Shuxin Yang d2eaf45af4 Fix a problem in alias analysis. It is about the misinterpretation of "Object".
This problem is exposed by r171325 which is already reverted. It is rather
hard to fabricate a testing case without it.

r171325 should *NOT* be resurrected as it has a potential problem although 
this problem dosen't directly contribute to PR14988.

The bug is tracked by:
  - rdar://13063553, and
  - http://llvm.org/bugs/show_bug.cgi?id=14988

Thank Arnold for coming up a better solution to this problem. After
comparing this solution and my original proposal, I decided to ditch mine.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176225 91177308-0d34-0410-b5e6-96231b3b80d8
2013-02-28 00:24:45 +00:00
..

Analysis Opportunities:

//===---------------------------------------------------------------------===//

In test/Transforms/LoopStrengthReduce/quadradic-exit-value.ll, the
ScalarEvolution expression for %r is this:

  {1,+,3,+,2}<loop>

Outside the loop, this could be evaluated simply as (%n * %n), however
ScalarEvolution currently evaluates it as

  (-2 + (2 * (trunc i65 (((zext i64 (-2 + %n) to i65) * (zext i64 (-1 + %n) to i65)) /u 2) to i64)) + (3 * %n))

In addition to being much more complicated, it involves i65 arithmetic,
which is very inefficient when expanded into code.

//===---------------------------------------------------------------------===//

In formatValue in test/CodeGen/X86/lsr-delayed-fold.ll,

ScalarEvolution is forming this expression:

((trunc i64 (-1 * %arg5) to i32) + (trunc i64 %arg5 to i32) + (-1 * (trunc i64 undef to i32)))

This could be folded to

(-1 * (trunc i64 undef to i32))

//===---------------------------------------------------------------------===//