通过tcc内联C代码，实现Lua程序性能显著提升。本文对比了使用Lua原生JSON解析器与通过tcc引入的yajl解析器的效率，结果显示yajl解析速度快33.5倍。
tcc 是一个很有趣的小型 C 编译器，其最大的特点是可以实现即时编译 C 代码，无需产生中间文件即可生成 native code，且将内部功能全部作为库函数暴露出来，可以很方便地嵌入到自己的应用中。
目前有一些现成的 lua 模块可以将 tcc 引入到 lua 里，用其即时生成 C-function 供 lua 使用。其中 lua-tcc 是最简单的一个，但代码年代较久，需要进行 patch 才可同最新的 tcc-0.9.25 一同工作，patch 后的代码参见这里 。
下面是一个例子，对比了 lua 原生的 json 解析器 json4lua 和通过 tcc 内联的 c json 解析器 yajl 的效率（需预先安装 json4lua 、lua-tcc 以及 yajl ）：
local json = require("json")
local tcc = require("lua_tcc")
yajl = tcc.compile([=[
#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>
#include <yajl/yajl_parse.h>
#include <stdlib.h>
#include <errno.h>
/* FIXME: maximum container nesting level hardcoded to 512 */
#define MAX_NEST_LEVEL 512
typedef struct {
lua_State *L;
int curr_level;
enum {not_in_container, in_map, in_array} env[MAX_NEST_LEVEL];
int start_of_container[MAX_NEST_LEVEL];
} parse_ctx_t;
int parse_null(void *ctx)
{
parse_ctx_t *c = (parse_ctx_t*)ctx;
lua_pushnil(c->L);
switch(c->env[c->curr_level]) {
case in_map:
/* nothing to do, push prev pair until the next pair key appears or map is end */
break;
case in_array:
lua_rawseti(c->L, -2, lua_objlen(c->L, -2)+1);
break;
default:
break;
}
return 1;
}
int parse_boolean(void *ctx, int bool_val)
{
parse_ctx_t *c = (parse_ctx_t*)ctx;
lua_pushboolean(c->L, bool_val);
switch(c->env[c->curr_level]) {
case in_map:
/* nothing to do, push prev pair until the next pair key appears or map is end */
break;
case in_array:
lua_rawseti(c->L, -2, lua_objlen(c->L, -2)+1);
break;
default:
break;
}
return 1;
}
int parse_integer(void *ctx, long int_val)
{
parse_ctx_t *c = (parse_ctx_t*)ctx;
lua_pushinteger(c->L, (lua_Integer)int_val);
switch(c->env[c->curr_level]) {
case in_map:
/* nothing to do, push prev pair until the next pair key appears or map is end */
break;
case in_array:
lua_rawseti(c->L, -2, lua_objlen(c->L, -2)+1);
break;
default:
break;
}
return 1;
}
int parse_double(void *ctx, double dbl_val)
{
parse_ctx_t *c = (parse_ctx_t*)ctx;
lua_pushnumber(c->L, (lua_Number)dbl_val);
switch(c->env[c->curr_level]) {
case in_map:
/* nothing to do, push prev pair until the next pair key appears or map is end */
break;
case in_array:
lua_rawseti(c->L, -2, lua_objlen(c->L, -2)+1);
break;
default:
break;
}
return 1;
}
int parse_string(void *ctx, const unsigned char *str_val, unsigned int str_len)
{
parse_ctx_t *c = (parse_ctx_t*)ctx;
lua_pushlstring(c->L, (const char*)str_val, str_len);
switch(c->env[c->curr_level]) {
case in_map:
/* nothing to do, push prev pair until the next pair key appears or map is end */
break;
case in_array:
lua_rawseti(c->L, -2, lua_objlen(c->L, -2)+1);
break;
default:
break;
}
return 1;
}
int parse_start_map(void *ctx)
{
parse_ctx_t *c = (parse_ctx_t*)ctx;
switch(c->env[c->curr_level]) {
case in_map:
case in_array:
default:
lua_newtable(c->L);
c->curr_level++;
c->env[c->curr_level] = in_map;
c->start_of_container[c->curr_level] = 1;
break;
}
return 1;
}
int parse_map_key(void *ctx, const unsigned char *key, unsigned int len)
{
parse_ctx_t *c = (parse_ctx_t*)ctx;
switch(c->env[c->curr_level]) {
case in_map:
/* store prev pair */
if(c->start_of_container[c->curr_level]) {
c->start_of_container[c->curr_level] = 0;
} else {
lua_rawset(c->L, -3);
}
/* push new pair's key */
lua_pushlstring(c->L, (const char*)key, len);
break;
case in_array:
default:
return 0;
}
return 1;
}
int parse_end_map(void *ctx)
{
parse_ctx_t *c = (parse_ctx_t*)ctx;
switch(c->env[c->curr_level]) {
case in_map:
/* store prev pair */
lua_rawset(c->L, -3);
c->curr_level--;
switch(c->env[c->curr_level]) {
case in_array:
lua_rawseti(c->L, -2, lua_objlen(c->L, -2)+1);
break;
case in_map:
default:
break;
}
break;
case in_array:
default:
return 0;
}
return 1;
}
int parse_start_array(void *ctx)
{
parse_ctx_t *c = (parse_ctx_t*)ctx;
switch(c->env[c->curr_level]) {
case in_map:
case in_array:
default:
lua_newtable(c->L);
c->curr_level++;
c->env[c->curr_level] = in_array;
c->start_of_container[c->curr_level] = 1;
break;
}
return 1;
}
int parse_end_array(void *ctx)
{
parse_ctx_t *c = (parse_ctx_t*)ctx;
switch(c->env[c->curr_level]) {
case in_array:
c->curr_level--;
switch(c->env[c->curr_level]) {
case in_array:
lua_rawseti(c->L, -2, lua_objlen(c->L, -2)+1);
break;
case in_map:
default:
break;
}
break;
case in_map:
default:
return 0;
}
return 1;
}
int parse_json(lua_State *L)
{
static yajl_parser_config _config = {
allowComments: 1,
checkUTF8: 1
};
static yajl_callbacks _callbacks = {
yajl_null: parse_null,
yajl_boolean: parse_boolean,
yajl_integer: parse_integer,
yajl_double: parse_double,
yajl_number: NULL,
yajl_string: parse_string,
yajl_start_map: parse_start_map,
yajl_map_key: parse_map_key,
yajl_end_map: parse_end_map,
yajl_start_array: parse_start_array,
yajl_end_array: parse_end_array
};
parse_ctx_t ctx;
size_t json_len;
const unsigned char *json_str = (const unsigned char*)luaL_checklstring(L, -1, &json_len);
yajl_handle yajl;
yajl_status status;
ctx.L = L;
ctx.curr_level = 0;
ctx.env[0] = not_in_container;
ctx.start_of_container[0] = 0;
yajl = yajl_alloc(&_callbacks, &_config, NULL, &ctx);
status = yajl_parse(yajl, json_str, json_len);
if(status == yajl_status_ok) {
status = yajl_parse_complete(yajl);
}
if(status != yajl_status_ok) {
unsigned char* err_msg = yajl_get_error(yajl, 1, json_str, json_len);
lua_pushfstring(L, "parsing error: %s", err_msg);
yajl_free_error(yajl, err_msg);
return lua_error(L);
}
yajl_free(yajl);
return 1;
}
]=],
{"parse_json"},
{"yajl"}
)
local ntimes = 300000
local st, ed
local str = '{"a":1234,"b":2.34,"c":true,"d":"hello","e":[1,2,{"a":{"b":{"c":"d"}}}]}'
st = os.time()
for i=1,ntimes do
local arr = yajl.parse_json(str)
end
ed = os.time()
print("yajl elapsed = ", (ed-st))
st = os.time()
for i=1,ntimes do
local arr = json.decode(str)
end
ed = os.time()
print("json4lua elapsed = ", (ed-st))
在我的 colinux 虚拟环境下用标准 lua 运行该程序时结果如下：
$ lua yajl.lua yajl elapsed = 4 json4lua elapsed = 134
相比之下通过 tcc 内联的 yajl 解析器解析相同的 json 串时比 json4lua 快了 33.5 倍！而换用 luajit-2.0 运行该程序时结果如下：
$ luajit yajl.lua yajl elapsed = 2 json4lua elapsed = 52
json4lua 表现好了不少，但 yajl 的解析速度仍是 json4lua 的 26 倍。由此可见，在恰当的地方通过 tcc 内联 c 代码替代 lua 处理数据可以收到相当好的性能提升效果，且由于模块代码以源代码形式分发，对于没有外部依赖库的代码不会存在传统 Lua-C 模块那样的跨平台二进制接口兼容性问题。
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