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, 4 insertions, 5 deletions

diff --git a/llvm/lib/Target/X86/X86InstrInfo.cpp b/llvm/lib/Target/X86/X86InstrInfo.cpp
index 1e6b21a3653..cbd86186c0a 100644
--- a/llvm/lib/Target/X86/X86InstrInfo.cpp
+++ b/llvm/lib/Target/X86/X86InstrInfo.cpp
@@ -5349,9 +5349,9 @@ bool X86InstrInfo::optimizeCompareInstr(MachineInstr &CmpInstr, unsigned SrcReg,
   // If the definition is in this basic block, RE points to the definition;
   // otherwise, RE is the rend of the basic block.
   MachineBasicBlock::reverse_iterator
-      RI = MachineBasicBlock::reverse_iterator(I),
+      RI = ++I.getReverse(),
       RE = CmpInstr.getParent() == MI->getParent()
-               ? MachineBasicBlock::reverse_iterator(++Def) /* points to MI */
+               ? Def.getReverse() /* points to MI */
                : CmpInstr.getParent()->rend();
   MachineInstr *Movr0Inst = nullptr;
   for (; RI != RE; ++RI) {
@@ -5497,9 +5497,8 @@ bool X86InstrInfo::optimizeCompareInstr(MachineInstr &CmpInstr, unsigned SrcReg,
   if (Movr0Inst) {
     // Look backwards until we find a def that doesn't use the current EFLAGS.
     Def = Sub;
-    MachineBasicBlock::reverse_iterator
-      InsertI = MachineBasicBlock::reverse_iterator(++Def),
-                InsertE = Sub->getParent()->rend();
+    MachineBasicBlock::reverse_iterator InsertI = Def.getReverse(),
+                                        InsertE = Sub->getParent()->rend();
     for (; InsertI != InsertE; ++InsertI) {
       MachineInstr *Instr = &*InsertI;
       if (!Instr->readsRegister(X86::EFLAGS, TRI) &&