[llvm] [RISCV] Order stack objects by access density (PR #217507)

[Matt Arsenault via llvm-commits]

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@@ -2846,6 +2846,82 @@ void RISCVFrameLowering::inlineStackProbe(MachineFunction &MF,
   }
 }
 
+namespace {
+/// Bookkeeping record used while ordering local stack objects by access
+/// density. One record is created for each frame index passed to
+/// orderFrameObjects().
+struct RISCVFrameSortingObject {
+  int ObjectIndex;
+  uint64_t ObjectSize;
+  Align ObjectAlign;
+  uint64_t NumUses = 0;
+};
+} // end anonymous namespace
+
+void RISCVFrameLowering::orderFrameObjects(
+    const MachineFunction &MF, SmallVectorImpl<int> &ObjectsToAllocate) const {
+  // PEI never adds callee-saved, dead, stack-protector, EH, scavenging,
+  // non-default-stack-ID or other special objects to ObjectsToAllocate, so
+  // the objects seen here are safe to reorder.
+  const MachineFrameInfo &MFI = MF.getFrameInfo();
+
+  DenseMap<int, unsigned> Index;
+  SmallVector<RISCVFrameSortingObject> Sorting;
+  Sorting.reserve(ObjectsToAllocate.size());
+
+  for (int FI : ObjectsToAllocate) {
+    assert(MFI.getStackID(FI) == TargetStackID::Default &&
+           "Only default stack objects should be reordered");
+    // Variable-sized objects report a size of zero. Give them a non-zero
+    // sentinel so the density comparison below does not degenerate. Their
+    // placement does not matter much in practice: they occupy no space in
+    // the static frame and are addressed by adjusting SP at runtime rather
+    // than through a frame index offset.
+    int64_t Size = MFI.getObjectSize(FI);
+    RISCVFrameSortingObject Obj{FI, Size > 0 ? static_cast<uint64_t>(Size) : 4,
+                                MFI.getObjectAlign(FI)};
+    Index[FI] = Sorting.size();
+    Sorting.push_back(Obj);
+  }
+
+  // Count static references to each frame index. This provides an access
+  // density heuristic without requiring profile data.
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arsenm wrote:

Why avoid that? The analysis is available to use?

https://github.com/llvm/llvm-project/pull/217507