[all-commits] [llvm/llvm-project] 38540d: [SCEV] Compute exit counts for unsigned IVs using ...

[Philip Reames via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 38540d71c74c2f63a77993e7bfb6e52e4b0da0fc
      https://github.com/llvm/llvm-project/commit/38540d71c74c2f63a77993e7bfb6e52e4b0da0fc
  Author: Philip Reames <listmail at philipreames.com>
  Date:   2021-06-07 (Mon, 07 Jun 2021)

  Changed paths:
    M llvm/include/llvm/Analysis/ScalarEvolution.h
    M llvm/lib/Analysis/ScalarEvolution.cpp
    A llvm/test/Analysis/ScalarEvolution/lt-overflow.ll

  Log Message:
  -----------
  [SCEV] Compute exit counts for unsigned IVs using mustprogress semantics

The motivation here is simple loops with unsigned induction variables w/non-one steps. A toy example would be:
for (unsigned i = 0; i < N; i += 2) { body; }

Given C/C++ semantics, we do not get the nuw flag on the induction variable. Given that lack, we currently can't compute a bound for this loop. We can do better for many cases, depending on the contents of "body".

The basic intuition behind this patch is as follows:
* A step which evenly divides the iteration space must wrap through the same numbers repeatedly. And thus, we can ignore potential cornercases where we exit after the n-th wrap through uint32_max.
* Per C++ rules, infinite loops without side effects are UB. We already have code in SCEV which relies on this.  In LLVM, this is tied to the mustprogress attribute.

Together, these let us conclude that the trip count of this loop must come before unsigned overflow unless the body would form a well defined infinite loop.

A couple notes for those reading along:
* I reused the loop properties code which is overly conservative for this case. I may follow up in another patch to generalize it for the actual UB rules.
* We could cache the n(s/u)w facts. I left that out because doing a pre-patch which cached existing inference showed a lot of diffs I had trouble fully explaining. I plan to get back to this, but I don't want it on the critical path.

Differential Revision: https://reviews.llvm.org/D103118