Re: Is gcc thread-unsafe?

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To: Linus Torvalds <torvalds@...>
Cc: Nick Piggin <nickpiggin@...>, Linux Kernel Mailing List <linux-kernel@...>
Date: Thursday, October 25, 2007 - 7:42 pm

On Friday 26 October 2007 01:32:53 Linus Torvalds wrote:

That is not what gcc did in that case. I don't think it tracks sets of values
over function calls (or even inside functions) at all.

The generated code was

          cmpl    $1, %eax                ; test res
          movl    acquires_count, %edx    ; load
          adcl    $0, %edx                ; maybe add 1
          movl    %edx, acquires_count    ; store

So it just added the result of a comparison into a variable
by (ab)using carry for this.

In theory such things can be done with CMOV too by redirecting
a store into a dummy variable to cancel it, but gcc doesn't
do that on its own.


The conditional add/sub using carry trick is not generally bogus. 
It's just bogus for memory addresses not pretty much guaranteed in L1
[aka small stack frame] because there the pipeline benefit is unlikely to 
offset the memory costs (and of course poor quality of implementation because of the 
missing thread safety). 

But for registers it's a fine optimization.

-Andi
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Messages in current thread:
Is gcc thread-unsafe?, Nick Piggin, (Wed Oct 24, 11:24 pm)
Re: Is gcc thread-unsafe?, Phillip Susi, (Wed Oct 31, 6:10 pm)
Re: Is gcc thread-unsafe?, Linus Torvalds, (Thu Oct 25, 10:55 am)
Re: Is gcc thread-unsafe?, Ismail , (Thu Oct 25, 6:26 pm)
Re: Is gcc thread-unsafe?, Jeff Garzik, (Thu Oct 25, 6:56 pm)
Re: Is gcc thread-unsafe?, Jeff Garzik, (Thu Oct 25, 7:04 pm)
RE: Is gcc thread-unsafe?, David Schwartz, (Thu Oct 25, 5:42 pm)
Re: Is gcc thread-unsafe?, Nick Piggin, (Thu Oct 25, 7:22 pm)
Re: Is gcc thread-unsafe?, Andrew Haley, (Fri Oct 26, 7:59 am)
Re: Is gcc thread-unsafe?, Andrew Haley, (Fri Oct 26, 7:59 am)
Re: Is gcc thread-unsafe?, Chris Friesen, (Fri Oct 26, 1:39 pm)
Re: Is gcc thread-unsafe?, Pekka Enberg, (Thu Oct 25, 11:12 am)
Re: Is gcc thread-unsafe?, Andi Kleen, (Thu Oct 25, 3:15 am)
Re: Is gcc thread-unsafe?, Nick Piggin, (Thu Oct 25, 6:49 pm)
Re: Is gcc thread-unsafe?, Andi Kleen, (Thu Oct 25, 7:09 pm)
Re: Is gcc thread-unsafe?, Nick Piggin, (Thu Oct 25, 7:43 pm)
Re: Is gcc thread-unsafe?, Andi Kleen, (Thu Oct 25, 7:55 pm)
Re: Is gcc thread-unsafe?, Nick Piggin, (Thu Oct 25, 7:57 pm)
Re: Is gcc thread-unsafe?, Linus Torvalds, (Thu Oct 25, 7:14 pm)
Re: Is gcc thread-unsafe?, Andi Kleen, (Thu Oct 25, 7:16 pm)
Re: Is gcc thread-unsafe?, Linus Torvalds, (Thu Oct 25, 7:32 pm)
Re: Is gcc thread-unsafe?, Andi Kleen, (Thu Oct 25, 7:42 pm)
Re: Is gcc thread-unsafe?, Willy Tarreau, (Fri Oct 26, 12:57 am)
Re: Is gcc thread-unsafe?, Linus Torvalds, (Thu Oct 25, 7:57 pm)
Re: Is gcc thread-unsafe?, Zachary Amsden, (Thu Oct 25, 9:15 pm)
Re: Is gcc thread-unsafe?, linux-os (Dick Johnson), (Thu Oct 25, 7:58 am)
Re: Is gcc thread-unsafe?, Andi Kleen, (Thu Oct 25, 8:16 am)
Re: Is gcc thread-unsafe?, Arjan van de Ven, (Wed Oct 24, 11:46 pm)
Re: Is gcc thread-unsafe?, Nick Piggin, (Wed Oct 24, 11:58 pm)
RE: Is gcc thread-unsafe?, David Schwartz, (Thu Oct 25, 12:29 am)
Re: Is gcc thread-unsafe?, Nick Piggin, (Thu Oct 25, 12:47 am)
Re: Is gcc thread-unsafe?, Samuel Tardieu, (Thu Oct 25, 5:44 am)
Re: Is gcc thread-unsafe?, Andi Kleen, (Thu Oct 25, 5:55 am)
Re: Is gcc thread-unsafe?, Arjan van de Ven, (Thu Oct 25, 12:35 am)