Previously, gccheckmark could only be enabled or disabled by calling
runtime.GCcheckmarkenable/GCcheckmarkdisable.  This was a necessary
hack because GODEBUG was broken.

Now that GODEBUG works again, move control over gccheckmark to a
GODEBUG variable and remove these runtime functions.  Currently,
gccheckmark is enabled by default (and will probably remain so for
much of the 1.5 development cycle).

Change-Id: I2bc6f30c21b795264edf7dbb6bd7354b050673ab
Reviewed-on: https://go-review.googlesource.com/2603
Reviewed-by: Rick Hudson <rlh@golang.org>

api/next.txt

diff --git a/api/next.txt b/api/next.txt
index b94bda1..eb21e80 100644
--- a/api/next.txt
+++ b/api/next.txt
@@ -247,5 +247,3 @@
 pkg runtime (openbsd-amd64-cgo), const EWOULDBLOCK ideal-int
 pkg runtime (openbsd-amd64-cgo), const HW_NCPU = 3
 pkg runtime (openbsd-amd64-cgo), const HW_NCPU ideal-int
-pkg runtime, func GCcheckmarkdisable()
-pkg runtime, func GCcheckmarkenable()

src/runtime/extern.go

diff --git a/src/runtime/extern.go b/src/runtime/extern.go
index f295b9b..58acbb3 100644
--- a/src/runtime/extern.go
+++ b/src/runtime/extern.go
@@ -66,6 +66,12 @@
 	problem with allocfreetrace=1 in order to understand the type
 	of the badly updated word.
 
+	gccheckmark: setting gccheckmark=1 enables verification of the
+	garbage collector's concurrent mark phase by performing a
+	second mark pass while the world is stopped.  If the second
+	pass finds a reachable object that was not found by concurrent
+	mark, the garbage collector will panic.
+
 The GOMAXPROCS variable limits the number of operating system threads that
 can execute user-level Go code simultaneously. There is no limit to the number of threads
 that can be blocked in system calls on behalf of Go code; those do not count against

src/runtime/malloc.go

diff --git a/src/runtime/malloc.go b/src/runtime/malloc.go
index bc14d22..fa59ce4 100644
--- a/src/runtime/malloc.go
+++ b/src/runtime/malloc.go
@@ -611,14 +611,6 @@
 	}
 }
 
-func GCcheckmarkenable() {
-	systemstack(gccheckmarkenable_m)
-}
-
-func GCcheckmarkdisable() {
-	systemstack(gccheckmarkdisable_m)
-}
-
 // gctimes records the time in nanoseconds of each phase of the concurrent GC.
 type gctimes struct {
 	sweepterm     int64 // stw

src/runtime/mgc.go

diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index 4d0900a..6d2470d 100644
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -236,10 +236,7 @@
 // When marking an object if the bool checkmark is true one uses the above
 // encoding, otherwise one uses the bitMarked bit in the lower two bits
 // of the nibble.
-var (
-	checkmark         = false
-	gccheckmarkenable = true
-)
+var checkmark = false
 
 // inheap reports whether b is a pointer into a (potentially dead) heap object.
 // It returns false for pointers into stack spans.
@@ -559,7 +556,7 @@
 			continue
 		}
 
-		if mheap_.shadow_enabled && debug.wbshadow >= 2 && gccheckmarkenable && checkmark {
+		if mheap_.shadow_enabled && debug.wbshadow >= 2 && debug.gccheckmark > 0 && checkmark {
 			checkwbshadow((*uintptr)(unsafe.Pointer(b + i)))
 		}
 
@@ -1856,7 +1853,7 @@
 // bitMarked bit that is not set then we throw.
 //go:nowritebarrier
 func gccheckmark_m(startTime int64, eagersweep bool) {
-	if !gccheckmarkenable {
+	if debug.gccheckmark == 0 {
 		return
 	}
 
@@ -1870,16 +1867,6 @@
 }
 
 //go:nowritebarrier
-func gccheckmarkenable_m() {
-	gccheckmarkenable = true
-}
-
-//go:nowritebarrier
-func gccheckmarkdisable_m() {
-	gccheckmarkenable = false
-}
-
-//go:nowritebarrier
 func finishsweep_m() {
 	// The world is stopped so we should be able to complete the sweeps
 	// quickly.
@@ -1987,6 +1974,9 @@
 	}
 
 	if !checkmark {
+		// TODO(austin) This is a noop beceause we should
+		// already have swept everything to the current
+		// sweepgen.
 		finishsweep_m() // skip during checkmark debug phase.
 	}
 
@@ -2107,7 +2097,7 @@
 		sysFree(unsafe.Pointer(&work.spans[0]), uintptr(len(work.spans))*unsafe.Sizeof(work.spans[0]), &memstats.other_sys)
 	}
 
-	if gccheckmarkenable {
+	if debug.gccheckmark > 0 {
 		if !checkmark {
 			// first half of two-pass; don't set up sweep
 			unlock(&mheap_.lock)

src/runtime/runtime1.go

diff --git a/src/runtime/runtime1.go b/src/runtime/runtime1.go
index e6510a8..6056a8d 100644
--- a/src/runtime/runtime1.go
+++ b/src/runtime/runtime1.go
@@ -317,6 +317,7 @@
 	scheddetail    int32
 	schedtrace     int32
 	wbshadow       int32
+	gccheckmark    int32
 }
 
 var dbgvars = []dbgVar{
@@ -329,9 +330,13 @@
 	{"scheddetail", &debug.scheddetail},
 	{"schedtrace", &debug.schedtrace},
 	{"wbshadow", &debug.wbshadow},
+	{"gccheckmark", &debug.gccheckmark},
 }
 
 func parsedebugvars() {
+	// gccheckmark is enabled by default for the 1.5 dev cycle
+	debug.gccheckmark = 1
+
 	for p := gogetenv("GODEBUG"); p != ""; {
 		field := ""
 		i := index(p, ",")