[LLVMdev] LLVM and OpenMP

Wojciech Matyjewicz wmatyjewicz at fastmail.fm
Thu Nov 29 09:52:05 PST 2007


In the beginning I'd like to say I am not very familiar with (llvm-)gcc
internals, so some of my assumptions and guesses may be wrong. Please,
correct me, then.

Anton Korobeynikov wrote:
> Internally OpenMP lowering in gcc is split into two phases. Funny, but
> llvm converter is run between these phases :) It looks like second phase
> uses some gcc analysis passes (like dominance info, etc). 
> In order to support OpenMP in llvm-gcc 4.2 we should:
> 1. Decide at which level we should handle openmp trees in llvm-gcc
> 2. Define a set of openmp builtins, which will need to be supported in
> llc codegen
> 3. Something I'm not aware of :)

I've found an interesting paper on the design and implementation of
OpenMP in GCC: http://people.restadhat.com/dnovillo/Papers/gcc2006.pdf
It describes how and when OpenMP pragmas are lowered. It seems the
second phase that Anton wrote about is the most important one. I believe
it is the "pass_expand_omp" mentioned in the paper. Its aim is to
extract parallel sections into new functions, expand directives into
built-in function calls, insert code to calculate iteration-space, etc.
I guess no GIMPLE codes used by OpenMP are left after this pass. If it
is possible to do the conversion to LLVM IR at this stage, the only
thing to do is to handle OpenMP built-ins. The simplest route would be
to replace them with calls to libgomp (an LGPL library accompanying
GCC). It is what GCC seems to do.

The second option is allowing to somehow express OpenMP directives
directly in the LLVM (or some general multiprocessing directives that
OpenMP ones can be lowered to). Then, a pass could be written that
handles them in a similar fashion to "pass_expand_omp" in GCC (see
above). Any frontend (i.e. clang) would have simpler job supporting
OpenMP then. That said, I admit I have no idea how to express these
directives. For example: how to annotate a loop that it should be turned
into a parallel one? I believe keeping the IR as simple as possible is
an important goal, so we would need some lightweight mechanism,
transparent to the existing passes.

I'm personally interested in the subject, because I am trying to develop
an automatic loop parallelization pass for LLVM. Initially, I thought I
would write anything from scratch. However, if there is a decision to
include some support for OpenMP (and similar techniques) lowering in the
LLVM core, I'am ready to help.


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