cmd/gc, cmd/ld, runtime: compact liveness bitmaps

Reduce footprint of liveness bitmaps by about 5x.

1. Mark all liveness bitmap symbols as 4-byte aligned
(they were aligned to a larger size by default).

2. The bitmap data is a bitmap count n followed by n bitmaps.
Each bitmap begins with its own count m giving the number
of bits. All the m's are the same for the n bitmaps.
Emit this bitmap length once instead of n times.

3. Many bitmaps within a function have the same bit values,
but each call site was given a distinct bitmap. Merge duplicate
bitmaps so that no bitmap is written more than once.

4. Many functions end up with the same aggregate bitmap data.
We used to name the bitmap data funcname.gcargs and funcname.gclocals.
Instead, name it gclocals.<md5 of data> and mark it dupok so
that the linker coalesces duplicate sets. This cut the bitmap
data remaining after step 3 by 40%; I was not expecting it to
be quite so dramatic.

Applied to "go build -ldflags -w code.google.com/p/go.tools/cmd/godoc":

                bitmaps           pclntab           binary on disk
before this CL  1326600           1985854           12738268
4-byte align    1154288 (0.87x)   1985854 (1.00x)   12566236 (0.99x)
one bitmap len   782528 (0.54x)   1985854 (1.00x)   12193500 (0.96x)
dedup bitmap     414748 (0.31x)   1948478 (0.98x)   11787996 (0.93x)
dedup bitmap set 245580 (0.19x)   1948478 (0.98x)   11620060 (0.91x)

While here, remove various dead blocks of code from plive.c.

Fixes #6929.
Fixes #7568.

LGTM=khr
R=khr
CC=golang-codereviews
https://golang.org/cl/83630044

4 files changed, 28 insertions, 39 deletions

diff --git a/src/pkg/runtime/heapdump.c b/src/pkg/runtime/heapdump.c
index bc0fd49c0a..f9bc4e559f 100644
--- a/src/pkg/runtime/heapdump.c
+++ b/src/pkg/runtime/heapdump.c
@@ -244,7 +244,7 @@ struct ChildInfo {
 	// the layout of the outargs region.
 	uintptr argoff;     // where the arguments start in the frame
 	uintptr arglen;     // size of args region
-	BitVector *args;    // if not nil, pointer map of args region
+	BitVector args;    // if args.n >= 0, pointer map of args region
 
 	byte *sp;           // callee sp
 	uintptr depth;      // depth in call stack (0 == most recent)
@@ -301,7 +301,7 @@ dumpframe(Stkframe *s, void *arg)
 	int32 pcdata;
 	StackMap *stackmap;
 	int8 *name;
-	BitVector *bv;
+	BitVector bv;
 
 	child = (ChildInfo*)arg;
 	f = s->fn;
@@ -320,13 +320,13 @@ dumpframe(Stkframe *s, void *arg)
 	stackmap = runtime·funcdata(f, FUNCDATA_LocalsPointerMaps);
 
 	// Dump any types we will need to resolve Efaces.
-	if(child->args != nil)
-		dumpbvtypes(child->args, (byte*)s->sp + child->argoff);
+	if(child->args.n >= 0)
+		dumpbvtypes(&child->args, (byte*)s->sp + child->argoff);
 	if(stackmap != nil && stackmap->n > 0) {
 		bv = runtime·stackmapdata(stackmap, pcdata);
-		dumpbvtypes(bv, s->varp - bv->n / BitsPerPointer * PtrSize);
+		dumpbvtypes(&bv, s->varp - bv.n / BitsPerPointer * PtrSize);
 	} else {
-		bv = nil;
+		bv.n = -1;
 	}
 
 	// Dump main body of stack frame.
@@ -343,8 +343,8 @@ dumpframe(Stkframe *s, void *arg)
 	dumpcstr(name);
 
 	// Dump fields in the outargs section
-	if(child->args != nil) {
-		dumpbv(child->args, child->argoff);
+	if(child->args.n >= 0) {
+		dumpbv(&child->args, child->argoff);
 	} else {
 		// conservative - everything might be a pointer
 		for(off = child->argoff; off < child->argoff + child->arglen; off += PtrSize) {
@@ -370,7 +370,7 @@ dumpframe(Stkframe *s, void *arg)
 	} else if(stackmap->n > 0) {
 		// Locals bitmap information, scan just the pointers in
 		// locals.
-		dumpbv(bv, s->varp - bv->n / BitsPerPointer * PtrSize - (byte*)s->sp);
+		dumpbv(&bv, s->varp - bv.n / BitsPerPointer * PtrSize - (byte*)s->sp);
 	}
 	dumpint(FieldKindEol);
 
@@ -383,7 +383,7 @@ dumpframe(Stkframe *s, void *arg)
 	if(stackmap != nil)
 		child->args = runtime·stackmapdata(stackmap, pcdata);
 	else
-		child->args = nil;
+		child->args.n = -1;
 	return true;
 }
 
@@ -421,7 +421,7 @@ dumpgoroutine(G *gp)
 	dumpint((uintptr)gp->panic);
 
 	// dump stack
-	child.args = nil;
+	child.args.n = -1;
 	child.arglen = 0;
 	child.sp = nil;
 	child.depth = 0;
diff --git a/src/pkg/runtime/malloc.h b/src/pkg/runtime/malloc.h
index 4b9cbc9787..9d478f82c1 100644
--- a/src/pkg/runtime/malloc.h
+++ b/src/pkg/runtime/malloc.h
@@ -602,12 +602,13 @@ typedef struct BitVector BitVector;
 struct BitVector
 {
 	int32 n; // # of bits
-	uint32 data[];
+	uint32 *data;
 };
 typedef struct StackMap StackMap;
 struct StackMap
 {
-	int32 n;
+	int32 n; // number of bitmaps
+	int32 nbit; // number of bits in each bitmap
 	uint32 data[];
 };
 enum {
@@ -626,7 +627,7 @@ enum {
 };
 // Returns pointer map data for the given stackmap index
 // (the index is encoded in PCDATA_StackMapIndex).
-BitVector*	runtime·stackmapdata(StackMap *stackmap, int32 n);
+BitVector	runtime·stackmapdata(StackMap *stackmap, int32 n);
 
 // defined in mgc0.go
 void	runtime·gc_m_ptr(Eface*);
@@ -636,4 +637,4 @@ void	runtime·memorydump(void);
 int32	runtime·setgcpercent(int32);
 
 // Value we use to mark dead pointers when GODEBUG=gcdead=1.
-#define PoisonPtr ((uintptr)0x6969696969696969LL)
+#define PoisonPtr ((uintptr)0xf9696969f9696969LL)
diff --git a/src/pkg/runtime/mgc0.c b/src/pkg/runtime/mgc0.c
index 26382f4142..d6eedfaa93 100644
--- a/src/pkg/runtime/mgc0.c
+++ b/src/pkg/runtime/mgc0.c
@@ -1403,24 +1403,12 @@ handoff(Workbuf *b)
 
 extern byte pclntab[]; // base for f->ptrsoff
 
-BitVector*
+BitVector
 runtime·stackmapdata(StackMap *stackmap, int32 n)
 {
-	BitVector *bv;
-	uint32 *ptr;
-	uint32 words;
-	int32 i;
-
-	if(n < 0 || n >= stackmap->n) {
+	if(n < 0 || n >= stackmap->n)
 		runtime·throw("stackmapdata: index out of range");
-	}
-	ptr = stackmap->data;
-	for(i = 0; i < n; i++) {
-		bv = (BitVector*)ptr;
-		words = ((bv->n + 31) / 32) + 1;
-		ptr += words;
-	}
-	return (BitVector*)ptr;
+	return (BitVector){stackmap->nbit, stackmap->data + n*((stackmap->nbit+31)/32)};
 }
 
 // Scans an interface data value when the interface type indicates
@@ -1533,7 +1521,7 @@ scanframe(Stkframe *frame, void *wbufp)
 {
 	Func *f;
 	StackMap *stackmap;
-	BitVector *bv;
+	BitVector bv;
 	uintptr size;
 	uintptr targetpc;
 	int32 pcdata;
@@ -1576,9 +1564,9 @@ scanframe(Stkframe *frame, void *wbufp)
 				runtime·throw("scanframe: bad symbol table");
 			}
 			bv = runtime·stackmapdata(stackmap, pcdata);
-			size = (bv->n * PtrSize) / BitsPerPointer;
+			size = (bv.n * PtrSize) / BitsPerPointer;
 			precise = true;
-			scanbitvector(f, true, frame->varp - size, bv, afterprologue, wbufp);
+			scanbitvector(f, true, frame->varp - size, &bv, afterprologue, wbufp);
 		}
 	}
 
@@ -1587,7 +1575,7 @@ scanframe(Stkframe *frame, void *wbufp)
 	stackmap = runtime·funcdata(f, FUNCDATA_ArgsPointerMaps);
 	if(stackmap != nil) {
 		bv = runtime·stackmapdata(stackmap, pcdata);
-		scanbitvector(f, precise, frame->argp, bv, true, wbufp);
+		scanbitvector(f, precise, frame->argp, &bv, true, wbufp);
 	} else
 		enqueue1(wbufp, (Obj){frame->argp, frame->arglen, 0});
 	return true;
diff --git a/src/pkg/runtime/stack.c b/src/pkg/runtime/stack.c
index f48b76c733..2c5e052365 100644
--- a/src/pkg/runtime/stack.c
+++ b/src/pkg/runtime/stack.c
@@ -436,7 +436,7 @@ adjustframe(Stkframe *frame, void *arg)
 	Func *f;
 	StackMap *stackmap;
 	int32 pcdata;
-	BitVector *bv;
+	BitVector bv;
 	uintptr targetpc;
 
 	adjinfo = arg;
@@ -462,7 +462,7 @@ adjustframe(Stkframe *frame, void *arg)
 		bv = runtime·stackmapdata(stackmap, pcdata);
 		if(StackDebug >= 3)
 			runtime·printf("      locals\n");
-		adjustpointers((byte**)frame->varp - bv->n / BitsPerPointer, bv, adjinfo, f);
+		adjustpointers((byte**)frame->varp - bv.n / BitsPerPointer, &bv, adjinfo, f);
 	}
 	// adjust inargs and outargs
 	if(frame->arglen != 0) {
@@ -472,7 +472,7 @@ adjustframe(Stkframe *frame, void *arg)
 		bv = runtime·stackmapdata(stackmap, pcdata);
 		if(StackDebug >= 3)
 			runtime·printf("      args\n");
-		adjustpointers((byte**)frame->argp, bv, adjinfo, nil);
+		adjustpointers((byte**)frame->argp, &bv, adjinfo, nil);
 	}
 	return true;
 }
@@ -491,7 +491,7 @@ adjustdefers(G *gp, AdjustInfo *adjinfo)
 	Func *f;
 	FuncVal *fn;
 	StackMap *stackmap;
-	BitVector *bv;
+	BitVector bv;
 
 	for(dp = &gp->defer, d = *dp; d != nil; dp = &d->link, d = *dp) {
 		if(adjinfo->oldstk <= (byte*)d && (byte*)d < adjinfo->oldbase) {
@@ -526,7 +526,7 @@ adjustdefers(G *gp, AdjustInfo *adjinfo)
 			if(stackmap == nil)
 				runtime·throw("runtime: deferred function has no arg ptr map");
 			bv = runtime·stackmapdata(stackmap, 0);
-			adjustpointers(d->args, bv, adjinfo, f);
+			adjustpointers(d->args, &bv, adjinfo, f);
 		}
 		d->argp += adjinfo->delta;
 	}