Date: Thu, 7 Jun 2018 13:27:33 +0000 From: Brooks Davis <brooks@freebsd.org> 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 Message-ID: <20180607132733.GA41876@spindle.one-eyed-alien.net> In-Reply-To: <468B8AB5-D2C7-4033-9F24-6E1F94DC7137@panasas.com> 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>
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[-- Attachment #1 --] 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 [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- iQEcBAEBAgAGBQJbGTLEAAoJEKzQXbSebgfA8cIH/RlCwFDXLPa31BCS4XMjzqxJ /31i3IEKOH7/4aQHxlf58rquy6xAi1w3nI0bLavyCU/9BFoJeAvwat1UkdVCK92x 6z3tdOP0XUK2hAJidT1T4/lgWT8VbRERnGOnQseL2fSV9Cb64PDkP6RLB1lEVBTE I4XzEM6oHOu5q6H74z2DDH4R0pNO1LU2K35pm4M4QTYGW5DWV7sXtnTzUONjnO07 6XSV8B/jFNiTUNEqY81o9jkcpk6ocCPKDsgIjwym6hDbEGrVrSmJFw6oDzoLoPI+ wkwMh1LDtm07VxgD5n8wt7RjUb/2a79GuQ/AcZhstzvnZekBP8iyGBJSXBgvJF0= =rAJw -----END PGP SIGNATURE-----
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