LoopVectorizer: decision when no vector instructions generated has changed

[Jonas Paulsson via llvm-commits]

Hi,

I find that now the LoopVectorizers behaviour has changed its behaviour 
regarding "Not considering vector loop of width X because it will not 
generate any vector instructions."

During my previous experiments with the Loop vectorizer, I found that it 
would actually vectorize a loop with no resulting vectorized 
instructions in the case where there is a memory access. I thought at 
first this was wrong, and tried changing it, only to find that this then 
actually made hundreds of loops then not
getting vectorized. Since those loops looked perhaps better vectorized, 
and this was so big of a change, that I concluded that it was probably 
on purpose, and later suggested a comment during review of that patch:

// Note: Even if all instructions are scalarized, return true if any memory
// accesses appear in the loop to get benefits from address folding etc.

I thought that most of those changed loops seemed somewhat better when 
vectorized (fully scalarized), giving a kind of unrolling effect. I 
wouldn't however really argue that this is needed.

So, I just want to point out that this affects hundreds of loops, in 
case this was changed unintentionally. Second, if this was not on 
purpose earlier and this really shouldn't be done, I guess the comment I 
added should just be removed.

/Jonas