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, 2 insertions, 9 deletions

diff --git a/llvm/lib/Target/ARM/MLxExpansionPass.cpp b/llvm/lib/Target/ARM/MLxExpansionPass.cpp
index 7f212403398..26745be947e 100644
--- a/llvm/lib/Target/ARM/MLxExpansionPass.cpp
+++ b/llvm/lib/Target/ARM/MLxExpansionPass.cpp
@@ -334,18 +334,15 @@ bool MLxExpansion::ExpandFPMLxInstructions(MachineBasicBlock &MBB) {
   unsigned Skip = 0;
   MachineBasicBlock::reverse_iterator MII = MBB.rbegin(), E = MBB.rend();
   while (MII != E) {
-    MachineInstr *MI = &*MII;
+    MachineInstr *MI = &*MII++;
 
-    if (MI->isPosition() || MI->isImplicitDef() || MI->isCopy()) {
-      ++MII;
+    if (MI->isPosition() || MI->isImplicitDef() || MI->isCopy())
       continue;
-    }
 
     const MCInstrDesc &MCID = MI->getDesc();
     if (MI->isBarrier()) {
       clearStack();
       Skip = 0;
-      ++MII;
       continue;
     }
 
@@ -365,13 +362,9 @@ bool MLxExpansion::ExpandFPMLxInstructions(MachineBasicBlock &MBB) {
         pushStack(MI);
       else {
         ExpandFPMLxInstruction(MBB, MI, MulOpc, AddSubOpc, NegAcc, HasLane);
-        E = MBB.rend(); // May have changed if MI was the 1st instruction.
         Changed = true;
-        continue;
       }
     }
-
-    ++MII;
   }
 
   return Changed;