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Merged
merged 2 commits into from
Sep 30, 2025
Merged

Add ref-counted string slices #1175

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d89658a
Add ref-counted string slices
bnoordhuis Sep 30, 2025
647d025
squash! JS_STRING_SLICE_LEN_MAX
bnoordhuis Sep 30, 2025
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112 changes: 90 additions & 22 deletions quickjs.c
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Original file line number Diff line number Diff line change
Expand Up @@ -211,6 +211,9 @@ typedef enum JSErrorEnum {
#define JS_MAX_LOCAL_VARS 65535
#define JS_STACK_SIZE_MAX 65534
#define JS_STRING_LEN_MAX ((1 << 30) - 1)
// 1,024 bytes is about the cutoff point where it starts getting
// more profitable to ref slice than to copy
#define JS_STRING_SLICE_LEN_MAX 1024 // in bytes

#define __exception __attribute__((warn_unused_result))

Expand Down Expand Up @@ -545,7 +548,12 @@ typedef enum {
JS_ATOM_KIND_PRIVATE,
} JSAtomKindEnum;

#define JS_ATOM_HASH_MASK ((1 << 30) - 1)
typedef enum {
JS_STRING_KIND_NORMAL,
JS_STRING_KIND_SLICE,
} JSStringKind;

#define JS_ATOM_HASH_MASK ((1 << 29) - 1)

struct JSString {
JSRefCountHeader header; /* must come first, 32-bit */
Expand All @@ -554,7 +562,8 @@ struct JSString {
/* for JS_ATOM_TYPE_SYMBOL: hash = 0, atom_type = 3,
for JS_ATOM_TYPE_PRIVATE: hash = 1, atom_type = 3
XXX: could change encoding to have one more bit in hash */
uint32_t hash : 30;
uint32_t hash : 29;
uint8_t kind : 1;
uint8_t atom_type : 2; /* != 0 if atom, JS_ATOM_TYPE_x */
uint32_t hash_next; /* atom_index for JS_ATOM_TYPE_SYMBOL */
JSWeakRefRecord *first_weak_ref;
Expand All @@ -563,14 +572,39 @@ struct JSString {
#endif
};

typedef struct JSStringSlice {
JSString *parent;
uint32_t start; // in characters, not bytes
} JSStringSlice;

static inline uint8_t *str8(JSString *p)
{
return (void *)(p + 1);
JSStringSlice *slice;

switch (p->kind) {
case JS_STRING_KIND_NORMAL:
return (void *)&p[1];
case JS_STRING_KIND_SLICE:
slice = (void *)&p[1];
return str8(slice->parent) + slice->start;
}
abort();
return NULL;
}

static inline uint16_t *str16(JSString *p)
{
return (void *)(p + 1);
JSStringSlice *slice;

switch (p->kind) {
case JS_STRING_KIND_NORMAL:
return (void *)&p[1];
case JS_STRING_KIND_SLICE:
slice = (void *)&p[1];
return str16(slice->parent) + slice->start;
}
abort();
return NULL;
}

typedef struct JSClosureVar {
Expand Down Expand Up @@ -2049,6 +2083,7 @@ static JSString *js_alloc_string_rt(JSRuntime *rt, int max_len, int is_wide_char
str->header.ref_count = 1;
str->is_wide_char = is_wide_char;
str->len = max_len;
str->kind = JS_STRING_KIND_NORMAL;
str->atom_type = 0;
str->hash = 0; /* optional but costless */
str->hash_next = 0; /* optional */
Expand All @@ -2069,18 +2104,28 @@ static JSString *js_alloc_string(JSContext *ctx, int max_len, int is_wide_char)
return p;
}

static inline void js_free_string0(JSRuntime *rt, JSString *str);

/* same as JS_FreeValueRT() but faster */
static inline void js_free_string(JSRuntime *rt, JSString *str)
{
if (--str->header.ref_count <= 0) {
if (str->atom_type) {
JS_FreeAtomStruct(rt, str);
} else {
if (--str->header.ref_count <= 0)
js_free_string0(rt, str);
}

static inline void js_free_string0(JSRuntime *rt, JSString *str)
{
if (str->atom_type) {
JS_FreeAtomStruct(rt, str);
} else {
#ifdef ENABLE_DUMPS // JS_DUMP_LEAKS
list_del(&str->link);
list_del(&str->link);
#endif
js_free_rt(rt, str);
if (str->kind == JS_STRING_KIND_SLICE) {
JSStringSlice *slice = (void *)&str[1];
js_free_string(rt, slice->parent); // safe, recurses only 1 level
}
js_free_rt(rt, str);
}
}

Expand Down Expand Up @@ -2962,6 +3007,7 @@ static JSAtom __JS_NewAtom(JSRuntime *rt, JSString *str, int atom_type)
p->header.ref_count = 1;
p->is_wide_char = str->is_wide_char;
p->len = str->len;
p->kind = JS_STRING_KIND_NORMAL;
#ifdef ENABLE_DUMPS // JS_DUMP_LEAKS
list_add_tail(&p->link, &rt->string_list);
#endif
Expand All @@ -2976,6 +3022,7 @@ static JSAtom __JS_NewAtom(JSRuntime *rt, JSString *str, int atom_type)
p->header.ref_count = 1;
p->is_wide_char = 1; /* Hack to represent NULL as a JSString */
p->len = 0;
p->kind = JS_STRING_KIND_NORMAL;
#ifdef ENABLE_DUMPS // JS_DUMP_LEAKS
list_add_tail(&p->link, &rt->string_list);
#endif
Expand Down Expand Up @@ -3680,13 +3727,37 @@ static JSValue js_new_string_char(JSContext *ctx, uint16_t c)

static JSValue js_sub_string(JSContext *ctx, JSString *p, int start, int end)
{
int len = end - start;
JSStringSlice *slice;
JSString *q;
int len;

len = end - start;
if (start == 0 && end == p->len) {
return js_dup(JS_MKPTR(JS_TAG_STRING, p));
}
if (len <= 0) {
return js_empty_string(ctx->rt);
}
if (len > (JS_STRING_SLICE_LEN_MAX >> p->is_wide_char)) {
if (p->kind == JS_STRING_KIND_SLICE) {
slice = (void *)&p[1];
p = slice->parent;
start += slice->start;
}
// allocate as 16 bit wide string to avoid wastage;
// js_alloc_string allocates 1 byte extra for 8 bit strings;
q = js_alloc_string(ctx, sizeof(*slice)/2, /*is_wide_char*/true);
if (!q)
return JS_EXCEPTION;
q->is_wide_char = p->is_wide_char;
q->kind = JS_STRING_KIND_SLICE;
q->len = len;
slice = (void *)&q[1];
slice->parent = p;
slice->start = start;
p->header.ref_count++;
return JS_MKPTR(JS_TAG_STRING, q);
}
if (p->is_wide_char) {
JSString *str;
int i;
Expand Down Expand Up @@ -5750,17 +5821,7 @@ static void js_free_value_rt(JSRuntime *rt, JSValue v)

switch(tag) {
case JS_TAG_STRING:
{
JSString *p = JS_VALUE_GET_STRING(v);
if (p->atom_type) {
JS_FreeAtomStruct(rt, p);
} else {
#ifdef ENABLE_DUMPS // JS_DUMP_LEAKS
list_del(&p->link);
#endif
js_free_rt(rt, p);
}
}
js_free_string0(rt, JS_VALUE_GET_STRING(v));
break;
case JS_TAG_OBJECT:
case JS_TAG_FUNCTION_BYTECODE:
Expand Down Expand Up @@ -58038,6 +58099,13 @@ uintptr_t js_std_cmd(int cmd, ...) {
*pv = ctx->error_back_trace;
ctx->error_back_trace = JS_UNDEFINED;
break;
case 3: // GetStringKind
ctx = va_arg(ap, JSContext *);
pv = va_arg(ap, JSValue *);
rv = -1;
if (JS_IsString(*pv))
rv = JS_VALUE_GET_STRING(*pv)->kind;
break;
default:
rv = -1;
}
Expand Down
17 changes: 17 additions & 0 deletions run-test262.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1700,15 +1700,32 @@ void update_stats(JSRuntime *rt, const char *filename) {
js_mutex_unlock(&stats_mutex);
}

static JSValue qjs_black_box(JSContext *ctx, JSValueConst this_val,
int argc, JSValueConst argv[], int magic)
{
return JS_NewInt32(ctx, js_std_cmd(magic, ctx, &argv[0]));
}

static const JSCFunctionListEntry qjs_methods[] = {
JS_CFUNC_MAGIC_DEF("getStringKind", 1, qjs_black_box, /*GetStringKind*/3),
};

static const JSCFunctionListEntry qjs_object =
JS_OBJECT_DEF("qjs", qjs_methods, countof(qjs_methods), JS_PROP_C_W_E);

JSContext *JS_NewCustomContext(JSRuntime *rt)
{
JSContext *ctx;
JSValue obj;

ctx = JS_NewContext(rt);
if (ctx && local) {
js_init_module_std(ctx, "qjs:std");
js_init_module_os(ctx, "qjs:os");
js_init_module_bjson(ctx, "qjs:bjson");
obj = JS_GetGlobalObject(ctx);
JS_SetPropertyFunctionList(ctx, obj, &qjs_object, 1);
JS_FreeValue(ctx, obj);
}
return ctx;
}
Expand Down
36 changes: 36 additions & 0 deletions tests/microbench.js
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Original file line number Diff line number Diff line change
Expand Up @@ -797,6 +797,39 @@ function string_build4(n)
return n * 100;
}

function string_slice1(n)
{
var i, j, s;
s = "x".repeat(1<<16);
for (i = 0; i < n; i++) {
for (j = 0; j < 1000; j++)
s.slice(-1); // too short for JSStringSlice
}
return n * 1000;
}

function string_slice2(n)
{
var i, j, s;
s = "x".repeat(1<<16);
for (i = 0; i < n; i++) {
for (j = 0; j < 1000; j++)
s.slice(-1024);
}
return n * 1000;
}

function string_slice3(n)
{
var i, j, s;
s = "x".repeat(1<<16);
for (i = 0; i < n; i++) {
for (j = 0; j < 1000; j++)
s.slice(1);
}
return n * 1000;
}

/* sort bench */

function sort_bench(text) {
Expand Down Expand Up @@ -1114,6 +1147,9 @@ function main(argc, argv, g)
string_build2,
//string_build3,
//string_build4,
string_slice1,
string_slice2,
string_slice3,
sort_bench,
int_to_string,
int_toString,
Expand Down
5 changes: 5 additions & 0 deletions tests/test_builtin.js
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Original file line number Diff line number Diff line change
Expand Up @@ -378,6 +378,11 @@ function test_string()
assert(eval('"\0"'), "\0");

assert("abc".padStart(Infinity, ""), "abc");

assert(qjs.getStringKind("xyzzy".slice(1)),
/*JS_STRING_KIND_NORMAL*/0);
assert(qjs.getStringKind("xyzzy".repeat(512).slice(1)),
/*JS_STRING_KIND_SLICE*/1);
}

function test_math()
Expand Down