CodeGen: Give MachineBasicBlock::reverse_iterator a handle to the current MI

Now that MachineBasicBlock::reverse_instr_iterator knows when it's at
the end (since r281168 and r281170), implement
MachineBasicBlock::reverse_iterator directly on top of an
ilist::reverse_iterator by adding an IsReverse template parameter to
MachineInstrBundleIterator.  This replaces another hard-to-reason-about
use of std::reverse_iterator on list iterators, matching the changes for
ilist::reverse_iterator from r280032 (see the "out of scope" section at
the end of that commit message).  MachineBasicBlock::reverse_iterator
now has a handle to the current node and has obvious invalidation
semantics.

r280032 has a more detailed explanation of how list-style reverse
iterators (invalidated when the pointed-at node is deleted) are
different from vector-style reverse iterators like std::reverse_iterator
(invalidated on every operation).  A great motivating example is this
commit's changes to lib/CodeGen/DeadMachineInstructionElim.cpp.

Note: If your out-of-tree backend deletes instructions while iterating
on a MachineBasicBlock::reverse_iterator or converts between
MachineBasicBlock::iterator and MachineBasicBlock::reverse_iterator,
you'll need to update your code in similar ways to r280032.  The
following table might help:

                  [Old]              ==>             [New]
        delete &*RI, RE = end()                   delete &*RI++
        RI->erase(), RE = end()                   RI++->erase()
      reverse_iterator(I)                 std::prev(I).getReverse()
      reverse_iterator(I)                          ++I.getReverse()
    --reverse_iterator(I)                            I.getReverse()
      reverse_iterator(std::next(I))                 I.getReverse()
                RI.base()                std::prev(RI).getReverse()
                RI.base()                         ++RI.getReverse()
              --RI.base()                           RI.getReverse()
     std::next(RI).base()                           RI.getReverse()

(For more details, have a look at r280032.)

llvm-svn: 281172

1 files changed, 1 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/IfConversion.cpp b/llvm/lib/CodeGen/IfConversion.cpp
index 9b30fa191a6..0bd40cf32a2 100644
--- a/llvm/lib/CodeGen/IfConversion.cpp
+++ b/llvm/lib/CodeGen/IfConversion.cpp
@@ -1802,8 +1802,7 @@ bool IfConverter::IfConvertDiamondCommon(
   // This is everything used+live in BB2 after the duplicated instructions. We
   // can compute this set by simulating liveness backwards from the end of BB2.
   DontKill.init(TRI);
-  for (const MachineInstr &MI :
-       make_range(MBB2.rbegin(), MachineBasicBlock::reverse_iterator(DI2)))
+  for (const MachineInstr &MI : make_range(MBB2.rbegin(), ++DI2.getReverse()))
     DontKill.stepBackward(MI);
 
   for (const MachineInstr &MI : make_range(MBB1.begin(), DI1)) {