[PATCH] D43256: [MBP] Move a latch block with conditional exit and multi predecessors to top of loop

[Gal Zohar via Phabricator via llvm-commits]

GalZohar added a comment.
Herald added subscribers: kerbowa, pengfei.

Hi,
Seems like this is supposed to increase the average number of fallthrough in a case of simple nested loops, however it seems to also increase the total number of branch instructions both in and outside the outer loop.
In our target this seems to do more harm than good as increasing the number of branches is significantly more harmful than having more conditional taken branches.
Instead of a conditional branch that is usually taken and jumps from end to start of outer loop, we have an unconditional branch that jumps from the end of the inner loop to the end of the outer loop, which then conditionally jumps out of the outer loop or falls through to the next iteration of the outer loop.
This increases the total number of executed branch instructions by 1.

Did I understand this correctly?

Is there a way to tweak this optimization to avoid generating additional branches? Or would this mean completely disabling it for targets that don't benefit from additional fallthroughs when it comes at the cost of additional branches?

Thanks


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https://reviews.llvm.org/D43256