asm volatile statement reordering

[Andrew Haley]

On 17/10/17 14:00, David Brown wrote:
> So how do you implement these macros in a way that is efficient,
> correct, and safe to use?  Including memory barriers in the form of
> memory clobbers certainly helps, though it can mean less optimal code -
> give the "normal" users sub-optimal but ease and safe macros, and let
> the advanced (or obsessive!) users fix the dependencies themselves.  But
> as we have seen, memory barriers are not necessarily enough.  There is
> no "execution barrier".

Indeed, yes, you'd want a way to insert an opaque barrier to GCC's
optimization horizon (i.e. how far GCC can see.)  But that is an
extremely violent thing to do, effectively even more than a memory
clobber.

In a way, I think that you're asking for something self-contradictory.
If you want a full execution barrier, you'll have to force all
calculations to complete before ENTER_CRITICAL(), even those you don't
need.

I think it would be possible to write versions of ENTER_CRITICAL() and
EXIT_CRITICAL() which take arguments, and these would be the values
which must be stable at these points.  You could do that with current
GCC with asm("" : "+r"(bar), "+r"(baz));.  But there's still nothing
to stop GCC from doing anything unrelated during the time that
interrupts are disabled.  If you really need that guarantee, a
noinline function is the only way I know to get it done.

-- 
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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