[PATCH] D51942: [InstCombine] Fold (C/x)>0 into x>0 if possible

[Martin Elshuber via Phabricator via llvm-commits]

marels created this revision.
marels added reviewers: john.brawn, majnemer.
Herald added a subscriber: llvm-commits.

This patch adds a simplification for floating point compares that
depend on the inversion of a variable. The comparison can be folded
such that it solely depends on the variable rather than the division.

E.g.:

  foo(double x) {
    double a = 1.0 / x;
    if (a < 0) ...
  }

>
=

  foo(double x) {
    double a = 1.0 / x;
    if (x < 0) ...
  }

The following conversion is applied if C!=0 where C is a compiler
known constant:

  (C/x < 0)
  (C*x < 0)       ... multiply by x*x; Note: x*x > 0 (see Remark 2)
                  ... divide by C
  (x < 0)         ... if C > 0
  (x > 0)         ... if C < 0

Remarks:

1. To avoid cases where x = NaN only the ordered predicates (ogt, olt, oge, ole) are folded.

2. The division requires the flag 'ninf' to be set. This also ensures there x!=0 can be assumed.

  Assume x=0. We know C!=0 (by definition) and C!=inf ('ninf' is set). Therefore C/x=inf (IEEE754_2008.pdf 7.2 and 7.3). But, because 'ninf' is set, the compile can assume x!=0

3. To avoid cases where C/x == 0.0, 'ninf' has to be set for the fdiv and fcmp instruction.

4. To allow the multiplication by x*x 'arcp' and 'ninf' has to be set for the fdiv and fcmp instruction.


Repository:
  rL LLVM

https://reviews.llvm.org/D51942

Files:
  lib/Transforms/InstCombine/InstCombineCompares.cpp
  test/Transforms/InstCombine/fcmp.ll

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