Date: Mon, 3 Jun 2013 20:57:45 -0700 From: Adrian Chadd <adrian@freebsd.org> To: Juli Mallett <jmallett@freebsd.org> Cc: Ed Schouten <ed@80386.nl>, "freebsd-mips@FreeBSD.org" <freebsd-mips@freebsd.org>, FreeBSD-arch <freebsd-arch@freebsd.org> Subject: Re: Kernelspace C11 atomics for MIPS Message-ID: <CAJ-Vmo=5%2BmWk4EWBuTdpF6vKx-%2BK=g1euJvZuRDF%2BvFkJNZZ4A@mail.gmail.com> In-Reply-To: <CACVs6=_X5vOfR%2BQOgvz6P-j3jUoNoK9hCFvz80fGRL3-PgBf5g@mail.gmail.com> References: <CAJOYFBD502MYbkVR2hnVDTYWOvOUr15=OPyjotNvv%2BZ09vQ1OQ@mail.gmail.com> <D02AF210-5129-40AB-9481-3F0A44575E98@bsdimp.com> <CAJ-Vmo=vNbT9majPCZ8ugzPsNzh46DTD4mMDX-cuxx9Og91ptw@mail.gmail.com> <CACVs6=_X5vOfR%2BQOgvz6P-j3jUoNoK9hCFvz80fGRL3-PgBf5g@mail.gmail.com>
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On 3 June 2013 20:55, Juli Mallett <jmallett@freebsd.org> wrote: > To drain the pipeline on certain deficient (and mostly older) CPUs by way of > guesswork and a little vague magic. Most CPUs we support, I would guess, do > not need this, and it continues to exist solely for hysterical reasons. How can I turn it off for my compiles? > I've certainly gotten rid of them and some other cargo cult synchronization > on Octeon for testing and had it survive under considerable load, and > occasionally with some slight speedups (for some more commonly-used or > slower things than Just a Bunch Of NOPs.) Right. Well, since it's happening on every inlined lock, it's a bit silly. > The trouble is that proving they aren't necessary requires being rigorous > and careful in understanding documentation and errata, and FUD about their > possible necessity is somewhat-intimidating. It's not an easy kind of > corruption/unreliability/etc., to prove the lack of empirically. I've checked the diassembly from gcc-4.mumble on linux; it doesn't include NOPs like this as far as I can tell. Adrian
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