Sanjoy Das 148e8c9b8b Add a new pass "inductive range check elimination" ...

IRCE eliminates range checks of the form

  0 <= A * I + B < Length

by splitting a loop's iteration space into three segments in a way
that the check is completely redundant in the middle segment.  As an
example, IRCE will convert

  len = < known positive >
  for (i = 0; i < n; i++) {
    if (0 <= i && i < len) {
      do_something();
    } else {
      throw_out_of_bounds();
    }
  }

to

  len = < known positive >
  limit = smin(n, len)
  // no first segment
  for (i = 0; i < limit; i++) {
    if (0 <= i && i < len) { // this check is fully redundant
      do_something();
    } else {
      throw_out_of_bounds();
    }
  }
  for (i = limit; i < n; i++) {
    if (0 <= i && i < len) {
      do_something();
    } else {
      throw_out_of_bounds();
    }
  }


IRCE can deal with multiple range checks in the same loop (it takes
the intersection of the ranges that will make each of them redundant
individually).

Currently IRCE does not do any profitability analysis.  That is a
TODO.

Please note that the status of this pass is *experimental*, and it is
not part of any default pass pipeline.  Having said that, I will love
to get feedback and general input from people interested in trying
this out.

This pass was originally r226201.  It was reverted because it used C++
features not supported by MSVC 2012.

Differential Revision: http://reviews.llvm.org/D6693



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226238 91177308-0d34-0410-b5e6-96231b3b80d8

2015-01-16 01:03:22 +00:00

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