Date: Thu, 07 Jun 2018 09:52:06 -0400 From: Ravi Pokala <rpokala@freebsd.org> To: Brooks Davis <brooks@freebsd.org> Cc: "Jonathan T. Looney" <jtl@freebsd.org>, Mateusz Guzik <mjguzik@gmail.com>, Mateusz Guzik <mjg@freebsd.org>, src-committers <src-committers@freebsd.org>, <svn-src-all@freebsd.org>, <svn-src-head@freebsd.org> Subject: Re: svn commit: r334702 - head/sys/sys Message-ID: <505A6291-6B3C-4857-8A9C-60E58EB50454@panasas.com> In-Reply-To: <20180607132733.GA41876@spindle.one-eyed-alien.net> References: <201806060508.w56586c9053686@repo.freebsd.org> <6E6E92B2-7536-4281-8EAF-72823E84902E@panasas.com> <CAGudoHF9Kw6gFyNkJADzCCg0vvSq-o%2BrWwxBX15cKszDSV5KiA@mail.gmail.com> <47E06039-234C-4078-A732-BFF230D2472B@panasas.com> <CADrOrmu5gYakgXu4bM9CMh5zQa2LZv6pA7tTEScoZH-hs9hMTQ@mail.gmail.com> <468B8AB5-D2C7-4033-9F24-6E1F94DC7137@panasas.com> <20180607132733.GA41876@spindle.one-eyed-alien.net>
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-----Original Message----- From: Brooks Davis <brooks@freebsd.org> Date: 2018-06-07, Thursday at 09:27 To: Ravi Pokala <rpokala@freebsd.org> Cc: "Jonathan T. Looney" <jtl@freebsd.org>, Mateusz Guzik <mjguzik@gmail.com>, Mateusz Guzik <mjg@freebsd.org>, src-committers <src-committers@freebsd.org>, <svn-src-all@freebsd.org>, <svn-src-head@freebsd.org> Subject: Re: svn commit: r334702 - head/sys/sys > On Thu, Jun 07, 2018 at 12:01:00AM -0400, Ravi Pokala wrote: >>> I believe the theory is that the compiler (remember, this is __builtin_memset) can optimize away portions of the zeroing, or can optimize zeroing for small sizes. >> >> Ahhh! I didn't consider that the compiler would be doing analysis of the larger context, and potentially skipping zeroing parts that are set immediately after the call. > > Clang does this. It does make for some quite interesting object code, > but the result is that zeroing with __builtin_memset() is basically free > for mostly-initialized structures. > > -- Brooks Yeah, it's a subtle but obvious optimization in hindsight. Thanks for the info. -Ravi (rpokala@)
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