[Openmp-dev] Knowing if an IR instruction will be executed by an OpenMP region

Simone Atzeni simone.at at gmail.com
Wed Feb 11 09:15:03 PST 2015

Hi Ronan,

thanks for you answer.
I think I will try to implement your first suggestion, some inter procedural control flow analysis.
I would like to do it statically and not involving any dynamic analysis.

My basic idea is that given an instruction we can check if it belongs or not to an OpenMP microtask, if yes it will be executed
in parallel, if not I recursively check if its parent function is called inside an OpenMP construct and so on.
In order to do this I was taking a look to llvm::CallGraph (http://llvm.org/docs/doxygen/html/classllvm_1_1CallGraph.html#details <http://llvm.org/docs/doxygen/html/classllvm_1_1CallGraph.html#details>) 
do you think it could help?


Simone Atzeni
simone.at at gmail.com
+1 (801) 696-8373

> On Feb 11, 2015, at 10:05, Ronan KERYELL <ronan-list at keryell.fr> wrote:
>>>>>> On Tue, 10 Feb 2015 11:19:39 +0000, "Cownie, James H" <james.h.cownie at intel.com> said:
>>> is there a way to understand if an LLVM IR instruction will be
>>> executed by an OpenMP constructs?
>    James> The simple answer is "No", you can't in general tell at
>    James> compile time whether code will be executed in parallel. (It
>    James> could be in a separate compilation unit, or even a dynamic
>    James> library which is only dlopened at runtime).
> Yes, this is why the OpenMP API provides functions like
> int omp_in_parallel(void);
> to test it.
>    James> Even without those practical examples, even knowing whether a
>    James> given IR instruction will ever be executed is clearly the
>    James> same as the halting problem...
> This is true but let's focus on the cases when we know it is not
> intractable... :-)
> You can apply some interprocedural control flow analysis in the compiler
> and in some cases be sure it is executed or not in parallel. Doing this
> in the LTO phase is a way to go.
> Another more pragmatic way is to rewrite some code bloc B by cloning it
> in the following way:
> if omp_in_parallel() {
>  // We know that here B is executed in a parallel context
>  B
> }
> else {
>  // We know that here B is not executed in a parallel context
>  B
> }
> and then go on with some optimizations on both paths.
> So this is a dynamic+static approach which may worth it if B is big enough.
> You may also use some partial evaluation late in the compilation phases
> to figure out that in some case omp_in_parallel() returns always true or
> false and then remove the dead code.
> -- 
>  Ronan KERYELL
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