[cfe-dev] Behaviour of loop pragmas and transformation options

[Philip Reames via cfe-dev]

On 8/7/19 12:25 AM, Sjoerd Meijer via cfe-dev wrote:
> We have recently added support for a new loop pragma
> 'vectorize_predicate' [1].
> During review of the LLVM part that does the actual transformation
> [2], the
> question was raised if 'vectorize_predicate' should imply
> 'vectorize(enable)'.
> I agree that this is indeed what users would most likely expect and so I
> created a patch for that [3].
>
> During review of [3], Florian mentioned bug report [4] supporting the
> case that
> users indeed expect that more specific pragmas such as e.g.
> vectorize_width(4)
> should imply vectorize(enable), but this is currently not the case.
>  But our
> docs for example mention: "The following example implicitly enables
> vectorization and interleaving by specifying a vector width and
> interleaving
> count", and so we have an inconsistency in the docs and
> implementation. But as
> I have e.g. no clue what the behaviour could be of "vectorize(disable)
> vectorize_predicate(enable)", I would say the docs are right and the
> implementation is wrong.
>
> Michael mentioned that the situation with for example loop unroll pragmas
> 'unroll(enable) unroll_count(4)' is not very different. Pragma
> 'unroll_count'
> also does not set 'llvm.loop.unroll.enable', but it is handled by the
> LoopUnroll pass itself and so we end up with checks like this:
>
>     bool ExplicitUnroll = PragmaCount > 0 || PragmaFullUnroll ||
>                           PragmaEnableUnroll || UserUnrollCount;
>
> And again, this is very similar to 'vectorize_width', and we have
> different
> checks to see if vectorization is enabled.
>
> Because of the many inconsistencies, we came to the conclusion in [3]
> that we
> don't know how know how to (best) implement that setting an transformation
> option implies setting the transformation.
>
> My proposal would be that the transformation options imply the
> transformations
> by setting the enable flag of that transformation. Thus, vectorize options
> vectorize_width, vectorize_predicate, etc., set vectorize(enable), and
> unroll
> option unroll count set unroll(enable).
I second this.  The complexities of which pragmas enable which
transforms should be handled entirely in Clang, with Clang responsible
for emitting a consistent set of metadata flags to LLVM. 
>
> Any thoughts on this welcome.
>
>
> [1] https://reviews.llvm.org/rL366989
> [2] https://reviews.llvm.org/D65197
> [3] https://reviews.llvm.org/D65776
> [4] https://bugs.llvm.org/show_bug.cgi?id=27643
> <https://bugs.llvm.org/show_bug.cgi?id=27643>
>
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