[PATCH] Add HLE target feature

[Michael Liao]

Hi Jeffery

Thanks for reminder. I will add documentation soon. So far, at IR level,
the only change is adding a metadata (!targetflags <N>) through the
patch. The flag word will be defined by target and not a generic one.

Target-specific flag should be helpful for other architectures, such as
Power. The similar HLE hint could be used by target backend to replace
regular atomic builtin with a HLE version if they have efficient TM
support.

Yours
- Michael

On Wed, 2013-02-20 at 17:28 -0800, Jeffrey Yasskin wrote:
> On Wed, Feb 20, 2013 at 5:08 PM, Chandler Carruth <chandlerc at google.com> wrote:
> > 1) The base case: we must have a single spec for the LLVM IR model. This
> > should be tailored to the needs of optimizers, general enough to represent
> > the various builtins, and still support efficient lowering to both X86 and
> > potentially PPC implementations.
> 
> Oh yeah, I'd forgotten this part. Michael, could you point us to your
> patch against http://llvm.org/docs/Atomics.html documenting your new
> IR attributes? I didn't look through every patches, but I didn't see
> any documentation in a brief scan.
> 
> Note that "Start a Hardware Lock Elision transaction with the atomic
> operation." won't be sufficient, even if these are just
> semantically-ignorable hints. I suspect that there's some
> actually-useful semantics here that would make these operations usable
> by other architectures if they ever add such instructions. For
> example, see the documentation of the !nontemporal metadata.
> 
> Thanks,
> Jeffrey