Lua: technical note 2

Technical Note 2

Minimal Lua 4.0 installations

This note explains how to build Lua for environments that do not have much memory, such as embedded systems. This is a version of Technical Note 1, updated for Lua 4.0.

As explicitly stated in the "about" page, one of the goals of our Lua implementation is low embedding cost. This means two things: first, it should be easy to embed Lua in an application; second, the additional code for Lua should not be too large.

The first requirement is fulfilled by the simplicity of Lua's C API. The second requirement is fulfilled by demonstration, as follows.

Here are a few numbers for Lua 4.0, compiled with the default options in an Intel machine running Linux (the numbers for other platforms will be different, but probably roughly the same in relative terms):

% size liblua.a
   text	   data	    bss	    dec	    hex	filename
   3328	      0	      0	   3328	    d00	lapi.o (ex liblua.a)
   4054	      0	      0	   4054	    fd6	lcode.o (ex liblua.a)
   3031	      0	      0	   3031	    bd7	ldebug.o (ex liblua.a)
   2372	      0	      0	   2372	    944	ldo.o (ex liblua.a)
    574	      0	      0	    574	    23e	lfunc.o (ex liblua.a)
   1874	      0	      0	   1874	    752	lgc.o (ex liblua.a)
   4909	      0	      0	   4909	   132d	llex.o (ex liblua.a)
    225	      0	      0	    225	     e1	lmem.o (ex liblua.a)
    734	      0	      0	    734	    2de	lobject.o (ex liblua.a)
   7634	      0	      0	   7634	   1dd2	lparser.o (ex liblua.a)
    598	      0	      0	    598	    256	lstate.o (ex liblua.a)
    953	      0	      0	    953	    3b9	lstring.o (ex liblua.a)
   1651	      0	      0	   1651	    673	ltable.o (ex liblua.a)
      0	      0	      0	      0	      0	ltests.o (ex liblua.a)
   1495	      0	      0	   1495	    5d7	ltm.o (ex liblua.a)
   2491	      0	      0	   2491	    9bb	lundump.o (ex liblua.a)
   5487	      0	      0	   5487	   156f	lvm.o (ex liblua.a)
    336	      0	      0	    336	    150	lzio.o (ex liblua.a)
% size liblualib.a
   text	   data	    bss	    dec	    hex	filename
   1437	      0	      0	   1437	    59d	lauxlib.o (ex liblualib.a)
   5619	      0	      0	   5619	   15f3	lbaselib.o (ex liblualib.a)
   1674	      0	      2	   1676	    68c	ldblib.o (ex liblualib.a)
   5288	      0	      0	   5288	   14a8	liolib.o (ex liblualib.a)
   2301	      0	      0	   2301	    8fd	lmathlib.o (ex liblualib.a)
   6209	      0	      0	   6209	   1841	lstrlib.o (ex liblualib.a)

In this listing, text actually is the size of the code in bytes. We conclude that the Lua core (liblua.a) takes 41746 bytes and that the Lua standard libraries (liblualib.a) take 22528 bytes. So, the whole Lua code takes 64274 bytes, or less than 63K. In other words, the impact of Lua in an application is 63K of additional code, which is quite small. (Of course, Lua will use memory at run-time -- but how much depends on the application.)

63K seem very little in these days that machines have many megabytes of main memory, but they might make a difference for someone trying to use Lua in a microwave oven or in a robot. So, let's see how to reduce these 63K to even less. (Even if you're not using Lua in embedded systems, you might learn something from the description below.)

The first thing is to remove any standard libraries that are not needed. For instance, ldblib.o will probably not be needed in most applications, and liolib.o will probably not make sense for a microwave oven. But removing standard libraries will not get you very far, because they are small already. So, let's look at the numbers for the core again, but now sorted by size:

text    %core   %whole   filename
   0     0%      0%      ltests.o
 225     1%      0%      lmem.o
 336     1%      1%      lzio.o
 574     1%      1%      lfunc.o
 598     1%      1%      lstate.o
 734     2%      1%      lobject.o
 953     2%      1%      lstring.o
1495     4%      2%      ltm.o
1651     4%      3%      ltable.o
1874     4%      3%      lgc.o
2372     6%      4%      ldo.o
2491     6%      4%      lundump.o
3031     7%      5%      ldebug.o
3328     8%      5%      lapi.o
4054    10%      6%      lcode.o
4909    12%      8%      llex.o
5487    13%      9%      lvm.o
7634    18%     12%      lparser.o

This listing tells us that the parsing modules -- the lexer llex.o, the parser lparser.o, and the code generator lcode.o -- represent 40% of the core (and 26% of the whole). So, they are the main candidates for removal. An application that does not need to compile Lua code at run-time does not need the parsing modules.

We have designed the code so that it is easy to remove these three modules. Only one module (ldo.o) calls the parser, which has only one public function (luaY_parser). The only module that calls the lexer is the parser, except for an initialization function (luaX_init) used in lua_open. The only module that calls the code generator is the parser, except that ldebug.o uses the array luaK_opproperties from lcode.o. So, to remove the parsing modules you only need to add the code below to your application (you can extract it from lua/src/luac/stubs.c, where it is disabled by default):

#include "llex.h"
#include "lparser.h"

void luaX_init(lua_State *L) {
  UNUSED(L);
}

Proto *luaY_parser(lua_State *L, ZIO *z) {
  UNUSED(z);
  lua_error(L,"parser not loaded");
  return NULL;
}

To remove the code generator too, you need to add #include "lcode.h" and copy luaK_opproperties from lcode.c into this code.

An application that contains the code above will not link the parsing modules and trying to load Lua source code will generate an error. But then, you ask, how does the application load Lua code at all? The answer is: by loading precompiled chunks instead of source code. Precompiled chunks are created with luac, which will contain the parsing modules, but which is an external application. The module that loads precompiled chunks is lundump.o, which is small enough.

Although lua_dofile and dofile automatically detect precompiled chunks, one convenient way is to use lua_dobuffer with precompiled chunks statically linked to your application (you will find lua/etc/bin2c.c useful for this), because embedded systems don't even have filesystems. (This is a quick solution, but it will increase the size of your application and it may be too inflexible for you.)

Removing the parsing modules leaves us with a core of just 25296 bytes, a little more than 24K. This is tiny indeed for a powerful language like Lua! Note also that this reduction was done without sacrificing any language features and without touching the source code; we just need a little help from the linker.

This note has focused on reducing the amount of code added to the application by the Lua library. Applications that need this will probably also prefer to use integers instead of floating-point numbers for the numbers in Lua. (Does a microwave oven need floating-point?) This should be simple to do, as described in lua/config, but the details will probably be discussed in another LTN.

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原文转自：http://www.ltesting.net