[PATCH] D111493: [IVUsers] Check for preheader instead of loop simplify form

Mon Oct 11 13:03:21 PDT 2021

nikic added a comment.

In D111493#3054181 <https://reviews.llvm.org/D111493#3054181>, @mkazantsev wrote:

> Looks fair. Just curious, what's the CT impact?

For the average case this is only a small improvement: https://llvm-compile-time-tracker.com/compare.php?from=943b3048484b7e3cf04f4d51c23c82fcece2185d&to=272c643a5149425ef577312ae494e726c8b15d9e&stat=instructions

For the pathological case I was looking at compile-time for LSR drops to 40s from previously [unknown but at least 10 minutes].

In D111493#3055286 <https://reviews.llvm.org/D111493#3055286>, @reames wrote:

> Glancing through the SCEVExpander code, I only see one unconditional use of a preheader assumption, and that looks easy to bail out of.  Are you sure we need this restriction at all?  (Landing this and then investigating is encouraged.)

Not sure, but I think the difference to those other usages is that they are only used for optimization, i.e. for basic LICM during expansion. For addrecs having no preheader can make expansion impossible. I guess one would have to check that in isSafeToExpand() instead?

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