[PATCH] D141823: [SCEV] More precise trip multiples

[Joshua Cao via Phabricator via llvm-commits]

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Small code updates


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We currently have getMinTrailingZeros(), from which we can get a SCEV's
multiple by computing 1 << MinTrailingZeroes. However, this only gets us
multiples that are a power of 2. This patch introduces a way to get max
constant multiples that are not just a power of 2. The logic is similar
to that of getMinTrailingZeros. getMinTrailingZerosImpl is replaced by
computing the max constant multiple, and counting the number of trailing
bits.

I have so far found this useful in two places:

1. Computing unsigned constant ranges. For example, if we have i8 {10,+,10}<nuw>, we know the max constant it can be is 250.

2. My original intent was to use this in getSmallConstantTripMultiples, but it has no effect right now due to change from D110587 <https://reviews.llvm.org/D110587>. For example, if we have backedge count `(6 * %N) - 1`, the trip count becomes `1 + zext((6 * %N) - 1)`, and we cannot say that 6 is a multiple of the SCEV. I plan to look further into this separately.

The implementation assumes the value is unsigned. It can probably be
extended to handle signed values as well.

If the code sees that a SCEV does not have <nuw>, it will fall back to
finding the max multiple that is a power of 2. Multiples that are a
power of 2 will still be a multiple even after the SCEV overflows. This
does not apply to other values.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D141823

Files:
  llvm/include/llvm/Analysis/ScalarEvolution.h
  llvm/lib/Analysis/ScalarEvolution.cpp
  llvm/test/Analysis/ScalarEvolution/nsw.ll
  llvm/test/Analysis/ScalarEvolution/ranges.ll

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