[PATCH] ScalarEvolution backedge taken count computation on LT and GT rewrite

[Michele Scandale]

Hi atrick,

Hi all,

here a little rewrite of the functions used to compute the backedge taken count of a loop on LT and GT comparisons.

I decided to split the handling of LT and GT cases becasue the trick "a > b == -a < -b" in some cases prevents the trip count computation due to the multiplication by -1 on the two operands of the comparison. This issue comes from the conservative computation of value range of SCEVs: taking the negative SCEV of an expression that have a small positive range (e.g. [0,31]), we would have a SCEV with a fullset as value range.

Indeed, in the new rewritten function I tried to better handle the maximum backedge taken count computation when MAX/MIN expression are used to handle the cases where no entry guard is found.

Some test have been modified in order to check the new value correctly (I manually check them and reasoning on possible overflow the new values seem correct).

I finally added a new test case related to the multiplication by -1 issue on GT comparisons.

Thanks in advance.

Best regards,
-Michele

http://llvm-reviews.chandlerc.com/D2008

Files:
  include/llvm/Analysis/ScalarEvolution.h
  lib/Analysis/ScalarEvolution.cpp
  test/Analysis/ScalarEvolution/2008-11-18-Stride1.ll
  test/Analysis/ScalarEvolution/2008-11-18-Stride2.ll
  test/Analysis/ScalarEvolution/2008-12-15-DontUseSDiv.ll
  test/Analysis/ScalarEvolution/bounded_decrement_loop.ll
  test/Analysis/ScalarEvolution/trip-count3.ll
  test/Analysis/ScalarEvolution/trip-count9.ll
  test/Transforms/IndVarSimplify/loop_evaluate_6.ll
  test/Transforms/IndVarSimplify/no-iv-rewrite.ll
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