From f7d740377612142c6a08e460b5c542ebe0d758b3 Mon Sep 17 00:00:00 2001
From: Austin Clements <[email protected]>
Date: Wed, 26 Aug 2015 13:54:26 -0400
Subject: [PATCH 10/63] [release-branch.go1.5] 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/13947
Reviewed-by: Russ Cox <[email protected]>
Reviewed-on: https://go-review.googlesource.com/14240
Reviewed-by: Austin Clements <[email protected]>
---
 src/runtime/extern.go   |  3 +++
 src/runtime/mgc.go      | 25 +++++++++++++------------
 src/runtime/mgcmark.go  |  2 +-
 src/runtime/runtime1.go |  6 ++++++
 4 files changed, 23 insertions(+), 13 deletions(-)

diff --git a/src/runtime/extern.go b/src/runtime/extern.go
index d346362..cdb66ba 100644
--- a/src/runtime/extern.go
+++ b/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.
diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index f7fd4e5..82b12b6 100644
--- a/src/runtime/mgc.go
+++ b/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.
 //
diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index 42aacb6..2e43830 100644
--- a/src/runtime/mgcmark.go
+++ b/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 {
diff --git a/src/runtime/runtime1.go b/src/runtime/runtime1.go
index a50e5b6..134c999 100644
--- a/src/runtime/runtime1.go
+++ b/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.
-- 
2.6.1