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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
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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.
>=20
> Ahhh! I didn't consider that the compiler would be doing analysis of the =
larger context, and potentially skipping zeroing parts that are set immedia=
tely 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

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