[llvm] [TableGen] Add support for sparse direct-lookup tables (PR #201158)

[Igor Wodiany via llvm-commits]

================
@@ -575,25 +577,70 @@ void SearchableTableEmitter::emitGenericTable(const GenericTable &Table,
 
   emitIfdef((Twine("GET_") + Table.PreprocessorGuard + "_IMPL").str(), OS);
 
+  SmallVector<const Record *, 0> SparseEntries;
+  ArrayRef<const Record *> Entries;
+  unsigned DirectLookupSlots = 0;
+  if (Table.AllowSparseTable) {
+    const auto *KeyBits = cast<BitsRecTy>(Table.PrimaryKey->Fields[0].RecType);
+    DirectLookupSlots = 1u << KeyBits->getNumBits();
+    SparseEntries.resize(DirectLookupSlots);
+    StringRef KeyFieldName = Table.PrimaryKey->Fields[0].Name;
+    for (const Record *Entry : Table.Entries) {
+      uint64_t Key = static_cast<uint64_t>(getInt(Entry, KeyFieldName));
+      assert(Key < DirectLookupSlots && "key exceeds table size");
+      if (SparseEntries[Key])
+        PrintFatalError(Entry, Twine("In table '") + Table.Name +
+                                   "', duplicate primary key value " +
+                                   Twine(Key));
+      SparseEntries[Key] = Entry;
+    }
+    Entries = SparseEntries;
+  } else {
+    Entries = Table.Entries;
+  }
+
   // The primary data table contains all the fields defined for this map.
   OS << "constexpr " << Table.CppTypeName << " " << Table.Name << "[] = {\n";
-  for (const auto &[Idx, Entry] : enumerate(Table.Entries)) {
+  for (const auto &[Idx, Entry] : enumerate(Entries)) {
     OS << "  { ";
-
     ListSeparator LS;
-    for (const auto &Field : Table.Fields)
-      OS << LS
-         << primaryRepresentation(Table.Locs[0], Field,
-                                  Entry->getValueInit(Field.Name));
-
+    if (Entry) {
+      for (const auto &Field : Table.Fields)
+        OS << LS
+           << primaryRepresentation(Table.Locs[0], Field,
+                                    Entry->getValueInit(Field.Name));
+    } else if (Table.AllowSparseTable) {
+      // For empty rows emit a sentinel value as a key, so we can return null
+      // during lookup. Value is constructed such that LookupKey != KeyField.
+      for (const auto &Field : Table.Fields) {
+        OS << LS;
+        if (Field.Name == Table.PrimaryKey->Fields[0].Name)
+          OS << "0x" << utohexstr((DirectLookupSlots - 1) ^ Idx);
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IgWod wrote:

Yes, it's good idea. I've updated it.

https://github.com/llvm/llvm-project/pull/201158