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Re: __sync_sychronize on ARM

Hi Ramana,

Thanks very much for getting back to me!

On Mon, 2012-01-30 at 08:50 +0000, Ramana Radhakrishnan wrote:
> On Mon, Jan 30, 2012 at 6:56 AM, Jon Masters <> wrote:
> > The __sync_synchronize "legacy" sync function is intended to be used to
> > perform an expensive data memory barrier operation. It is defined within
> > libgcc in such a way that I *believe* means that, on most architectures,
> > it is replaced with an inline assembly code emitted that performs a sync
> > operation. On ARM, and some other architectures with mixed ISAs wherein
> > there may not be a sync function nor one way to do this, this function
> > (__sync_synchronize) can be a real function call. In that case, it might
> > cause inline assembly generation, or e.g. call a kernel VDSO helper.
> On ARM we don't have a kernel VDSO

You're right of course! I was confusing the VDSO-like user mode mapped
helpers (Documentation/arm/kernel_user_helpers.txt) with full VDSO. I
apologize for my mistake. Nonetheless, I believe for the purposes of
this thread, we can consider the behavior I described roughly consistent
with reality, because a kernel helper will be called in a VDSO-like way.

> sync_synchronize for older versions of
> the architecture ( anything prior to armv6k) should result in a call
> to sync_synchronize
> in libgcc.a which should take care of calling the kernel helper function.

This is what's confusing me :) Is one supposed (from some random source)
to be calling __sync_synchronize or sync_synchronize? Convention
suggests the latter, but I was sufficiently confused by the aliasing of
the names in the source vs. the documentation, so I'd like to ask you :)

> Therefore I'm assuming this is a breakage you face when building for
> armv5te

It is indeed. Thanks for noting that. 

> > The icu package contains a direct call to __sync_sychronize, especially
> > in the iotest test cases. I believe that this compiles fine on x86
> > because there is no function call. However, on ARM, the code fails to
> > link because the __sync_synchronize function is HIDDEN and not exported
> > (or so goes my understanding - is that correct?).
> No, the HIDDEN shouldn't cause a link failure in this case - you
> should be able to pull this
> in when you link against the static libgcc where this should be defined.
> I don't know what your linker command line is  so maybe that's a place
> to start investigating from.

Thanks! You're the second person to suggest that, so I'll look some
more. Could you let me know about the correct function name, above?

Appreciate the help.


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