Date: Thu, 25 Jul 2013 12:43:49 +0100 From: David Chisnall <theraven@FreeBSD.org> To: Hans Petter Selasky <hps@bitfrost.no> Cc: svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org, Tim Kientzle <kientzle@FreeBSD.org>, src-committers@FreeBSD.org, Bruce Evans <brde@optusnet.com.au> Subject: Re: svn commit: r253636 - head/sys/vm Message-ID: <73FCA347-5EB9-445F-A25C-D06CA137CBEE@FreeBSD.org> In-Reply-To: <51F0DDB0.7080102@bitfrost.no> References: <201307250348.r6P3mbsG049595@svn.freebsd.org> <20130725171038.O841@besplex.bde.org> <51F0DDB0.7080102@bitfrost.no>
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On 25 Jul 2013, at 09:11, Hans Petter Selasky <hps@bitfrost.no> wrote:
> The structure looks like some size, so bzero() might run faster than memset() depending on the compiler settings. Should be profiled before changed!
They will generate identical code for small structures with known sizes. Both clang and gcc have a simplify libcalls pass that recognises both functions and will elide the call in preference to a small set of inline stores.
However(), memset is to be preferred in this idiom because the compiler provides better diagnostics in the case of error:
bzero.c:9:22: warning: 'memset' call operates on objects of type 'struct foo'
while the size is based on a different type 'struct foo *'
[-Wsizeof-pointer-memaccess]
memset(f, 0, sizeof(f));
~ ^
bzero.c:9:22: note: did you mean to dereference the argument to 'sizeof' (and
multiply it by the number of elements)?
memset(f, 0, sizeof(f));
^
The same line with bzero(f, sizeof(f)) generates no error.
David
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