[llvm] [LoopRotate] Rotate loop if this makes exit count computable (PR #162654)

[via llvm-commits]

annamthomas wrote:

> this form helps with runtime loop unrolling (see test) and also LoopVectorization requires rotation to make the loop bottom-tested (unconditional latch -> rotate the loop to make latch conditional). There are other passes as well which prefers this "bottom-tested" notation for loops

Just to clarify, the main reason for loop rotation has always been to make the loop bottom tested. . What is being proposed here is a stronger heuristic: make the bottom tested loop countable (if possible). 

As shown in the testcase, this helps in loop-unrolling.  We also can potentially help vectorize more early-exit loops (as seen in the code referenced in the description).


https://github.com/llvm/llvm-project/pull/162654