cmd/gc: make liveness ~10x faster

1) The arrayindexof lookup function is O(n). Replace with O(1) lookups.

2) The checkptxt function is O(n²) and is purely for debugging.
Only run when the debugging flags are turned on.

3) Iterating over sparse bitmaps can be done faster word by word.
Introduce and use bvnext for that.

Run times before and after, on my 2.5 GHz Core i5 MacBook Pro.

x.go       9.48  0.84  issue 8259

x100.go    0.01  0.01  issue 8354
x1000.go   0.10  0.10
x2000.go   0.62  0.19
x3000.go   1.33  0.34
x4000.go   2.29  0.49
x5000.go   3.89  0.67
x6000.go   5.00  0.90
x7000.go   6.70  1.13
x8000.go   9.44  1.38
x9000.go  11.23  1.87
x10000.go 13.78  2.09

Fixes #8259.
Fixes #8354.

LGTM=iant, r
R=golang-codereviews, iant, r
CC=golang-codereviews
https://golang.org/cl/125720043

src/cmd/gc/array.c

@@ -108,20 +108,6 @@ arrayadd(Array *array, void *element)
 	arrayset(array, array->length - 1, element);
 }
 
-int32
-arrayindexof(Array *array, void *element)
-{
-	void *p;
-	int32 i;
-
-	for(i = 0; i < array->length; i++) {
-		p = arrayget(array, i);
-		if(memcmp(p, &element, array->size) == 0)
-			return i;
-	}
-	return -1;
-}
-
 void
 arraysort(Array *array, int (*cmp)(const void*, const void*))
 {

src/cmd/gc/bv.c

@@ -9,6 +9,8 @@
 enum {
 	WORDSIZE = sizeof(uint32),
 	WORDBITS = 32,
+	WORDMASK = WORDBITS - 1,
+	WORDSHIFT = 5,
 };
 
 static uintptr
@@ -94,13 +96,35 @@ bvconcat(Bvec *src1, Bvec *src2)
 int
 bvget(Bvec *bv, int32 i)
 {
-	uint32 mask, word;
-
 	if(i < 0 || i >= bv->n)
 		fatal("bvget: index %d is out of bounds with length %d\n", i, bv->n);
-	mask = 1U << (i % WORDBITS);
-	word = bv->b[i / WORDBITS] & mask;
-	return word ? 1 : 0;
+	return (bv->b[i>>WORDSHIFT] >> (i&WORDMASK)) & 1;
+}
+
+// bvnext returns the smallest index >= i for which bvget(bv, i) == 1.
+// If there is no such index, bvnext returns -1.
+int
+bvnext(Bvec *bv, int32 i)
+{
+	uint32 w;
+
+	// Jump i ahead to next word with bits.
+	if((bv->b[i>>WORDSHIFT]>>(i&WORDMASK)) == 0) {
+		i &= ~WORDMASK;
+		i += WORDBITS;
+		while(i < bv->n && bv->b[i>>WORDSHIFT] == 0)
+			i += WORDBITS;
+	}
+	if(i >= bv->n)
+		return -1;
+	
+	// Find 1 bit.
+	w = bv->b[i>>WORDSHIFT]>>(i&WORDMASK);
+	while((w&1) == 0) {
+		w>>=1;
+		i++;
+	}
+	return i;
 }
 
 int
@@ -109,7 +133,7 @@ bvisempty(Bvec *bv)
 	int32 i;
 
 	for(i = 0; i < bv->n; i += WORDBITS)
-		if(bv->b[i / WORDBITS] != 0)
+		if(bv->b[i>>WORDSHIFT] != 0)
 			return 0;
 	return 1;
 }

src/cmd/gc/go.h

@@ -1017,7 +1017,6 @@ int32	arraylength(Array *array);
 void*	arrayget(Array *array, int32 index);
 void	arrayset(Array *array, int32 index, void *element);
 void	arrayadd(Array *array, void *element);
-int32	arrayindexof(Array* array, void *element);
 void	arraysort(Array* array, int (*cmp)(const void*, const void*));
 
 /*
@@ -1043,6 +1042,7 @@ int	bvcmp(Bvec *bv1, Bvec *bv2);
 void	bvcopy(Bvec *dst, Bvec *src);
 Bvec*	bvconcat(Bvec *src1, Bvec *src2);
 int	bvget(Bvec *bv, int32 i);
+int32	bvnext(Bvec *bv, int32 i);
 int	bvisempty(Bvec *bv);
 void	bvnot(Bvec *bv);
 void	bvor(Bvec *dst, Bvec *src1, Bvec *src2);

src/cmd/gc/plive.c

@@ -283,13 +283,30 @@ getvariables(Node *fn)
 			// For arguments and results, the bitmap covers all variables,
 			// so we must include all the variables, even the ones without
 			// pointers.
+			//
+			// The Node.opt field is available for use by optimization passes.
+			// We use it to hold the index of the node in the variables array, plus 1
+			// (so that 0 means the Node is not in the variables array).
+			// Each pass should clear opt when done, but you never know,
+			// so clear them all ourselves too.
+			// The Node.curfn field is supposed to be set to the current function
+			// already, but for some compiler-introduced names it seems not to be,
+			// so fix that here.
+			// Later, when we want to find the index of a node in the variables list,
+			// we will check that n->curfn == curfn and n->opt > 0. Then n->opt - 1
+			// is the index in the variables list.
+			ll->n->opt = nil;
+			ll->n->curfn = curfn;
 			switch(ll->n->class) {
 			case PAUTO:
-				if(haspointers(ll->n->type))
+				if(haspointers(ll->n->type)) {
+					ll->n->opt = (void*)(uintptr)(arraylength(result)+1);
 					arrayadd(result, &ll->n);
+				}
 				break;
 			case PPARAM:
 			case PPARAMOUT:
+				ll->n->opt = (void*)(uintptr)(arraylength(result)+1);
 				arrayadd(result, &ll->n);
 				break;
 			}
@@ -718,14 +735,16 @@ progeffects(Prog *prog, Array *vars, Bvec *uevar, Bvec *varkill, Bvec *avarinit)
 	}
 	if(info.flags & (LeftRead | LeftWrite | LeftAddr)) {
 		from = &prog->from;
-		if (from->node != nil && from->sym != nil) {
+		if (from->node != nil && from->sym != nil && from->node->curfn == curfn) {
 			switch(from->node->class & ~PHEAP) {
 			case PAUTO:
 			case PPARAM:
 			case PPARAMOUT:
-				pos = arrayindexof(vars, from->node);
+				pos = (int)(uintptr)from->node->opt - 1; // index in vars
 				if(pos == -1)
 					goto Next;
+				if(pos >= arraylength(vars) || *(Node**)arrayget(vars, pos) != from->node)
+					fatal("bad bookkeeping in liveness %N %d", from->node, pos);
 				if(from->node->addrtaken) {
 					bvset(avarinit, pos);
 				} else {
@@ -741,14 +760,16 @@ progeffects(Prog *prog, Array *vars, Bvec *uevar, Bvec *varkill, Bvec *avarinit)
 Next:
 	if(info.flags & (RightRead | RightWrite | RightAddr)) {
 		to = &prog->to;
-		if (to->node != nil && to->sym != nil) {
+		if (to->node != nil && to->sym != nil && to->node->curfn == curfn) {
 			switch(to->node->class & ~PHEAP) {
 			case PAUTO:
 			case PPARAM:
 			case PPARAMOUT:
-				pos = arrayindexof(vars, to->node);
+				pos = (int)(uintptr)to->node->opt - 1; // index in vars
 				if(pos == -1)
 					goto Next1;
+				if(pos >= arraylength(vars) || *(Node**)arrayget(vars, pos) != to->node)
+					fatal("bad bookkeeping in liveness %N %d", to->node, pos);
 				if(to->node->addrtaken) {
 					if(prog->as != AVARKILL)
 						bvset(avarinit, pos);
@@ -1020,6 +1041,9 @@ checkptxt(Node *fn, Prog *firstp)
 {
 	Prog *p;
 
+	if(debuglive == 0)
+		return;
+
 	for(p = firstp; p != P; p = p->link) {
 		if(0)
 			print("analyzing '%P'\n", p);
@@ -1172,21 +1196,17 @@ twobitlivepointermap(Liveness *lv, Bvec *liveout, Array *vars, Bvec *args, Bvec
 	vlong xoffset;
 	int32 i;
 
-	for(i = 0; i < arraylength(vars); i++) {
+	for(i = 0; (i = bvnext(liveout, i)) >= 0; i++) {
 		node = *(Node**)arrayget(vars, i);
 		switch(node->class) {
 		case PAUTO:
-			if(bvget(liveout, i)) {
-				xoffset = node->xoffset + stkptrsize;
-				twobitwalktype1(node->type, &xoffset, locals);
-			}
+			xoffset = node->xoffset + stkptrsize;
+			twobitwalktype1(node->type, &xoffset, locals);
 			break;
 		case PPARAM:
 		case PPARAMOUT:
-			if(bvget(liveout, i)) {
-				xoffset = node->xoffset;
-				twobitwalktype1(node->type, &xoffset, args);
-			}
+			xoffset = node->xoffset;
+			twobitwalktype1(node->type, &xoffset, args);
 			break;
 		}
 	}
@@ -1944,6 +1964,7 @@ liveness(Node *fn, Prog *firstp, Sym *argssym, Sym *livesym)
 	Array *cfg, *vars;
 	Liveness *lv;
 	int debugdelta;
+	NodeList *l;
 
 	// Change name to dump debugging information only for a specific function.
 	debugdelta = 0;
@@ -1984,6 +2005,9 @@ liveness(Node *fn, Prog *firstp, Sym *argssym, Sym *livesym)
 	twobitwritesymbol(lv->argslivepointers, argssym);
 
 	// Free everything.
+	for(l=fn->dcl; l != nil; l = l->next)
+		if(l->n != N)
+			l->n->opt = nil;
 	freeliveness(lv);
 	arrayfree(vars);
 	freecfg(cfg);