Re: __builtin_expect in clang?
Re: __builtin_expect in clang?
- Subject: Re: __builtin_expect in clang?
- From: Andreas Grosam <email@hidden>
- Date: Sat, 12 Nov 2011 11:57:37 +0100
Thank you all for the responses and interesting information,
On Nov 10, 2011, at 2:51 PM, Andreas Grosam wrote:
> I can't see any performance differences in a very hot code spot, when using it in branches, or omitting it, or even when giving the compiler *false* hints.
> So, I'm wondering whether __builtin_expect is implemented in clang?
On Nov 12, 2011, at 4:49 AM, Wim Lewis wrote:
> I think it isn't. I ran clang (and llvm-gcc) with -O3 -S and compared the resulting assembly for opposite values of expect, and I couldn't come up with input for which there were any differences. gcc-4.2.1 makes some fairly obvious changes based on that hint, though (moving unlikely code out of an inner loop, for example).
>
On Nov 11, 2011, at 5:22 PM, Don Quixote de la Mancha wrote:
> I don't have the first clue whether CLang or LLVM implement
> __builtin_expect, but if they did, and their implementation was
> complete, and your use of __builtin_expect was both widespread
> throughout your code and configured correctly, I would expect it to
> make a measurable difference even for instruction sets that don't have
> branch prediction bits.
For x86_64 I'm not seeing any difference in the runtime. The code is however already written such that the compiler should be able to produce fast code. When searching for __builtin_expect in clang related forums it appears that __builtin_expect is implemented. Bit to what extent?
So, I'm still wondering, whether I actually *should* observe at least some minor difference in performance, or whether there is an unnoticed bug in the optimizer or code generator.
Andreas _______________________________________________
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