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List:       busybox
Subject:    Re: [PATCH] top: fix parsing of /proc/meminfo
From:       Rich Felker <dalias () libc ! org>
Date:       2014-07-22 17:30:42
Message-ID: 20140722173042.GY17402 () brightrain ! aerifal ! cx
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On Tue, Jul 22, 2014 at 08:24:30PM +0300, Timo Teras wrote:
> On Tue, 22 Jul 2014 13:07:15 -0400
> "Cathey, Jim" <jcathey@ciena.com> wrote:
> 
> > >Am I missing something here?  There is no structure, just a
> > >character pointer.  If you leave off "static" it will be compiled as
> > >an instruction that pushes a constant onto the stack, by any/every
> > >compiler.
> > 
> > Which is probably _larger_ code than just referring to something
> > stored in the .text/.data segment.  It's copying something that is
> > found in text/data into the stack space, then referring to that
> > address thereafter.  The compiler has to do:
> > 
> > 	Copy A->B
> > 	Then use &B.
> > 
> > -vs-
> > 
> > 	Use &A.
> > 
> > You do this when you want the ability to modify B.
> > If A is fixed and inviolate, you mark it static const
> > and be done with it.  What this turns into, exactly,
> > depends upon your compiler, CPU, and ABI, but I can't
> > think of any case where the extra copy is going to
> > turn out _better_ than the simpler case.
> 
> Usage of 'static' depends whether it's "const char *" or "const
> char[]". When defining "const char *" static does not make sense, as
> in theory it should enforce *also* the pointer value to go to .rodata
> (the string literal of course goes there too). So you might end up
> wasting one extra pointer in .rodata.
> 
> OTOH, with "const char[]" you definitely want the "static" as other
> wise you'd end up with the copy described above.
> 
> I only suggested removing 'static' since I had it a pointer. The latest
> patch has array, and thus is static.

I agree that using a static const array is the best approach here, and
that with a *single* pointer, making a static object to hold the
pointer rather than just an automatic const-qualified variable is a
pessimization, if anything. However static const pointers are useful
when they're arrays of pointers, or part of structures, etc., but
their usefulness is limited except with static linking and non-PIE
main programs, since they'll need relocations for PIC/PIE which would
throw them out of rodata.

Rich
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