[UnitTests/Vectorizer] Add unit tests for find_first_of pattern

[hazzlim]

This patch adds runtime test cases for vectorization of the find_first_of idiom:

for (; first != last; ++first)
for (s_it = s_first; s_it != s_last; ++s_it)
if (*first == *s_it)
return first;
return last;

NOTE: this idiom is currently only vectorized by LoopIdiomVectorize, which is not affected by #pragma clang loop vectorize(enable).

[fhahn]

NOTE: this idiom is currently only vectorized by LoopIdiomVectorize, which is not affected by #pragma clang loop vectorize(enable).

So doe sthat mean that the current version does not generating a scalar version for ScalarFn?

[hazzlim]

NOTE: this idiom is currently only vectorized by LoopIdiomVectorize, which is not affected by #pragma clang loop vectorize(enable).

So doe sthat mean that the current version does not generating a scalar version for ScalarFn?

Yes that is correct - when compiling for AArch64 with +sve2 the ScalarFn will still be vectorized with the MATCH idiom by LoopIdiomVectorizePass.

There was a bit of discussion here - llvm/llvm-project#179187 (comment) - I'm not sure there's a good way to make the ScalarFn not vectorize via a source-level attribute besides __attribute__((optnone)).

From https://clang.llvm.org/docs/LanguageExtensions.html#extensions-for-loop-hint-optimizations it doesn't seem obvious to me that the vectorize(enable/disable) pragma is intended/defined to only affect the LoopVectorizer... I don't know if it would be correct/acceptable/reasonable to make it also switch LoopIdiomVectorizePass off when vectorize(disable)?

[fhahn]

I think it would make sense for the pragma to apply there as well.

[hazzlim]

I think it would make sense for the pragma to apply there as well.

Agreed - I can look into this and open a PR

[hazzlim]

I think it would make sense for the pragma to apply there as well.

I have added llvm/llvm-project#181142.

Also updated this PR to put pragmas on the outer loop, as this makes more sense given that it is the entire nested loop which the vectorization applies to.