This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: Old compiler optimizations in installed headers
- From: Richard Henderson <rth at twiddle dot net>
- To: OndÅej BÃlka <neleai at seznam dot cz>, Joseph Myers <joseph at codesourcery dot com>
- Cc: libc-alpha at sourceware dot org
- Date: Wed, 27 May 2015 08:15:51 -0700
- Subject: Re: Old compiler optimizations in installed headers
- Authentication-results: sourceware.org; auth=none
- References: <alpine dot DEB dot 2 dot 10 dot 1505221433160 dot 16611 at digraph dot polyomino dot org dot uk> <20150524133125 dot GA19929 at domone>
On 05/24/2015 06:31 AM, OndÅej BÃlka wrote:
> While good idea in general I would be wary of signbit et al. macros.
> Using gcc will cause performance regression in some cases as they
> generate suboptimal branchless code. Adding branches is correct as
> performance savings from branchless code are in branch misprediction
> which doesn't happen. If you have more than 5% inputs NaN to offset
> single cycle penalty of going branchless your problem is that you are
> producing garbage instead performance.
I beg your pardon? Why would you *ever* have a branch implementing signbit?
r~