[SDAG] Remove the reliance on MI's allocation strategy for

`MachineMemOperand` pointers attached to `MachineSDNodes` and instead
have the `SelectionDAG` fully manage the memory for this array.

Prior to this change, the memory management was deeply confusing here --
The way the MI was built relied on the `SelectionDAG` allocating memory
for these arrays of pointers using the `MachineFunction`'s allocator so
that the raw pointer to the array could be blindly copied into an
eventual `MachineInstr`. This creates a hard coupling between how
`MachineInstr`s allocate their array of `MachineMemOperand` pointers and
how the `MachineSDNode` does.

This change is motivated in large part by a change I am making to how
`MachineFunction` allocates these pointers, but it seems like a layering
improvement as well.

This would run the risk of increasing allocations overall, but I've
implemented an optimization that should avoid that by storing a single
`MachineMemOperand` pointer directly instead of allocating anything.
This is expected to be a net win because the vast majority of uses of
these only need a single pointer.

As a side-effect, this makes the API for updating a `MachineSDNode` and
a `MachineInstr` reasonably different which seems nice to avoid
unexpected coupling of these two layers. We can map between them, but we
shouldn't be *surprised* at where that occurs. =]

Differential Revision: https://reviews.llvm.org/D50680

llvm-svn: 339740

1 files changed, 2 insertions, 3 deletions

diff --git a/llvm/lib/Target/XCore/XCoreISelDAGToDAG.cpp b/llvm/lib/Target/XCore/XCoreISelDAGToDAG.cpp
index 1a56d1fd6e2..1688c38efc1 100644
--- a/llvm/lib/Target/XCore/XCoreISelDAGToDAG.cpp
+++ b/llvm/lib/Target/XCore/XCoreISelDAGToDAG.cpp
@@ -150,11 +150,10 @@ void XCoreDAGToDAGISel::Select(SDNode *N) {
       SDNode *node = CurDAG->getMachineNode(XCore::LDWCP_lru6, dl, MVT::i32,
                                             MVT::Other, CPIdx,
                                             CurDAG->getEntryNode());
-      MachineSDNode::mmo_iterator MemOp = MF->allocateMemRefsArray(1);
-      MemOp[0] =
+      MachineMemOperand *MemOp =
           MF->getMachineMemOperand(MachinePointerInfo::getConstantPool(*MF),
                                    MachineMemOperand::MOLoad, 4, 4);
-      cast<MachineSDNode>(node)->setMemRefs(MemOp, MemOp + 1);
+      CurDAG->setNodeMemRefs(cast<MachineSDNode>(node), {MemOp});
       ReplaceNode(N, node);
       return;
     }