runtime: eliminate allspans snapshot

Now that sweeping and span marking use the sweep list, there's no need
for the work.spans snapshot of the allspans list. This change
eliminates the few remaining uses of it, which are either dead code or
can use allspans directly, and removes work.spans and its support
functions.

Change-Id: Id5388b42b1e68e8baee853d8eafb8bb4ff95bb43
Reviewed-on: https://go-review.googlesource.com/30537
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -815,11 +815,6 @@ var work struct {
 	// mode is the concurrency mode of the current GC cycle.
 	mode gcMode
 
-	// Snapshot of mheap.allspans for marker or sweeper created by
-	// gcCopySpans. Unlike allspans, this does not change except
-	// at controlled GC transition points.
-	spans []*mspan
-
 	// totaltime is the CPU nanoseconds spent in GC since the
 	// program started if debug.gctrace > 0.
 	totaltime int64
@@ -981,7 +976,7 @@ func gcStart(mode gcMode, forceTrigger bool) {
 	systemstack(stopTheWorldWithSema)
 	// Finish sweep before we start concurrent scan.
 	systemstack(func() {
-		finishsweep_m(true)
+		finishsweep_m()
 	})
 	// clearpools before we start the GC. If we wait they memory will not be
 	// reclaimed until the next GC cycle.
@@ -1018,10 +1013,6 @@ func gcStart(mode gcMode, forceTrigger bool) {
 		// possible.
 		setGCPhase(_GCmark)
 
-		// markrootSpans uses work.spans, so make sure
-		// it is up to date.
-		gcCopySpans()
-
 		gcBgMarkPrepare() // Must happen before assist enable.
 		gcMarkRootPrepare()
 
@@ -1222,7 +1213,7 @@ func gcMarkTermination() {
 			// they have gcscanvalid==true and gcworkdone==true.
 			// Reset these so that all stacks will be rescanned.
 			gcResetMarkState()
-			finishsweep_m(true)
+			finishsweep_m()
 
 			// Still in STW but gcphase is _GCoff, reset to _GCmarktermination
 			// At this point all objects will be found during the gcMark which
@@ -1565,8 +1556,6 @@ func gcMark(start_time int64) {
 	}
 	work.tstart = start_time
 
-	gcCopySpans() // TODO(rlh): should this be hoisted and done only once? Right now it is done for normal marking and also for checkmarking.
-
 	// Queue root marking jobs.
 	gcMarkRootPrepare()
 
@@ -1679,7 +1668,6 @@ func gcSweep(mode gcMode) {
 	if gcphase != _GCoff {
 		throw("gcSweep being done but phase is not GCoff")
 	}
-	gcCopySpans()
 
 	lock(&mheap_.lock)
 	mheap_.sweepgen += 2
@@ -1735,24 +1723,6 @@ func gcSweep(mode gcMode) {
 	unlock(&sweep.lock)
 }
 
-func gcCopySpans() {
-	// Cache runtime.mheap_.allspans in work.spans to avoid conflicts with
-	// resizing/freeing allspans.
-	// New spans can be created while GC progresses, but they are not garbage for
-	// this round:
-	//  - new stack spans can be created even while the world is stopped.
-	//  - new malloc spans can be created during the concurrent sweep
-	// Even if this is stop-the-world, a concurrent exitsyscall can allocate a stack from heap.
-	lock(&mheap_.lock)
-	// Free the old cached mark array if necessary.
-	if work.spans != nil && &work.spans[0] != &mheap_.allspans[0] {
-		sysFree(unsafe.Pointer(&work.spans[0]), uintptr(len(work.spans))*unsafe.Sizeof(work.spans[0]), &memstats.other_sys)
-	}
-	// Cache the current array for sweeping.
-	work.spans = mheap_.allspans
-	unlock(&mheap_.lock)
-}
-
 // gcResetMarkState resets global state prior to marking (concurrent
 // or STW) and resets the stack scan state of all Gs.
 //

src/runtime/mgcmark.go

@@ -1421,7 +1421,7 @@ var useCheckmark = false
 //go:nowritebarrier
 func initCheckmarks() {
 	useCheckmark = true
-	for _, s := range work.spans {
+	for _, s := range mheap_.allspans {
 		if s.state == _MSpanInUse {
 			heapBitsForSpan(s.base()).initCheckmarkSpan(s.layout())
 		}
@@ -1430,7 +1430,7 @@ func initCheckmarks() {
 
 func clearCheckmarks() {
 	useCheckmark = false
-	for _, s := range work.spans {
+	for _, s := range mheap_.allspans {
 		if s.state == _MSpanInUse {
 			heapBitsForSpan(s.base()).clearCheckmarkSpan(s.layout())
 		}

src/runtime/mgcsweep.go

@@ -28,8 +28,13 @@ type sweepdata struct {
 	pacertracegen uint32
 }
 
+// finishsweep_m ensures that all spans are swept.
+//
+// The world must be stopped. This ensures there are no sweeps in
+// progress.
+//
 //go:nowritebarrier
-func finishsweep_m(stw bool) {
+func finishsweep_m() {
 	// Sweeping must be complete before marking commences, so
 	// sweep any unswept spans. If this is a concurrent GC, there
 	// shouldn't be any spans left to sweep, so this should finish
@@ -39,20 +44,6 @@ func finishsweep_m(stw bool) {
 		sweep.npausesweep++
 	}
 
-	// There may be some other spans being swept concurrently that
-	// we need to wait for. If finishsweep_m is done with the world stopped
-	// this is not required because the STW must have waited for sweeps.
-	//
-	// TODO(austin): As of this writing, we always pass true for stw.
-	// Consider removing this code.
-	if !stw {
-		sg := mheap_.sweepgen
-		for _, s := range work.spans {
-			if s.sweepgen != sg && s.state == _MSpanInUse {
-				s.ensureSwept()
-			}
-		}
-	}
 	nextMarkBitArenaEpoch()
 }
 

src/runtime/mheap.go

@@ -285,9 +285,7 @@ func recordspan(vh unsafe.Pointer, p unsafe.Pointer) {
 		}
 		oldAllspans := h.allspans
 		h.allspans = new
-		// Don't free the old array if it's referenced by sweep.
-		// See the comment in mgc.go.
-		if len(oldAllspans) != 0 && &oldAllspans[0] != &work.spans[0] {
+		if len(oldAllspans) != 0 {
 			sysFree(unsafe.Pointer(&oldAllspans[0]), uintptr(cap(oldAllspans))*unsafe.Sizeof(oldAllspans[0]), &memstats.other_sys)
 		}
 	}