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Re: [Bug libc/18402] FPU use in application before main()
- From: OndÅej BÃlka <neleai at seznam dot cz>
- To: "hjl.tools at gmail dot com" <sourceware-bugzilla at sourceware dot org>
- Cc: glibc-bugs at sourceware dot org
- Date: Tue, 12 May 2015 20:06:28 +0200
- Subject: Re: [Bug libc/18402] FPU use in application before main()
- Authentication-results: sourceware.org; auth=none
- References: <bug-18402-131 at http dot sourceware dot org/bugzilla/> <bug-18402-131-Yw9VTznuoW at http dot sourceware dot org/bugzilla/>
On Tue, May 12, 2015 at 04:46:31PM +0000, hjl.tools at gmail dot com wrote:
> https://sourceware.org/bugzilla/show_bug.cgi?id=18402
>
> --- Comment #10 from H.J. Lu <hjl.tools at gmail dot com> ---
> (In reply to Dave Hansen from comment #9)
> >
> > There's also the matter that the kernel tries to be lazy about allocating
> > and saving/restoring FPU state. If the task is virtually guaranteed to have
> > FPU state, then we should reconsider the lazy mode being the default.
>
> Since most, if not all, applications will call string/memory functions,
> which are optimized with SSE, kernel needs to prepare for that.
>
Yes, lazy FPU is relict from 486 days. Now it only decreases performance
as every application uses sse. Also gcc vectorizer will likely introduce sse
instruction to improve performance of vectorizable loop.
I would mark this as invalid as I don't see why its problem.