[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, 10 insertions, 26 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
index f7bd8847bee..8df30fb0349 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
@@ -3598,38 +3598,22 @@ void SelectionDAGISel::SelectCodeCommon(SDNode *NodeToMatch,
         bool mayLoad = MCID.mayLoad();
         bool mayStore = MCID.mayStore();
 
-        unsigned NumMemRefs = 0;
-        for (SmallVectorImpl<MachineMemOperand *>::const_iterator I =
-               MatchedMemRefs.begin(), E = MatchedMemRefs.end(); I != E; ++I) {
-          if ((*I)->isLoad()) {
+        // We expect to have relatively few of these so just filter them into a
+        // temporary buffer so that we can easily add them to the instruction.
+        SmallVector<MachineMemOperand *, 4> FilteredMemRefs;
+        for (MachineMemOperand *MMO : MatchedMemRefs) {
+          if (MMO->isLoad()) {
             if (mayLoad)
-              ++NumMemRefs;
-          } else if ((*I)->isStore()) {
+              FilteredMemRefs.push_back(MMO);
+          } else if (MMO->isStore()) {
             if (mayStore)
-              ++NumMemRefs;
+              FilteredMemRefs.push_back(MMO);
           } else {
-            ++NumMemRefs;
+            FilteredMemRefs.push_back(MMO);
           }
         }
 
-        MachineSDNode::mmo_iterator MemRefs =
-          MF->allocateMemRefsArray(NumMemRefs);
-
-        MachineSDNode::mmo_iterator MemRefsPos = MemRefs;
-        for (SmallVectorImpl<MachineMemOperand *>::const_iterator I =
-               MatchedMemRefs.begin(), E = MatchedMemRefs.end(); I != E; ++I) {
-          if ((*I)->isLoad()) {
-            if (mayLoad)
-              *MemRefsPos++ = *I;
-          } else if ((*I)->isStore()) {
-            if (mayStore)
-              *MemRefsPos++ = *I;
-          } else {
-            *MemRefsPos++ = *I;
-          }
-        }
-
-        Res->setMemRefs(MemRefs, MemRefs + NumMemRefs);
+        CurDAG->setNodeMemRefs(Res, FilteredMemRefs);
       }
 
       LLVM_DEBUG(if (!MatchedMemRefs.empty() && Res->memoperands_empty()) dbgs()