[llvm-dev] vectorize.enable

[Michael Kruse via llvm-dev]

Am Mi., 2. Okt. 2019 um 07:08 Uhr schrieb Florian Hahn via llvm-dev
<llvm-dev at lists.llvm.org>:
> The other thing is that with the patch behaviour is slightly changed and we could get a diagnostic we didn't get before:
>
> warning: loop not vectorized: the optimizer was unable to
>       perform the requested transformation; the transformation might be disabled or
>       specified as part of an unsupported transformation ordering
>       [-Wpass-failed=transform-warning]
>
> For the example given in revert 858a1ae, in both cases before and after my commit, the loop vectoriser was bailing because "Not vectorizing: The exiting block is not the loop latch".

The source looks like a straightforward canonical loop. What pass
transformed it to have code between the exiting block and the latch?


> But the difference is that vectorize_width() now implies vectorize(enable), and so this is now marked as forced vectorisation which wasn't the case before. Because of this forced vectorization, and that the transformation wasn't applied, we now emit this diagnostic. The first part of this diagnostic is spot on: "the optimizer was unable to perform the requested transformation". We could argue about the suggestions given as to why the transformations didn't happen in this case, but overall I think this is an improvement.

The patch that added the warning as-is was by me
(https://reviews.llvm.org/D55288). It changed to emitted message from
"loop not vectorized: failed explicitly specified loop vectorization"
to the lengthier description following review feedback.

It's done by the WarnMissedTransformation and just looks for
transformation metadata that is still in the IR after all passes that
should have transformed them have ran. That is, it does not know why
it is still there -- it could be because the LoopVectorize pass is not
even in the pipeline -- and we cannot be more specific in the message.
However, -Rpass-missed=loop-vectorize may give more information.


> The additional warning makes sense to me and I think is also beneficial to the user.
>
> Before, we silently ignored vector_width() in the example [1] and I suppose the user was expecting vectorize_width(4) to be honored. Now we are more transparent in informing the user what is happening: we were not able to honor their requested pragma and I assume they would be interested in knowing.

As already mentioned, the loop indeed was never vectorized. It is
again the question whether vectorize_width(4) means "vectorize with
simd length 4" or "if this is vectorized, use simd length 4". Since
the decided on the former (which is also what the docs say), the
warning is correct.

That is, IMHO, Chrome should either remove the #pragma (since it has
no effect), add -Wno-pass-failed. We could also wait for the
LoopVectorize pass to support this, Philip Reames is currently working
on it.

Michael