Use ArrayRecycler for MachineInstr operand lists.

Instead of an std::vector<MachineOperand>, use MachineOperand arrays
from an ArrayRecycler living in MachineFunction.

This has several advantages:

- MachineInstr now has a trivial destructor, making it possible to
  delete them in batches when destroying MachineFunction. This will be
  enabled in a later patch.

- Bypassing malloc() and free() can be faster, depending on the system
  library.

- MachineInstr objects and their operands are allocated from the same
  BumpPtrAllocator, so they will usually be next to each other in
  memory, providing better locality of reference.

- Reduce MachineInstr footprint. A std::vector is 24 bytes, the new
  operand array representation only uses 8+4+1 bytes in MachineInstr.

- Better control over operand array reallocations. In the old
  representation, the use-def chains would be reordered whenever a
  std::vector reached its capacity. The new implementation never changes
  the use-def chain order.

Note that some decisions in the code generator depend on the use-def
chain orders, so this patch may cause different assembly to be produced
in a few cases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171598 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 5 insertions, 0 deletions

diff --git a/include/llvm/CodeGen/MachineOperand.h b/include/llvm/CodeGen/MachineOperand.h
index 606833c..66a56ed 100644
--- a/include/llvm/CodeGen/MachineOperand.h
+++ b/include/llvm/CodeGen/MachineOperand.h
@@ -35,6 +35,11 @@ class MCSymbol;
 
 /// MachineOperand class - Representation of each machine instruction operand.
 ///
+/// This class isn't a POD type because it has a private constructor, but its
+/// destructor must be trivial. Functions like MachineInstr::addOperand(),
+/// MachineRegisterInfo::moveOperands(), and MF::DeleteMachineInstr() depend on
+/// not having to call the MachineOperand destructor.
+///
 class MachineOperand {
 public:
   enum MachineOperandType {