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Re: [PATCH] Kill __bzero()

To: Franck Bui-Huu <vagabon.xyz@gmail.com>
Subject: Re: [PATCH] Kill __bzero()
From: Ralf Baechle <ralf@linux-mips.org>
Date: Mon, 5 Nov 2007 23:18:18 +0000
Cc: Franck Bui-Huu <fbuihuu@gmail.com>, linux-mips <linux-mips@linux-mips.org>
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References: <472D8058.5080209@gmail.com> <20071105112429.GC27893@linux-mips.org> <472F906F.7080205@gmail.com>
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On Mon, Nov 05, 2007 at 10:51:43PM +0100, Franck Bui-Huu wrote:

> > Memset is almost always only ever invoked with a zero argument.  So the
> > idea was to have something like this:
> > 
> > extern void *__memset(void *__s, int __c, size_t __count);
> > extern void *bzero(void *__s, size_t __count);
> > 
> > static inline void *memset(void *s, int c, size_t count)
> > {
> >     if (__builtin_constant_p(c) && c == 0) {
> >             bzero(s, count);
> >             return s;
> >     } else
> >             return __memset(s, __c, count);
> > }
> > 
> > But that was never quite implemented like this as you noticed.
> 
> Well I'm not sure we really need this. bzero() is not part of the
> Linux string API, so it can only be used by MIPS specific code. And
> with the current implementation of bzero(), $a1 needs to be setup to 0
> anyway. That's why I simply killed it...
> 
> BTW, can memset() be an inlined function ?

It can be anything, macro, inline or outline function.  In the kernel
there are fewer restrictions than for a standards compliant library in
userspace.

You may take the i386 implementation in include/asm-x86/string_32.h as
an extreme example.

Older gcc used to generate significantly worse code for inline functions
than for macros so Linux became a fairly excessive user of macros.  This
has very much improved since, so these days inlines are prefered over
macros where possible.

> Yes I noticed this. Actually I'm wondering if we couldn't add a new
> function, fill_user() like the following:
> 
> extern size_t fill_user(void __user *to, int c, size_t len);

That's much better function name than the old __bzero - except that
__bzero effectivly took a long argument for the 2nd argument so 32-bit
on 32-bit kernels and 64-bit on 64-bit kernels.

> This could be used by both memset() and clear_user():
> 
> #define memset(s,c,l) ({ (void)fill(s,c,l); s; })
> #define clear_user(t,l)       fill_user(t,0,l)
> 
> Therefore the definition of clear_user() could be saner.

Looks alot nicer that way though an inline is probably preferable as
expressed above.

  Ralf

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