[llvm] [llvm-objdump] Optimize live element tracking (PR #158763)

[James Henderson via llvm-commits]

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@@ -205,14 +231,55 @@ unsigned LiveElementPrinter::moveToFirstVarColumn(formatted_raw_ostream &OS) {
   return FirstUnprintedLogicalColumn;
 }
 
-unsigned LiveElementPrinter::findFreeColumn() {
-  for (unsigned ColIdx = 0; ColIdx < ActiveCols.size(); ++ColIdx)
-    if (!ActiveCols[ColIdx].isActive())
-      return ColIdx;
+unsigned LiveElementPrinter::getOrCreateColumn(unsigned ElementIdx) {
+  // Check if the element already has an assigned column.
+  auto it = ElementToColumn.find(ElementIdx);
+  if (it != ElementToColumn.end())
+    return it->second;
+
+  unsigned ColIdx;
+  if (!FreeCols.empty()) {
+    // Get the smallest available index from the set.
+    ColIdx = *FreeCols.begin();
+    // Remove the index from the set.
+    FreeCols.erase(FreeCols.begin());
+  } else {
+    // No free columns, so create a new one.
+    ColIdx = ActiveCols.size();
+    ActiveCols.emplace_back();
+  }
 
-  size_t OldSize = ActiveCols.size();
-  ActiveCols.grow(std::max<size_t>(OldSize * 2, 1));
-  return OldSize;
+  // Assign the element to the column and update the map.
+  ElementToColumn[ElementIdx] = ColIdx;
+  ActiveCols[ColIdx].ElementIdx = ElementIdx;
+  return ColIdx;
+}
+
+void LiveElementPrinter::freeColumn(unsigned ColIdx) {
+  unsigned ElementIdx = ActiveCols[ColIdx].ElementIdx;
+
+  // Clear the column's data and add it to the free list.
+  ActiveCols[ColIdx].ElementIdx = Column::NullElementIdx;
+  ActiveCols[ColIdx].LiveIn = false;
+  ActiveCols[ColIdx].LiveOut = false;
+  ActiveCols[ColIdx].MustDrawLabel = false;
+
+  // Remove the element's entry from the map and add the column to the free
+  // list.
+  ElementToColumn.erase(ElementIdx);
+  FreeCols.insert(ColIdx);
+}
+
+std::vector<unsigned>
+LiveElementPrinter::getSortedActiveElementIndices() const {
+  // Get all element indexes that currently have an assigned column.
+  std::vector<unsigned> Indices;
+  for (const auto &Pair : ElementToColumn)
+    Indices.push_back(Pair.first);
+
+  // Sort by the DWARF discovery order.
+  llvm::stable_sort(Indices);
----------------
jh7370 wrote:

How costly is this sort? It feels like it doesn't fit, given all the other efforts to improve the performance you've been making.

There are a few other places where we sort by DWARF discovery order too, so similar questions apply in those cases too.

https://github.com/llvm/llvm-project/pull/158763