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Re: basic asm and memory clobbers
- From: Segher Boessenkool <segher at kernel dot crashing dot org>
- To: Richard Henderson <rth at redhat dot com>
- Cc: David Wohlferd <dw at LimeGreenSocks dot com>, Andrew Haley <aph at redhat dot com>, Jeff Law <law at redhat dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>, rth at gcc dot gnu dot org, pinskia at gcc dot gnu dot org, Sandra Loosemore <sandra at codesourcery dot com>
- Date: Fri, 20 Nov 2015 10:21:07 -0600
- Subject: Re: basic asm and memory clobbers
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- References: <564AC155 dot 4040601 at LimeGreenSocks dot com> <564B9CB1 dot 1060001 at redhat dot com> <564E762B dot 6070705 at LimeGreenSocks dot com> <564EF338 dot 4030703 at redhat dot com> <564EF7FF dot 1070107 at LimeGreenSocks dot com> <564F008B dot 8040703 at redhat dot com> <564F1436 dot 2060005 at LimeGreenSocks dot com> <564F1A7D dot 9050607 at redhat dot com> <20151120152039 dot GA15922 at gate dot crashing dot org> <564F3C6E dot 5010908 at redhat dot com>
On Fri, Nov 20, 2015 at 04:29:50PM +0100, Richard Henderson wrote:
> >>It seems to me that it would be better to remove the feature, forcing what
> >>must be an extremely small number of users to audit and update to extended
> >Should asm("bla"); then be an extended asm with no input, no outputs,
> >no (non-automatic) clobbers? That would be the most straightforward and
> >logical semantics, but will it break user code?
> I'm suggesting that we don't accept that at all inside a function. One
> must audit the source and make a conscious decision to write asm("bla" : );
Ah, or excepting asm("bla") and treating it just like asm("bla" : ), but
giving a warning? That will get people to migrate at least.
> Accepting basic asm outside of a function is perfectly ok, since that's
> just a mechanism by which one can inject complete assembly routines into a
> C translation unit.
Of course. You cannot have extended asm outside of functions at all.