Messages in this thread | | | Date | Mon, 23 Jul 2007 22:13:01 +0530 (IST) | From | Satyam Sharma <> | Subject | Re: [PATCH 7/8] i386: bitops: Kill needless usage of __asm__ __volatile__ |
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Hi Jeremy,
On Mon, 23 Jul 2007, Jeremy Fitzhardinge wrote:
> Satyam Sharma wrote: > > From: Satyam Sharma <ssatyam@cse.iitk.ac.in> > > > > [7/8] i386: bitops: Kill needless usage of __asm__ __volatile__ > > > > Another oddity I noticed in this file. The semantics of __volatile__ > > when used to qualify inline __asm__ are that the compiler will not > > (1) elid, or, (2) reorder, or, (3) intersperse, our inline asm with > > the rest of the generated code. > > > > "asm volatile" does not mean that at all. It only guarantees (1),
Actually, you're probably right about (2), but (3)?
From the gcc manual:
<quote>
Similarly, you can't expect a sequence of volatile asm instructions to remain perfectly consecutive. If you want consecutive output, use a single asm. Also GCC will perform some optimizations across a volatile asm instruction, GCC does not "forget everything" when it encounters a volatile asm instruction the way some other compilers do.
</quote>
I'm reading "Similarly, you can't expect a sequence of volatile asm instructions to remain perfectly consecutive" to mean they're talking about something like:
asm volatile(...); asm volatile(...); asm volatile(...);
But "If you want consecutive output, use a single asm" probably means:
asm volatile(... multiple instructions here ...);
would actually ensure the code written in there would not be interspersed ... at least that's how I read it.
[ BTW "Also GCC will perform some optimizations across a volatile asm instruction ..." is exactly the kind of optimizations we must allow the compiler to do, but that's not related to point at hand. ]
> and > only then if the asm is ever reachable.
Yup, of course.
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