runtime: add GODEBUG for stack barriers at every frame

Currently enabling the debugging mode where stack barriers are
installed at every frame requires recompiling the runtime. However,
this is potentially useful for field debugging and for runtime tests,
so make this mode a GODEBUG.

Updates #12238.

Change-Id: I6fb128f598b19568ae723a612e099c0ed96917f5
Reviewed-on: https://go-review.googlesource.com/13947Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/extern.go

@@ -47,6 +47,9 @@ It is a comma-separated list of name=val pairs setting these named variables:
 	that allow the garbage collector to avoid repeating a stack scan during the
 	mark termination phase.
 
+	gcstackbarrierall: setting gcstackbarrierall=1 installs stack barriers
+	in every stack frame, rather than in exponentially-spaced frames.
+
 	gcstoptheworld: setting gcstoptheworld=1 disables concurrent garbage collection,
 	making every garbage collection a stop-the-world event. Setting gcstoptheworld=2
 	also disables concurrent sweeping after the garbage collection finishes.

src/runtime/mgc.go

@@ -133,18 +133,7 @@ const (
 	_RootFlushCaches = 4
 	_RootCount       = 5
 
-	// firstStackBarrierOffset is the approximate byte offset at
-	// which to place the first stack barrier from the current SP.
-	// This is a lower bound on how much stack will have to be
-	// re-scanned during mark termination. Subsequent barriers are
-	// placed at firstStackBarrierOffset * 2^n offsets.
-	//
-	// For debugging, this can be set to 0, which will install a
-	// stack barrier at every frame. If you do this, you may also
-	// have to raise _StackMin, since the stack barrier
-	// bookkeeping will use a large amount of each stack.
-	firstStackBarrierOffset = 1024
-	debugStackBarrier       = false
+	debugStackBarrier = false
 
 	// sweepMinHeapDistance is a lower bound on the heap distance
 	// (in bytes) reserved for concurrent sweeping between GC
@@ -152,6 +141,18 @@ const (
 	sweepMinHeapDistance = 1024 * 1024
 )
 
+// firstStackBarrierOffset is the approximate byte offset at
+// which to place the first stack barrier from the current SP.
+// This is a lower bound on how much stack will have to be
+// re-scanned during mark termination. Subsequent barriers are
+// placed at firstStackBarrierOffset * 2^n offsets.
+//
+// For debugging, this can be set to 0, which will install a
+// stack barrier at every frame. If you do this, you may also
+// have to raise _StackMin, since the stack barrier
+// bookkeeping will use a large amount of each stack.
+var firstStackBarrierOffset = 1024
+
 // heapminimum is the minimum heap size at which to trigger GC.
 // For small heaps, this overrides the usual GOGC*live set rule.
 //

src/runtime/mgcmark.go

@@ -341,7 +341,7 @@ func scanstack(gp *g) {
 	switch gcphase {
 	case _GCscan:
 		// Install stack barriers during stack scan.
-		barrierOffset = firstStackBarrierOffset
+		barrierOffset = uintptr(firstStackBarrierOffset)
 		nextBarrier = sp + barrierOffset
 
 		if debug.gcstackbarrieroff > 0 {

src/runtime/runtime1.go

@@ -310,6 +310,7 @@ var debug struct {
 	gcpacertrace      int32
 	gcshrinkstackoff  int32
 	gcstackbarrieroff int32
+	gcstackbarrierall int32
 	gcstoptheworld    int32
 	gctrace           int32
 	invalidptr        int32
@@ -327,6 +328,7 @@ var dbgvars = []dbgVar{
 	{"gcpacertrace", &debug.gcpacertrace},
 	{"gcshrinkstackoff", &debug.gcshrinkstackoff},
 	{"gcstackbarrieroff", &debug.gcstackbarrieroff},
+	{"gcstackbarrierall", &debug.gcstackbarrierall},
 	{"gcstoptheworld", &debug.gcstoptheworld},
 	{"gctrace", &debug.gctrace},
 	{"invalidptr", &debug.invalidptr},
@@ -382,6 +384,10 @@ func parsedebugvars() {
 	if islibrary || isarchive {
 		traceback_cache |= 1
 	}
+
+	if debug.gcstackbarrierall > 0 {
+		firstStackBarrierOffset = 0
+	}
 }
 
 // Poor mans 64-bit division.