Messages in this thread | | | Date | Sun, 27 Oct 2013 20:56:41 +0100 | From | Maarten Lankhorst <> | Subject | Re: [GIT PULL] locking fix |
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op 27-10-13 20:51, Linus Torvalds schreef: > On Sun, Oct 27, 2013 at 12:37 PM, Maarten Lankhorst > <maarten.lankhorst@canonical.com> wrote: >> I would love for a compiler to become that smart though, but I do not think it's likely. > Dammit, even if that is true, then write the conditional *correctly*. > > As mentioned, the conditional > > __builtin_constant_p(ww_ctx) && ww_ctx == NULL > > is actually sensible, in a way the original one was *not*. It actually > tests what you apparently intended to test, and is more readable to > humans to boot. Yeah that mail arrived after I sent mine, I agree that this would have been more sensible. > And no, it still isn't actually guaranteed to do what you want it to > do. Historically, in gcc, __builtin_constant_p() really only ever > worked in macros, because by the time you use it in inline functions, > a constant NULL in the caller will have been turned into a argument > variable in the inline function, and __builtin_constant_p() would be > done before that was optimized away. Over the years, gcc has pushed > some of the builtin evaluation deeper down, and these days it actually > works within inline functions, but my point that > __builtin_constant_p() is about a certain level of compiler > optimization is very much true: you're actually testing for a compiler > optimization detail. > > I know the LLVM people had similar issues with this comparison, so > these days it's not even just about gcc versions. We may never have > cared very much about icc, but llvm is actually an interesting target > compiler. > And this is why ww_ctx == NULL is now passed as an inline argument. :)
~Maarten
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