[PATCH] InstCombineSimplifyDemanded: Remove nsw/nuw flags when optimizing demanded bits

[Matthias Braun]

Hi majnemer,

When optimizing demanded bits of the operands of an Add we have to
remove the nsw/nuw flags as we have no guarantee anymore that we don't
wrap.  This is legal here because the top bit is not demanded.  In fact
this operaion was already performed but missed in the case of an Add
with a constant on the right side.  To fix this this patch refactors the
code to unify the code paths in SimplifyDemandedUseBits() handling of
Add/Sub:

- The transformation of Add->Or is removed from the simplify demand
  code because the equivalent transformation exists in
  InstCombiner::visitAdd()
- KnownOnes/KnownZero are not adjusted for Add x, C anymore as
  computeKnownBits() already performs these computations.
- The simplification of the operands is unified. In this new version
  constant on the right side of a Sub are shrunk now as I could not find
  a reason why not to do so.
- The special case for clearing nsw/nuw in ShrinkDemandedConstant() is
  not necessary anymore as the caller does that already.

This also fixes PR11449.

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D9415

Files:
  lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
  test/Transforms/InstCombine/demand_shrink_nsw.ll

EMAIL PREFERENCES
  http://reviews.llvm.org/settings/panel/emailpreferences/
-------------- next part --------------
A non-text attachment was scrubbed...
Name: D9415.24768.patch
Type: text/x-patch
Size: 8658 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150430/42089ad6/attachment.bin>