[PATCH] Enable ExitValue rewrite only when expansion cost is low.

[Wei Mi]

Hi qcolombet, hfinkel, atrick,

The patch is to enable ExitValue rewrite (In IndVarSimplify pass) only when the cost of expansion is low, just like LinearFunctionTestReplace does right now. The problem is described in https://llvm.org/bugs/show_bug.cgi?id=23538.

It includes these changes:
1. Add an option. -replexitval=cheap is the default.
  -replexitval      - Choose the strategy to replace exit value in IndVarSimplify
    =never          -   never replace exit value
    =cheap          -   only replace exit value when the cost is cheap
    =always         -   always replace exit value whenever possible

2. The existing logic of SCEVExpander::isHighCostExpansionHelper is problematic. It cannot find out the udiv operator inside SCEVNAryExpr other than SCEVAddExpr. It doesn't check the sub SCEV inside SCEVTruncateExpr/SCEVZeroExtendExpr/SCEVSignExtendExpr. Like the SCEV: (-12 + (-12 * ((-12 + %len) /u 12)) + %len), SCEVExpander::isHighCostExpansionHelper will return false for it now. Fix the logic.

The patch improved an internal benchmark by 10%. It improved bzip2 in spec2000 by 1%.

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D9800

Files:
  lib/Analysis/ScalarEvolutionExpander.cpp
  lib/Transforms/Scalar/IndVarSimplify.cpp
  test/Transforms/IndVarSimplify/exit_value_test2.ll
  test/Transforms/IndVarSimplify/exit_value_tests.ll
  test/Transforms/IndVarSimplify/lcssa-preservation.ll
  test/Transforms/IndVarSimplify/loop_evaluate_1.ll
  test/Transforms/IndVarSimplify/loop_evaluate_6.ll

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