Friday, June 05, 2020
A Lua module in assembly, why not?
I've been known to dabble in assembly language from time to time,
and there's been this one instruction on the Intel Pentium that I've wanted to play around with—RDTSC
.
It's used to read the internal time-stamp counter which is incremented every clock cycle.
On my system,
this counter is incremented 2,660,000,000 times per second
(the computer is running at 2.66GHz)
and this makes for a nice way to time code,
as the instruction is available in userspace
(at least on Linux).
I wanted to use this to time some Lua code, which means I need to wrap this instruction into a function that can be called. I could have used some inline assembly in C to do this, but
- the code is non-portable anyway;
- I wanted to avoid as much C overhead as possible;
- and I thought it would be amusing to write an entire Lua module in assembly.
It wasn't hard:
;*************************************************************************** ; ; Copyright 2020 by Sean Conner. ; ; This library is free software; you can redistribute it and/or modify it ; under the terms of the GNU Lesser General Public License as published by ; the Free Software Foundation; either version 3 of the License, or (at your ; option) any later version. ; ; This library is distributed in the hope that it will be useful, but ; WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY ; or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public ; License for more details. ; ; You should have received a copy of the GNU Lesser General Public License ; along with this library; if not, see <http://www.gnu.org/licenses/>. ; ; Comments, questions and criticisms can be sent to: sean@conman.org ; ;*************************************************************************** bits 32 global luaopen_rdtsc extern lua_pushinteger extern lua_pushcclosure ;*************************************************************************** section .text ldl_rdtsc: rdtsc push edx push eax push dword [esp + 12] call lua_pushinteger ; lua_pushinteger(L,rdtsc); xor eax,eax ; return 1 inc eax lea esp,[esp + 12] ret ;--------------------------------------------------------------------------- luaopen_rdtsc: xor eax,eax push eax push ldl_rdtsc push dword [esp + 12] call lua_pushcclosure ; lua_pushcclosure(L,ldl_rdtsc,0); xor eax,eax ; return 1 inc eax lea esp,[esp + 12] ret
I'll leave it up to the reader to convert this to 64-bit code.