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[LV/LoopAccess] Check statically if an unknown dependence distance can be…

Authored by dorit on Feb 12 2017, 1:32 AM.

Description

[LV/LoopAccess] Check statically if an unknown dependence distance can be proven larger than the loop-count

This fixes PR31098: Try to resolve statically data-dependences whose
compile-time-unknown distance can be proven larger than the loop-count,
instead of resorting to runtime dependence checking (which are not always
possible).

For vectorization it is sufficient to prove that the dependence distance
is >= VF; But in some cases we can prune unknown dependence distances early,
and even before selecting the VF, and without a runtime test, by comparing
the distance against the loop iteration count. Since the vectorized code
will be executed only if LoopCount >= VF, proving distance >= LoopCount
also guarantees that distance >= VF. This check is also equivalent to the
Strong SIV Test.

Reviewers: mkuper, anemet, sanjoy

Differential Revision: https://reviews.llvm.org/D28044

llvm-svn: 294892