[PATCH] ARM: allow inline atomics on Cortex M

[Phoebe Buckheister]

Hi,

On Mon, 16 Jun 2014 15:21:59 -0600, Jonathan Roelofs wrote:
> 
> 
> On 6/16/14, 3:00 PM, Phoebe Buckheister wrote:
> > Current code to determine availability of atomics seems a bit
> > incorrect, or at least weird.
> >
> > I don't quite understand why clang looks at the OS specified in the
> > triple, as availability of atomic instructions doesn't depend on the
> > OS; it only depends on the arch version and the execution mode.
> > Atomics are there in ARM mode on v6 and later, and in Thumb mode on
> > v7 and later (but no ldrexd/strexd in the M profile).
> Clang is looking at the OS because these OSes support calling a
> kernel provided cmpxchg, which is what you need to make the __sync_*
> builtins work on pre v6 hardware (and this is how they are
> implemented in libgcc).
> 
> Given that, I think that this check and comment:
[…]
> ought to really be placed as a fallback if the HW is older than v6.

Thanks, I'll put it back. I had assumed that in those cases, clang
would just always generate inline atomics and let LLVM fix it up if the
actual hardware doesn't support it.

> Cheers,
> 
> Jon
> 
> >
> > This patch is an attempt to rectify the situation: check only the
> > arch version - correctly, as thumbv7em fails the old check - and
> > allow atomics up to 32 bits on the ARMv7 M profiles.
> >
> > If the OS check has a reason I missed, I'll add it back - but bare
> > metal wants atomics too ...
> >
> >
> >
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> >
> 
> -- 
> Jon Roelofs
> jonathan at codesourcery.com
> CodeSourcery / Mentor Embedded