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Re: The speed of the compiler, was: Re: Combine four insns
On Tue, Aug 10, 2010 at 7:48 AM, Andi Kleen <andi@firstfloor.org> wrote:
> Chris Lattner <clattner@apple.com> writes:
>>
>> ? e. General speedups: Clang's preprocessor is roughly 2x faster than GCC's and the frontend is generally much faster. ?For example, it uses hash tables instead of lists where appropriate, so it doesn't get N^2 cases in silly situations as often. ?I don't what what else GCC is doing wrong, I haven't looked at its frontends much.
>
> I looked at this a weekend or two ago. The two hot functions in the
> preprocessor are cpp_clean_line and the lexer.
>
> At least cpp_clean_line was pretty easy to speed up using SSE 4.2
> string instructions and vectorizing it.
>
> That change made it drop down from top 10 in a unoptimized build to
> lower top 40 or so. I suspect with that change the clang advantage
> is much less than 2x.
>
> Drawback: the patch broke some of the PCH test cases in the test
> suite and I never quite figured out why (that's why I didn't post
> the patch)
>
> Other drawback: the optimization only helps on x86 systems
> that support SSE 4.2 (but presumably that's a popular build system)
>
> Here's the patch if anyone is interested.
>
> Vectorizing the lexer might be possible too, but it's somewhat
> harder.
>
> The other problem I found is that cpplib is not using profile
> feedback, that is likely giving some performance away too.
>
> -Andi
>
>
You can use IFUNC to automatically enable it on Linux.
--
H.J.