Date: Fri, 16 Jan 2009 11:21:39 +0300 From: Stanislav Sedov <stas@FreeBSD.org> To: "M. Warner Losh" <imp@bsdimp.com> Cc: sobomax@FreeBSD.org, danfe@FreeBSD.org, src-committers@FreeBSD.org, svn-src-all@FreeBSD.org, gonzo@FreeBSD.org, svn-src-head@FreeBSD.org, christoph.mallon@gmx.de Subject: Re: svn commit: r187251 - head/sys/mips/malta Message-ID: <20090116112139.fd8f0c4a.stas@FreeBSD.org> In-Reply-To: <20090115.175500.692488694.imp@bsdimp.com> References: <20090115114407.GA67726@FreeBSD.org> <496F24D8.2040104@gmx.de> <496FC32F.3040104@FreeBSD.org> <20090115.175500.692488694.imp@bsdimp.com>
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-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On Thu, 15 Jan 2009 17:55:00 -0700 (MST) "M. Warner Losh" <imp@bsdimp.com> mentioned: > In message: <496FC32F.3040104@FreeBSD.org> > Maxim Sobolev <sobomax@FreeBSD.org> writes: > : Christoph Mallon wrote: > : > Alexey Dokuchaev schrieb: > : >> On Wed, Jan 14, 2009 at 07:05:27PM -0700, M. Warner Losh wrote: > : >>> In message: <20090115020752.52566769.stas@FreeBSD.org> > : >>> Stanislav Sedov <stas@FreeBSD.org> writes: > : >>> : > + shift = 8 * (reg & 3); > : >>> : > : : Would it make sense to replace this with > : >>> : > + shift = (reg & 3) << 3; > : >>> : : to not rely on possible compiler optimizations? > : >>> > : >>> I don't think that it matters all that much these days... > : >> > : >> But the name "shift" kinda suggests << instead of *, no? > : > > : > The value *is* a shift amount (see its uses a few lines down). Its name > : > does not imply the way it is calculated, but what it is used for. > : > > : > BTW: Even the most cheap compilers emit shift instructions for > : > multiplication by a power of two. The new code also is clearly faster > : > then the old - quite some code gets generated for switches. > : > : I believe Warner's point is that the code is not in the hot path, so > : that it should not really matter either way. > > The code is clear the way it is, there's little reason to > hyper-optimize this path in the face of bogus compiler optimizations > or not, and the effort to optimize the hot-path should be aided by > dtrace or kernel profiling rather than reading the code. > Heh, that was just a suggestion, as the author decided to optimize the code anyway.:-) Of course, this optimization doesn't matter here much. - -- Stanislav Sedov ST4096-RIPE -----BEGIN PGP SIGNATURE----- iEYEARECAAYFAklwQ5cACgkQK/VZk+smlYH9xQCffKoCymcGgyBUKBT2HHi4jLiM L3IAn2QNrh3FsdQGMdD1H6rKMmc7YhzE =qfKk -----END PGP SIGNATURE----- !DSPAM:49704399967005350815758!
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