[LLVMdev] [RFC] Parallelization metadata and intrinsics in LLVM (for OpenMP, etc.)

[Chris Lattner]

On Oct 1, 2012, at 10:37 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> On Mon, 01 Oct 2012 21:26:54 -0700
> Chris Lattner <clattner at apple.com> wrote:
> 
>> 
>> On Oct 1, 2012, at 6:16 PM, greened at obbligato.org wrote:
>> 
>>> Sanjoy Das <sanjoy at playingwithpointers.com> writes:
>>> 
>>>> In short, I propose a intrinsic based approach which hinges on the
>>>> concept of a "parallel map".  The immediate effect of using
>>>> intrinsics is that we no longer have to worry about missing
>>>> metadata.  Moreover, we are still free to lower the intrinsics in
>>>> a variety of ways -- including vectorizing them or lowering them
>>>> to calls to an actual openmp backend.
>>> 
>>> I'll re-ask here since this is in its own thread.
>>> 
>>> Why can't we just make ordinary function calls to runtime routines?
>> 
>> I agree.  I can't imagine any practical way that a metadata-based
>> approach could be preserved by optimizers.
> 
> Regarding the metadata approach, it depends on what you mean by
> preserved. The trick is to make sure that transformations that don't
> understand the metadata can't cause miscompiles. The specific scheme
> that I proposed used a combination of procedurization and
> cross-referencing metadata such that invalidated parallel metadata can
> be detected and the entire enclosing parallel region can be dropped.
> 
> The proposal from Intel, which more-heavily uses intrinsics, has other
> advantages, but will require more modifications to existing passes to
> realize its potential optimization benefits.

My comment was mostly in response to the Intel proposal, which effectively translates OpenMP pragmas directly into llvm intrinsics + metadata.  I can't imagine a way to make this work *correctly* without massive changes to the optimizer.

-Chris