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, 8 deletions

diff --git a/llvm/lib/CodeGen/DeadMachineInstructionElim.cpp b/llvm/lib/CodeGen/DeadMachineInstructionElim.cpp
index 0b8dc7a86ad..17c229a216a 100644
--- a/llvm/lib/CodeGen/DeadMachineInstructionElim.cpp
+++ b/llvm/lib/CodeGen/DeadMachineInstructionElim.cpp
@@ -122,7 +122,7 @@ bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
     // liveness as we go.
     for (MachineBasicBlock::reverse_iterator MII = MBB.rbegin(),
          MIE = MBB.rend(); MII != MIE; ) {
-      MachineInstr *MI = &*MII;
+      MachineInstr *MI = &*MII++;
 
       // If the instruction is dead, delete it!
       if (isDead(MI)) {
@@ -133,9 +133,6 @@ bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
         MI->eraseFromParentAndMarkDBGValuesForRemoval();
         AnyChanges = true;
         ++NumDeletes;
-        MIE = MBB.rend();
-        // MII is now pointing to the next instruction to process,
-        // so don't increment it.
         continue;
       }
 
@@ -169,10 +166,6 @@ bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
           }
         }
       }
-
-      // We didn't delete the current instruction, so increment MII to
-      // the next one.
-      ++MII;
     }
   }