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POWhen to use Test&Set or Test&Test&Set?
 

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  1. POWhen to use Test&Set or Test&Test&Set?
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    <p>Parallel programming under x86 can be hard job especially under multi-core CPU. Let say that we have multi-core x86 CPU and more different multithread communication combinations.</p> <ol> <li>Single writer and single reader</li> <li>Single reader multiple writers</li> <li>Multiple readers and single writer</li> <li>Multiple readers and multiple writers</li> </ol> <p>So which one model is better (more efficient) for locking shared memory region: <strong>Test&amp;Set</strong> or <strong>Test&amp;Test&amp;Set</strong> and when to use it!</p> <p>Here I have two simple (no time limited) test procedures written in under Delphi IDE in x86 assembler:</p> <pre><code>procedure TestAndSet(const oldValue, newValue: cardinal; var destination); asm //eax = oldValue //edx = NewLockValue //ecx = destination = 32 bit pointer on lock variable 4 byte aligned @RepeatSpinLoop: push eax //Save lock oldValue (compared) pause //CPU spin-loop hint lock cmpxchg dword ptr [ecx], edx pop eax //Restore eax as oldValue jnz @RepeatSpinLoop //Repeat if cmpxchg wasn't successful end; procedure TestAndTestAndSet(const oldValue, newValue: cardinal; var destination); asm //eax = oldValue //edx = NewLockValue //ecx = destination = 32 bit pointer on lock variable 4 byte aligned @RepeatSpinLoop: push eax //Save lock oldValue (compared) @SpinLoop: pause //CPU spin-loop hint cmp dword ptr [ecx], eax //Test betfore test&amp;set jnz @SpinLoop lock cmpxchg dword ptr [ecx], edx pop eax //Restore eax as oldValue jnz @RepeatSpinLoop //Repeat if cmpxchg wasn't successful end; </code></pre> <p><strong>EDIT:</strong></p> <p>Intel in documentation mention two approach <strong>Test&amp;Set or Test&amp;Test&amp;Set</strong>. I' wont to establish in which case is someone approach better, so when to use it. Check: <a href="http://software.intel.com/sites/products/documentation/hpc/compilerpro/en-us/cpp/win/compiler_c/intref_cls/common/intref_sse2_pause.htm?wapkw=%28pause%29" rel="nofollow">Intel</a></p>
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      1. POWhen to use Test&Set or Test&Test&Set?
      1. USJeroen Wiert Pluimers
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    1. COwhat exactly is your question?
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      1. USDavid Heffernan
    2. COI hope that my answer was clear: which approach is better and when, of course if somebody have any expirience about.
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    3. CO@GJ: the sample you have linked to uses `xchg`, not `cmpxchg`.
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      1. USAndras Vass
 

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