[LLVMdev] interest in support for Transactional Memory?

[Duncan Sands]

Hi Torvald,

> I would like to know whether the community is interested in getting support
> for Transactional Memory (TM) merged in upstream LLVM.

I guess not :(

TM basically gives you
> transaction properties (eg, virtually atomic + isolated execution) for
> ordinary program code. Thus, to make incrementing a counter thread-safe, you
> could say __transaction { counter++; } and the compiler would transform this
> code so that it uses a TM library, which in turn does concurrency control for
> the memory accesses in a transaction. Recent studies support the assumption
> that shared-memory synchronization with transactions is supposed to be a lot
> easier than when using locking, for example.

Why does this require special LLVM support rather than, say, having the front
end lower everything to library calls and so forth, like gcc does for OpenMP?

Ciao,

Duncan.

>
> There seems to be interest by several large companies towards TM. Intel,
> Oracle, IBM, Red Hat, and others have been working on a specification for
> memory transactions in C++:
> http://software.intel.com/file/21569
> Intel, Red Hat, and my group have been working on specifying a TM library
> ABI:
> http://software.intel.com/file/8097/
> Intel has published an ICC prototype version with TM support, and Red Hat is
> working on TM support in the gcc "trans-memory" branch.
>
> My group has been working on compiler support for TM for LLVM during the last
> couple of years, and we have built an LLVM-based compiler, DTMC, that covers
> both the frontend part (ie, parsing the __transaction statements and
> translating txn boundaries to IR markers) and the IR transformations
> (transforming all transactional code). The former is a patch to llvm-gcc
> (based, in turn, on gcc's TM support), but we're currently also looking at
> other options for the frontend. The latter is a normal module pass, and the
> larger part of the work.
> We have also built a TM library, TinySTM++ (but TM libraries are not compiler
> specific). You can have a look at the last stable releases of the code
> (compiler and library) at http://tm.inf.tu-dresden.de
>
>
> Torvald
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