[Patch][LoopVectorize] Late evaluate of runtime pointer check's threshold

[Tyler Nowicki via llvm-commits]

Hi,

This patch extends the patches in 'Late evaluation of vectorization requirements’ (still in review). This makes the num runtime pointer checks test into a late diagnostic so we can give the user a better diagnostic message. Clang appends the diagnostic with options for getting the loop vectorized. Here is an example of the diagnostic the user will see:

"remark: loop not vectorized: cannot prove pointers refer to independent arrays in memory. The loop requires 9 runtime independence checks to safely vectorize the loop, but that would exceed the limit of 8 checks; increase the number of runtime pointer checks with the compiler option ‘-mllvm -runtime-memory-check-threshold=’. To avoid runtime pointer checking when you know the arrays will always be independent specify ‘#pragma clang loop vectorize(assume_safety)’ before the loop or specify ‘restrict’ on the array arguments. Erroneous results will occur if these options are incorrectly applied."

The diagnostic is produced when the command line option -Rpass-analysis=loop-vectorize is provided.

Reviews/comments are much appreciated!

Tyler

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