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Quickie: Is iCounter += iSomeVal; thread-safe?

CWizard · 2002-07-28T17:37:37

It's all in the subject. Will a incremental C statement be thread-safe, when the variable incremented is shared among threads? To narrow the question, leave out multi-processor systems. If I have understood it right, the (stupid, ed. note ) intel processor cannot do memory-2-memory operations in a single instruction. Is it so, or is the compiler likely to not use such instruction? There are a few (main) methods I can think of that the compiler could implement the following C-code, and give some sort of pseudo-assembly code for it (am no good at intel's syntax). // global int iCounter = 0; ... // A thread executes this iCounter += iSomeVal; add <iCounter>, <iSomeVal> move eax, <iSomeVal> add <iCounter>, eax move eax, <iCounter> add eax, <iSomeVal> move <iCounter>, eax The first and second method will be thread safe (in single-processor environment), but the last one won't. I know I could check the disassembly, but my experience tell me that you cannot be sure that it always do it that way. I thought this would be a short question, but it got a bit longer. Thanks for reading, and more so if you provide some aspect of this. EDIT: some formatting probs. [edited by - CWizard on July 28, 2002 3:26:54 PM]

General and Gameplay Programming Programming

Started by CWizard July 28, 2002 02:24 PM

9 comments, last by CWizard 21 years, 8 months ago

CWizard

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July 28, 2002 05:37 PM

This thread should probably just die out, but just one more question here. I checked the disassembly of the InterlockedExchangeAdd(), which was:

77E76B5C   mov         ecx,dword ptr [esp+4]77E76B60   mov         eax,dword ptr [esp+8]77E76B64   lock xadd   dword ptr [ecx],eax77E76B68   ret         8

. But, before that, a lot of overhead. Could one instead make this a inline unsing a macro? What instruction is that "lock xadd", and which processors support it?

Quickie: Is iCounter += iSomeVal; thread-safe?

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Quickie: Is iCounter += iSomeVal; thread-safe?

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