[TableGen] Simplify generated code for isSubclass (#117351)

Implement isSubclass with direct lookup into some tables instead of
nested switches.

Part of the motivation for this is improving compile time when clang-18
is used as a host compiler, since it seems to have trouble with very
large switch statements.

Author: jayfoad

llvm/utils/TableGen/AsmMatcherEmitter.cpp

@@ -2547,53 +2547,58 @@ static void emitIsSubclass(CodeGenTarget &Target,
   OS << "  if (A == B)\n";
   OS << "    return true;\n\n";
 
-  bool EmittedSwitch = false;
+  // TODO: Use something like SequenceToOffsetTable to allow sequences to
+  // overlap in this table.
+  SmallVector<bool> SuperClassData;
+
+  OS << "  [[maybe_unused]] static constexpr struct {\n";
+  OS << "    uint32_t Offset;\n";
+  OS << "    uint16_t Start;\n";
+  OS << "    uint16_t Length;\n";
+  OS << "  } Table[] = {\n";
+  OS << "    {0, 0, 0},\n"; // InvalidMatchClass
+  OS << "    {0, 0, 0},\n"; // OptionalMatchClass
   for (const auto &A : Infos) {
-    std::vector<StringRef> SuperClasses;
-    if (A.IsOptional)
-      SuperClasses.push_back("OptionalMatchClass");
-    for (const auto &B : Infos) {
-      if (&A != &B && A.isSubsetOf(B))
-        SuperClasses.push_back(B.Name);
-    }
-
-    if (SuperClasses.empty())
-      continue;
-
-    // If this is the first SuperClass, emit the switch header.
-    if (!EmittedSwitch) {
-      OS << "  switch (A) {\n";
-      OS << "  default:\n";
-      OS << "    return false;\n";
-      EmittedSwitch = true;
-    }
-
-    OS << "\n  case " << A.Name << ":\n";
-
-    if (SuperClasses.size() == 1) {
-      OS << "    return B == " << SuperClasses.back() << ";\n";
-      continue;
-    }
-
-    if (!SuperClasses.empty()) {
-      OS << "    switch (B) {\n";
-      OS << "    default: return false;\n";
-      for (StringRef SC : SuperClasses)
-        OS << "    case " << SC << ": return true;\n";
-      OS << "    }\n";
-    } else {
-      // No case statement to emit
-      OS << "    return false;\n";
-    }
+    SmallVector<bool> SuperClasses;
+    SuperClasses.push_back(false);        // InvalidMatchClass
+    SuperClasses.push_back(A.IsOptional); // OptionalMatchClass
+    for (const auto &B : Infos)
+      SuperClasses.push_back(&A != &B && A.isSubsetOf(B));
+
+    // Trim leading and trailing zeros.
+    auto End = find_if(reverse(SuperClasses), [](bool B) { return B; }).base();
+    auto Start =
+        std::find_if(SuperClasses.begin(), End, [](bool B) { return B; });
+
+    unsigned Offset = SuperClassData.size();
+    SuperClassData.append(Start, End);
+
+    OS << "    {" << Offset << ", " << (Start - SuperClasses.begin()) << ", "
+       << (End - Start) << "},\n";
   }
+  OS << "  };\n\n";
 
-  // If there were case statements emitted into the string stream write the
-  // default.
-  if (EmittedSwitch)
-    OS << "  }\n";
-  else
+  if (SuperClassData.empty()) {
     OS << "  return false;\n";
-
+  } else {
+    // Dump the boolean data packed into bytes.
+    SuperClassData.append(-SuperClassData.size() % 8, false);
+    OS << "  static constexpr uint8_t Data[] = {\n";
+    for (unsigned I = 0, E = SuperClassData.size(); I < E; I += 8) {
+      unsigned Byte = 0;
+      for (unsigned J = 0; J < 8; ++J)
+        Byte |= (unsigned)SuperClassData[I + J] << J;
+      OS << formatv("    {:X2},\n", Byte);
+    }
+    OS << "  };\n\n";
+
+    OS << "  auto &Entry = Table[A];\n";
+    OS << "  unsigned Idx = B - Entry.Start;\n";
+    OS << "  if (Idx >= Entry.Length)\n";
+    OS << "    return false;\n";
+    OS << "  Idx += Entry.Offset;\n";
+    OS << "  return (Data[Idx / 8] >> (Idx % 8)) & 1;\n";
+  }
   OS << "}\n\n";
 }