Winboard Forum

I just rewrote Project Invincible's spinlock implementation. The new implementation is pure C++11 (based on atomic flags). It allowed me to get rid of all remaining assembly in my codebase.

The most important advantage for me is that it simplifies Project Invincible's build systems. In particular, for GNU/Linux I distribute source code rather than binaries (i.e. end users will have to compile the game themselves), but I had decided to pre-assemble my previous spinlock implementation to allow players to compile Project Invincible without having an assembler installed. That required me to implement a way to detect the architecture of the OS kernel so that the game will be linked to the right object code file...

I decided to license my new spinlock implementation, unlike the rest of Project Invincible's codebase, as public domain. (It is probably too short to be copyrightable anyway.)

Here it is: http://code.google.com/p/project-invincible/source/browse/trunk/common/spinlock.cpp?spec=svn418&r=418.

Atomic flags are guaranteed to be lock-free, i.e. to be implemented with atomic processor instructions instead of mutexes or other locks. For example, here is the assembly MSVC 2012 generated for the acquire() member function:

Code: Select all

; Function compile flags: /Ogtp
; File f:\project invincible\project invincible\common\spinlock.cpp
; File c:\program files (x86)\microsoft visual studio 11.0\vc\include\xatomic.h
; File c:\program files (x86)\microsoft visual studio 11.0\vc\include\atomic
; File f:\project invincible\project invincible\common\spinlock.cpp
;   COMDAT ?acquire@Spinlock@@QAEXXZ
_TEXT   SEGMENT
?acquire@Spinlock@@QAEXXZ PROC            ; Spinlock::acquire, COMDAT
; _this$ = ecx

; 18   : {

$LL2@acquire:
; File c:\program files (x86)\microsoft visual studio 11.0\vc\include\xatomic.h

; 2818 :          return (_interlockedbittestandset(_Flag, 0));   /* set bit 0 */

   lock    bts    DWORD PTR [ecx], 0
   setb   al
   movzx   eax, al
; File c:\program files (x86)\microsoft visual studio 11.0\vc\include\atomic

; 202  :    return _ATOMIC_FLAG_TEST_AND_SET(&_My_flag, _Order);

   test   eax, eax
; File f:\project invincible\project invincible\common\spinlock.cpp

; 19   :    while (lock.test_and_set(memory_order_acquire));

   jne   SHORT $LL2@acquire

; 20   : }

   ret   0
?acquire@Spinlock@@QAEXXZ ENDP            ; Spinlock::acquire
_TEXT   ENDS