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Re: basic asm and memory clobbers
- From: Richard Henderson <rth at redhat dot com>
- To: Segher Boessenkool <segher at kernel dot crashing dot org>
- 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 16:29:50 +0100
- Subject: Re: basic asm and memory clobbers
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On 11/20/2015 04:20 PM, Segher Boessenkool wrote:
On Fri, Nov 20, 2015 at 02:05:01PM +0100, Richard Henderson wrote:
I'd be perfectly happy to deprecate and later completely remove basic asm
Because IMO it's essentially useless. It has no inputs, no outputs, and no
way to tell the compiler what machine state has been changed. We can say
that "it clobbers everything", but that's not actually useful, and quite
difficult as you're finding out.
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" : ); instead.
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