[cfe-dev] Why doesn't pragma vectorize warn by default on failure

[Tyler Nowicki]

> I would also like to get a full diagnostic about why vectorization failed. That is, have -Rpass-missed=loop-vectorize and -Rpass-analysis=loop-vectorize enabled by default.

That’s a good idea, I think it would solve the problem you are seeing. We should enable the remarks for code with a pragma hint. Thinking about the implementation, I can’t say off the top of my head how to pass that information to the BackendConsumer where remarks are emitted. If I get time I’ll do this, but I’m pretty swamped right now.


> > Its possible there is a bug in the code for generating the warning or we missed generating it somewhere. Could you provide the loop body?
> 
> I provide a minimal example below (in Appendix A). It seems the diagnostic triggers or not depending on the optimization level selected, which makes sense now that I looked more into it. For example:
> 
> When compiling with O0, O2, O3, and Ofast I get no diagnostic, and in fact, the loop is not vectorized because it is actually eliminated.
> When compiling with O1 I get by default without passing any extra flags the diagnostic I wanted: 
> 
> > file.cpp:13:1: error: loop not vectorized: failed explicitly specified loop
> >      vectorization [-Werror,-Wpass-failed]
> >}
> >^
> >
> >1 error generated.
> 
> It is also not hard to get "no diagnostic" even for loops that could be vectorized (godbolt+assembly: http://goo.gl/nfeXOX <http://goo.gl/nfeXOX>). In that example, the code is vectorized for a size of 100. But changing the size of 8 removes the vectorization "silently". This is good and makes sense. For a size of 8 it is probably not worth it to vectorize the loop, and whatever cost model the vectorizer uses knows this. 

Its an interesting example. When SIZE=8 the loop unroller removes the loop. Actually the vectorizer doesn’t even see it! That makes it difficult to generate a diagnostic in the loop vectorizer. And there are many passes that can remove a loop entirely before it gets to the vectorizer causing the “no diagnostic” problem. Some of these passes generate a remark. For example: -Rpass=unroll says the loop in your example was “completely unrolled loop with 8 iterations”. We should extend the diagnostics to all passes that could remove a loop, such as the LoopDelete pass. In combination with your suggestion above that should resolve the “no diagnostics” problem.

Thanks for taking the time to generate the example and think about a solution. It's a lot of help!

Tyler

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