runtime: smarter slice grow

When growing slice take into account size of the allocated memory block.
Also apply the same optimization to string->[]byte conversion.
Fixes #6307.

benchmark                    old ns/op    new ns/op    delta
BenchmarkAppendGrowByte        4541036      4434108   -2.35%
BenchmarkAppendGrowString     59885673     44813604  -25.17%

LGTM=khr
R=khr
CC=golang-codereviews, iant, rsc
https://golang.org/cl/53340044

src/pkg/runtime/append_test.go

@@ -19,6 +19,25 @@ func BenchmarkAppend(b *testing.B) {
 	}
 }
 
+func BenchmarkAppendGrowByte(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		var x []byte
+		for j := 0; j < 1<<20; j++ {
+			x = append(x, byte(j))
+		}
+	}
+}
+
+func BenchmarkAppendGrowString(b *testing.B) {
+	var s string
+	for i := 0; i < b.N; i++ {
+		var x []string
+		for j := 0; j < 1<<20; j++ {
+			x = append(x, s)
+		}
+	}
+}
+
 func benchmarkAppendBytes(b *testing.B, length int) {
 	b.StopTimer()
 	x := make([]byte, 0, N)

src/pkg/runtime/malloc.h

@@ -273,6 +273,7 @@ extern MStats mstats;
 //	making new objects in class i
 
 int32	runtime·SizeToClass(int32);
+uintptr	runtime·roundupsize(uintptr);
 extern	int32	runtime·class_to_size[NumSizeClasses];
 extern	int32	runtime·class_to_allocnpages[NumSizeClasses];
 extern	int8	runtime·size_to_class8[1024/8 + 1];

src/pkg/runtime/msize.c

@@ -162,3 +162,18 @@ dump:
 	}
 	runtime·throw("InitSizes failed");
 }
+
+// Returns size of the memory block that mallocgc will allocate if you ask for the size.
+uintptr
+runtime·roundupsize(uintptr size)
+{
+	if(size < MaxSmallSize) {
+		if(size <= 1024-8)
+			return runtime·class_to_size[runtime·size_to_class8[(size+7)>>3]];
+		else
+			return runtime·class_to_size[runtime·size_to_class128[(size-1024+127) >> 7]];
+	}
+	if(size + PageSize < size)
+		return size;
+	return ROUND(size, PageSize);
+}

src/pkg/runtime/slice.c

@@ -8,6 +8,7 @@
 #include "typekind.h"
 #include "malloc.h"
 #include "race.h"
+#include "stack.h"
 #include "../../cmd/ld/textflag.h"
 
 enum
@@ -92,26 +93,53 @@ runtime·growslice(SliceType *t, Slice old, int64 n, Slice ret)
 static void
 growslice1(SliceType *t, Slice x, intgo newcap, Slice *ret)
 {
-	intgo m;
+	intgo newcap1;
+	uintptr capmem, lenmem;
+	int32 flag;
+	Type *typ;
+
+	typ = t->elem;
+	if(typ->size == 0) {
+		*ret = x;
+		ret->cap = newcap;
+		return;
+	}
 
-	m = x.cap;
+	newcap1 = x.cap;
 	
 	// Using newcap directly for m+m < newcap handles
 	// both the case where m == 0 and also the case where
 	// m+m/4 wraps around, in which case the loop
 	// below might never terminate.
-	if(m+m < newcap)
-		m = newcap;
+	if(newcap1+newcap1 < newcap)
+		newcap1 = newcap;
 	else {
 		do {
 			if(x.len < 1024)
-				m += m;
+				newcap1 += newcap1;
 			else
-				m += m/4;
-		} while(m < newcap);
+				newcap1 += newcap1/4;
+		} while(newcap1 < newcap);
 	}
-	makeslice1(t, x.len, m, ret);
-	runtime·memmove(ret->array, x.array, ret->len * t->elem->size);
+
+	if(newcap1 > MaxMem/typ->size)
+		runtime·panicstring("growslice: cap out of range");
+	capmem = runtime·roundupsize(newcap1*typ->size);
+	flag = FlagNoZero;
+	if(typ->kind&KindNoPointers)
+		flag |= FlagNoScan;
+	// Here we allocate with FlagNoZero but potentially w/o FlagNoScan,
+	// GC must not see this blocks until memclr below.
+	m->locks++;
+	ret->array = runtime·mallocgc(capmem, (uintptr)typ|TypeInfo_Array, flag);
+	ret->len = x.len;
+	ret->cap = capmem/typ->size;
+	lenmem = x.len*typ->size;
+	runtime·memmove(ret->array, x.array, lenmem);
+	runtime·memclr(ret->array+lenmem, capmem-lenmem);
+	m->locks--;
+	if(m->locks == 0 && g->preempt)  // restore the preemption request in case we've cleared it in newstack
+		g->stackguard0 = StackPreempt;
 }
 
 // copy(to any, fr any, wid uintptr) int

src/pkg/runtime/string.goc

@@ -78,6 +78,7 @@ runtime·gostringn(byte *str, intgo l)
 	return s;
 }
 
+// used by cmd/cgo
 Slice
 runtime·gobytes(byte *p, intgo n)
 {
@@ -278,10 +279,15 @@ func slicebytetostring(b Slice) (s String) {
 }
 
 func stringtoslicebyte(s String) (b Slice) {
-	b.array = runtime·mallocgc(s.len, 0, FlagNoScan|FlagNoZero);
+	uintptr cap;
+
+	cap = runtime·roundupsize(s.len);
+	b.array = runtime·mallocgc(cap, 0, FlagNoScan|FlagNoZero);
 	b.len = s.len;
-	b.cap = s.len;
+	b.cap = cap;
 	runtime·memmove(b.array, s.str, s.len);
+	if(cap != b.len)
+		runtime·memclr(b.array+b.len, cap-b.len);
 }
 
 func slicerunetostring(b Slice) (s String) {
@@ -316,6 +322,7 @@ func stringtoslicerune(s String) (b Slice) {
 	intgo n;
 	int32 dum, *r;
 	uint8 *p, *ep;
+	uintptr mem;
 
 	// two passes.
 	// unlike slicerunetostring, no race because strings are immutable.
@@ -327,13 +334,16 @@ func stringtoslicerune(s String) (b Slice) {
 		n++;
 	}
 
-	b.array = runtime·mallocgc(n*sizeof(r[0]), 0, FlagNoScan|FlagNoZero);
+	mem = runtime·roundupsize(n*sizeof(r[0]));
+	b.array = runtime·mallocgc(mem, 0, FlagNoScan|FlagNoZero);
 	b.len = n;
-	b.cap = n;
+	b.cap = mem/sizeof(r[0]);
 	p = s.str;
 	r = (int32*)b.array;
 	while(p < ep)
 		p += runtime·charntorune(r++, p, ep-p);
+	if(b.cap > b.len)
+		runtime·memclr(b.array+b.len*sizeof(r[0]), (b.cap-b.len)*sizeof(r[0]));
 }
 
 enum