Late evaluation of vectorization requirements

[Tyler Nowicki]

Hi,

To vectorize fp reductions fast-math is required, but this check is done while identifying the reduction causing a very cryptical analysis message. Also it should be possible to provide a loop hint rather than specifying fast-math for the whole module.

For float reductions the current analysis message reads:
- value that could not be identified as reduction is used outside the loop

This patch moves the verification of fast-math, adds a check for a loop hint, and improves the analysis message.

The improved message reads:
- vectorization requires changes in the order of operations, however IEEE 754 floating-point operations are not commutative; allow commutativity by specifying ‘#pragma clang loop vectorize(enable)’ before the loop or by providing the compiler option ‘-ffast-math’

In the future I will extend this to include pointer aliasing checks and perhaps cost-model checks.

This patch builds off of my other diagnostic changes that are currently in review (http://reviews.llvm.org/D10714 <http://reviews.llvm.org/D10714>) and “Renaming and Diagnostics for Loop Interleaving". Please make sure to apply patches 0002 and 0003 if you want to try this patch.

Comments are much appreciated!

Tyler

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