[llvm-debuginfo-analyzer] Apply various memory savings in Core/LVxxx base classes (#144399)

This small changelist reduces memory footprint of instances of the Core
classes. Specifically,

- For `LVProperties`, use underlying type of `uint32_t` if there are at
most 32 properties to keep track of. Otherwise, fallback to the generic
`std::bitset<N>`.
The use of `llvm::SmallBitVector` is disregarded in this case, as the
upper bound on the size of the bitset can be determined statically (no
heap alloc ever needed).
- Reorder members in `LVElement` s.t. padding between members is
reduced.
- `LVScopeCompileUnit`: fix a couple of members which should be `static
constexpr` instead.

llvm/include/llvm/DebugInfo/LogicalView/Core/LVElement.h

@@ -107,18 +107,18 @@ class LLVM_ABI LVElement : public LVObject {
     IsAnonymous,
     LastEntry
   };
-  // Typed bitvector with properties for this element.
-  LVProperties<Property> Properties;
   static LVElementDispatch Dispatch;
 
-  /// RTTI.
-  const LVSubclassID SubclassID;
-
   // Indexes in the String Pool.
   size_t NameIndex = 0;
   size_t QualifiedNameIndex = 0;
   size_t FilenameIndex = 0;
 
+  // Typed bitvector with properties for this element.
+  LVProperties<Property> Properties;
+  /// RTTI.
+  const LVSubclassID SubclassID;
+
   uint16_t AccessibilityCode : 2; // DW_AT_accessibility.
   uint16_t InlineCode : 2;        // DW_AT_inline.
   uint16_t VirtualityCode : 2;    // DW_AT_virtuality.

llvm/include/llvm/DebugInfo/LogicalView/Core/LVScope.h

@@ -473,7 +473,7 @@ class LLVM_ABI LVScopeCompileUnit final : public LVScope {
 
   // Record scope sizes indexed by lexical level.
   // Setting an initial size that will cover a very deep nested scopes.
-  const size_t TotalInitialSize = 8;
+  static constexpr size_t TotalInitialSize = 8;
   using LVTotalsEntry = std::pair<unsigned, float>;
   SmallVector<LVTotalsEntry> Totals;
   // Maximum seen lexical level. It is used to control how many entries
@@ -510,7 +510,7 @@ public:
   void addMapping(LVLine *Line, LVSectionIndex SectionIndex);
   LVLineRange lineRange(LVLocation *Location) const;
 
-  LVNameInfo NameNone = {UINT64_MAX, 0};
+  static constexpr LVNameInfo NameNone = {UINT64_MAX, 0};
   void addPublicName(LVScope *Scope, LVAddress LowPC, LVAddress HighPC) {
     PublicNames.emplace(std::piecewise_construct, std::forward_as_tuple(Scope),
                         std::forward_as_tuple(LowPC, HighPC - LowPC));

llvm/include/llvm/DebugInfo/LogicalView/Core/LVSupport.h

@@ -13,7 +13,6 @@
 #ifndef LLVM_DEBUGINFO_LOGICALVIEW_CORE_LVSUPPORT_H
 #define LLVM_DEBUGINFO_LOGICALVIEW_CORE_LVSUPPORT_H
 
-#include "llvm/ADT/SmallBitVector.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/DebugInfo/LogicalView/Core/LVStringPool.h"
 #include "llvm/Support/Compiler.h"
@@ -21,9 +20,11 @@
 #include "llvm/Support/Format.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/raw_ostream.h"
+#include <bitset>
 #include <cctype>
 #include <map>
 #include <sstream>
+#include <type_traits>
 
 namespace llvm {
 namespace logicalview {
@@ -38,14 +39,32 @@ using LVLexicalIndex =
 
 // Used to record specific characteristics about the objects.
 template <typename T> class LVProperties {
-  SmallBitVector Bits = SmallBitVector(static_cast<unsigned>(T::LastEntry) + 1);
+  static constexpr unsigned N_PROPS = static_cast<unsigned>(T::LastEntry);
+  // Use uint32_t as the underlying type if the `T` enum has at most 32
+  // enumerators; otherwise, fallback to the generic `std::bitset` case.
+  std::conditional_t<(N_PROPS > 32), std::bitset<N_PROPS>, uint32_t> Bits{};
 
 public:
   LVProperties() = default;
 
-  void set(T Idx) { Bits[static_cast<unsigned>(Idx)] = 1; }
-  void reset(T Idx) { Bits[static_cast<unsigned>(Idx)] = 0; }
-  bool get(T Idx) const { return Bits[static_cast<unsigned>(Idx)]; }
+  void set(T Idx) {
+    if constexpr (std::is_same_v<decltype(Bits), uint32_t>)
+      Bits |= 1 << static_cast<unsigned>(Idx);
+    else
+      Bits.set(static_cast<unsigned>(Idx));
+  }
+  void reset(T Idx) {
+    if constexpr (std::is_same_v<decltype(Bits), uint32_t>)
+      Bits &= ~(1 << static_cast<unsigned>(Idx));
+    else
+      Bits.reset(static_cast<unsigned>(Idx));
+  }
+  bool get(T Idx) const {
+    if constexpr (std::is_same_v<decltype(Bits), uint32_t>)
+      return Bits & (1 << static_cast<unsigned>(Idx));
+    else
+      return Bits[static_cast<unsigned>(Idx)];
+  }
 };
 
 // Generate get, set and reset 'bool' functions for LVProperties instances.