[PATCH] D25607: [SCEV] More accurate calculation of max backedge count of some less-than loops

[John Brawn via llvm-commits]

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In loops that look something like

  i = n;
  do {
   ...
  } while(i++ < n+k);

where k is a constant, the maximum backedge count is k (in fact the backedge count will be either 0 or k, depending on whether n+k wraps). More generally for LHS < RHS if RHS-(LHS of first comparison) is a constant then the loop will iterate either 0 or that constant number of times.

This allows for more loop unrolling with the recent upper bound loop unrolling changes, and I'm working on a patch that will let loop unrolling additionally make use of the loop being executed either 0 or k times (we need to retain the loop comparison only on the first unrolled iteration).


Repository:
  rL LLVM

https://reviews.llvm.org/D25607

Files:
  lib/Analysis/ScalarEvolution.cpp
  test/Analysis/ScalarEvolution/trip-count13.ll
  test/Analysis/ScalarEvolution/trip-count14.ll

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