runtime: introduce helper persistentalloc() function

It is a caching wrapper around SysAlloc() that can allocate small chunks.
Use it for symtab allocations. Reduces number of symtab walks from 4 to 3
(reduces buildfuncs time from 10ms to 7.5ms on a large binary,
reduces initial heap size by 680K on the same binary).
Also can be used for type info allocation, itab allocation.
There are also several places in GC where we do the same thing,
they can be changed to use persistentalloc().
Also can be used in FixAlloc, because each instance of FixAlloc allocates
in 128K regions, which is too eager.
Reincarnation of committed and rolled back https://golang.org/cl/9805043
The latent bugs that it revealed are fixed:
https://golang.org/cl/9837049
https://golang.org/cl/9778048

R=golang-dev, khr
CC=golang-dev
https://golang.org/cl/9778049

src/pkg/runtime/malloc.goc

@@ -498,6 +498,53 @@ runtime·MHeap_SysAlloc(MHeap *h, uintptr n)
 	return p;
 }
 
+static struct
+{
+	Lock;
+	byte*	pos;
+	byte*	end;
+} persistent;
+
+enum
+{
+	PersistentAllocChunk	= 256<<10,
+	PersistentAllocMaxBlock	= 64<<10,  // VM reservation granularity is 64K on windows
+};
+
+// Wrapper around SysAlloc that can allocate small chunks.
+// There is no associated free operation.
+// Intended for things like function/type/debug-related persistent data.
+// If align is 0, uses default align (currently 8).
+void*
+runtime·persistentalloc(uintptr size, uintptr align)
+{
+	byte *p;
+
+	if(align) {
+		if(align&(align-1))
+			runtime·throw("persistentalloc: align is now a power of 2");
+		if(align > PageSize)
+			runtime·throw("persistentalloc: align is too large");
+	} else
+		align = 8;
+	if(size >= PersistentAllocMaxBlock)
+		return runtime·SysAlloc(size);
+	runtime·lock(&persistent);
+	persistent.pos = (byte*)ROUND((uintptr)persistent.pos, align);
+	if(persistent.pos + size > persistent.end) {
+		persistent.pos = runtime·SysAlloc(PersistentAllocChunk);
+		if(persistent.pos == nil) {
+			runtime·unlock(&persistent);
+			runtime·throw("runtime: cannot allocate memory");
+		}
+		persistent.end = persistent.pos + PersistentAllocChunk;
+	}
+	p = persistent.pos;
+	persistent.pos += size;
+	runtime·unlock(&persistent);
+	return p; 
+}
+
 static Lock settype_lock;
 
 void

src/pkg/runtime/malloc.h

@@ -447,6 +447,7 @@ void	runtime·MHeap_MapSpans(MHeap *h);
 void	runtime·MHeap_Scavenger(void);
 
 void*	runtime·mallocgc(uintptr size, uint32 flag, int32 dogc, int32 zeroed);
+void*	runtime·persistentalloc(uintptr size, uintptr align);
 int32	runtime·mlookup(void *v, byte **base, uintptr *size, MSpan **s);
 void	runtime·gc(int32 force);
 void	runtime·markallocated(void *v, uintptr n, bool noptr);

src/pkg/runtime/symtab.c

@@ -69,11 +69,6 @@ struct Sym
 
 static uintptr mainoffset;
 
-// A dynamically allocated string containing multiple substrings.
-// Individual strings are slices of hugestring.
-static String hugestring;
-static int32 hugestring_len;
-
 extern void main·main(void);
 
 static uintptr
@@ -303,7 +298,6 @@ makepath(byte *buf, int32 nbuf, byte *path)
 	return p - buf;
 }
 
-// appends p to hugestring
 static String
 gostringn(byte *p, int32 l)
 {
@@ -311,13 +305,8 @@ gostringn(byte *p, int32 l)
 
 	if(l == 0)
 		return runtime·emptystring;
-	if(hugestring.str == nil) {
-		hugestring_len += l;
-		return runtime·emptystring;
-	}
-	s.str = hugestring.str + hugestring.len;
 	s.len = l;
-	hugestring.len += s.len;
+	s.str = runtime·persistentalloc(l, 1);
 	runtime·memmove(s.str, p, l);
 	return s;
 }
@@ -348,8 +337,6 @@ dosrcline(Sym *sym)
 	switch(sym->symtype) {
 	case 't':
 	case 'T':
-		if(hugestring.str == nil)
-			break;
 		if(runtime·strcmp(sym->name, (byte*)"etext") == 0)
 			break;
 		f = &func[nfunc++];
@@ -574,11 +561,12 @@ runtime·symtabinit(void)
 	walksymtab(dofunc);
 
 	// Initialize tables.
-	// Can use FlagNoPointers - all pointers either point into sections of the executable
-	// or point into hugestring.
-	func = runtime·mallocgc((nfunc+1)*sizeof func[0], FlagNoPointers, 0, 1);
+	// Memory obtained from runtime·persistentalloc() is not scanned by GC,
+	// this is fine because all pointers either point into sections of the executable
+	// or also obtained from persistentmalloc().
+	func = runtime·persistentalloc((nfunc+1)*sizeof func[0], 0);
 	func[nfunc].entry = (uint64)etext;
-	fname = runtime·mallocgc(nfname*sizeof fname[0], FlagNoPointers, 0, 1);
+	fname = runtime·persistentalloc(nfname*sizeof fname[0], 0);
 	nfunc = 0;
 	lastvalue = 0;
 	walksymtab(dofunc);
@@ -588,15 +576,9 @@ runtime·symtabinit(void)
 
 	// record src file and line info for each func
 	files = runtime·malloc(maxfiles * sizeof(files[0]));
-	walksymtab(dosrcline);  // pass 1: determine hugestring_len
-	hugestring.str = runtime·mallocgc(hugestring_len, FlagNoPointers, 0, 0);
-	hugestring.len = 0;
-	walksymtab(dosrcline);  // pass 2: fill and use hugestring
+	walksymtab(dosrcline);
 	files = nil;
 
-	if(hugestring.len != hugestring_len)
-		runtime·throw("buildfunc: problem in initialization procedure");
-
 	m->nomemprof--;
 }