runtime: preparation for parallel GC

make MHeap.allspans an array instead on a linked-list,
it's required for parallel for

benchmark                              old ns/op    new ns/op    delta

garbage.BenchmarkTree                  494435529    487962705   -1.31%
garbage.BenchmarkTree-2                499652705    485358000   -2.86%
garbage.BenchmarkTree-4                468482117    454093117   -3.07%
garbage.BenchmarkTree-8                488533235    471872470   -3.41%
garbage.BenchmarkTree-16               507835176    492558470   -3.01%

garbage.BenchmarkTree2                  31453900     31404300   -0.16%
garbage.BenchmarkTree2-2                21440600     21477000   +0.17%
garbage.BenchmarkTree2-4                10982000     11117400   +1.23%
garbage.BenchmarkTree2-8                 7544700      7456700   -1.17%
garbage.BenchmarkTree2-16                7049500      6805700   -3.46%

garbage.BenchmarkParser               4448988000   4453264000   +0.10%
garbage.BenchmarkParser-2             4086045000   4057948000   -0.69%
garbage.BenchmarkParser-4             3677365000   3661246000   -0.44%
garbage.BenchmarkParser-8             3517253000   3540190000   +0.65%
garbage.BenchmarkParser-16            3506562000   3463478000   -1.23%

garbage.BenchmarkTreePause              20969784     21100238   +0.62%
garbage.BenchmarkTreePause-2            20215875     20139572   -0.38%
garbage.BenchmarkTreePause-4            17240709     16683624   -3.23%
garbage.BenchmarkTreePause-8            18196386     17639306   -3.06%
garbage.BenchmarkTreePause-16           20621158     20215056   -1.97%

garbage.BenchmarkTree2Pause            173992142    173872380   -0.07%
garbage.BenchmarkTree2Pause-2          131281904    131366666   +0.06%
garbage.BenchmarkTree2Pause-4           93484952     95109619   +1.74%
garbage.BenchmarkTree2Pause-8           88950523     86533333   -2.72%
garbage.BenchmarkTree2Pause-16          86071238     84089190   -2.30%

garbage.BenchmarkParserPause           135815000    135255952   -0.41%
garbage.BenchmarkParserPause-2          92691523     91451428   -1.34%
garbage.BenchmarkParserPause-4          53392190     51611904   -3.33%
garbage.BenchmarkParserPause-8          36059523     35116666   -2.61%
garbage.BenchmarkParserPause-16         30174300     27340600   -9.39%

garbage.BenchmarkTreeLastPause          28420000     29142000   +2.54%
garbage.BenchmarkTreeLastPause-2        23514000     26779000  +13.89%
garbage.BenchmarkTreeLastPause-4        21773000     18660000  -14.30%
garbage.BenchmarkTreeLastPause-8        24072000     21276000  -11.62%
garbage.BenchmarkTreeLastPause-16       25149000     28541000  +13.49%

garbage.BenchmarkTree2LastPause        314491000    313982000   -0.16%
garbage.BenchmarkTree2LastPause-2      214363000    214715000   +0.16%
garbage.BenchmarkTree2LastPause-4      109778000    111115000   +1.22%
garbage.BenchmarkTree2LastPause-8       75390000     74522000   -1.15%
garbage.BenchmarkTree2LastPause-16      70333000     67880000   -3.49%

garbage.BenchmarkParserLastPause       327247000    326815000   -0.13%
garbage.BenchmarkParserLastPause-2     217039000    212529000   -2.08%
garbage.BenchmarkParserLastPause-4     119722000    111535000   -6.84%
garbage.BenchmarkParserLastPause-8      70806000     69613000   -1.68%
garbage.BenchmarkParserLastPause-16     62813000     48009000  -23.57%

R=rsc, r
CC=golang-dev
https://golang.org/cl/5992055

src/pkg/runtime/malloc.h

@@ -306,7 +306,6 @@ struct MSpan
 {
 	MSpan	*next;		// in a span linked list
 	MSpan	*prev;		// in a span linked list
-	MSpan	*allnext;	// in the list of all spans
 	PageID	start;		// starting page number
 	uintptr	npages;		// number of pages in span
 	MLink	*freelist;	// list of free objects
@@ -351,7 +350,9 @@ struct MHeap
 	Lock;
 	MSpan free[MaxMHeapList];	// free lists of given length
 	MSpan large;			// free lists length >= MaxMHeapList
-	MSpan *allspans;
+	MSpan **allspans;
+	uint32	nspan;
+	uint32	nspancap;
 
 	// span lookup
 	MSpan *map[1<<MHeapMap_Bits];

src/pkg/runtime/mgc0.c

@@ -123,7 +123,7 @@ static struct {
 	Note	alldone;
 	Lock	markgate;
 	Lock	sweepgate;
-	MSpan	*spans;
+	uint32	spanidx;
 
 	Lock;
 	byte	*chunk;
@@ -728,16 +728,18 @@ static void sweepspan(MSpan *s);
 static void
 sweep(void)
 {
-	MSpan *s;
+	MSpan *s, **allspans;
 	int64 now;
+	uint32 spanidx, nspan;
 
 	now = runtime·nanotime();
+	nspan = runtime·mheap.nspan;
+	allspans = runtime·mheap.allspans;
 	for(;;) {
-		s = work.spans;
-		if(s == nil)
+		spanidx = runtime·xadd(&work.spanidx, 1) - 1;
+		if(spanidx >= nspan)
 			break;
-		if(!runtime·casp(&work.spans, s, s->allnext))
-			continue;
+		s = allspans[spanidx];
 
 		// Stamp newly unused spans. The scavenger will use that
 		// info to potentially give back some pages to the OS.
@@ -969,7 +971,7 @@ runtime·gc(int32 force)
 		mark(debug_scanblock);
 	t1 = runtime·nanotime();
 
-	work.spans = runtime·mheap.allspans;
+	work.spanidx = 0;
 	runtime·unlock(&work.sweepgate);  // let the helpers in
 	sweep();
 	if(work.nproc > 1)

src/pkg/runtime/mheap.c

@@ -27,11 +27,24 @@ RecordSpan(void *vh, byte *p)
 {
 	MHeap *h;
 	MSpan *s;
+	MSpan **all;
+	uint32 cap;
 
 	h = vh;
 	s = (MSpan*)p;
-	s->allnext = h->allspans;
-	h->allspans = s;
+	if(h->nspan >= h->nspancap) {
+		cap = 64*1024/sizeof(all[0]);
+		if(cap < h->nspancap*3/2)
+			cap = h->nspancap*3/2;
+		all = (MSpan**)runtime·SysAlloc(cap*sizeof(all[0]));
+		if(h->allspans) {
+			runtime·memmove(all, h->allspans, h->nspancap*sizeof(all[0]));
+			runtime·SysFree(h->allspans, h->nspancap*sizeof(all[0]));
+		}
+		h->allspans = all;
+		h->nspancap = cap;
+	}
+	h->allspans[h->nspan++] = s;
 }
 
 // Initialize the heap; fetch memory using alloc.