[llvm] [llvm-debuginfo-analyzer] Reduce size of `LVProperties` where possible (PR #144399)

[Javier Lopez-Gomez via llvm-commits]

https://github.com/jalopezg-git updated https://github.com/llvm/llvm-project/pull/144399

>From 4e4b961209d5474bacba009276ea06887a350f8c Mon Sep 17 00:00:00 2001
From: Javier Lopez-Gomez <javier.lopez.gomez at proton.me>
Date: Mon, 16 Jun 2025 22:52:57 +0200
Subject: [PATCH 1/2] [llvm-debuginfo-analyzer] Reduce size of `LVProperties`
 where possible

Use underlying type of `uint32_t` if there are at most 32 properties
to keep track of.  Otherwise, switch to `std::bitset<N>`.
This effectively reduces the size of `LVObject` from 48 to 40 bytes.
---
 .../DebugInfo/LogicalView/Core/LVSupport.h    | 29 +++++++++++++++----
 1 file changed, 24 insertions(+), 5 deletions(-)

diff --git a/llvm/include/llvm/DebugInfo/LogicalView/Core/LVSupport.h b/llvm/include/llvm/DebugInfo/LogicalView/Core/LVSupport.h
index 01137f80c0f8b..058ca2da9a960 100644
--- a/llvm/include/llvm/DebugInfo/LogicalView/Core/LVSupport.h
+++ b/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.

>From ed5fcb95f254a2abe261f20f341d8ae3ea373f75 Mon Sep 17 00:00:00 2001
From: Javier Lopez-Gomez <javier.lopez.gomez at proton.me>
Date: Mon, 16 Jun 2025 22:52:57 +0200
Subject: [PATCH 2/2] [llvm-debuginfo-analyzer] Reorder LVElement members in
 order to minimize padding

---
 .../llvm/DebugInfo/LogicalView/Core/LVElement.h        | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/llvm/include/llvm/DebugInfo/LogicalView/Core/LVElement.h b/llvm/include/llvm/DebugInfo/LogicalView/Core/LVElement.h
index b4501db190fe5..0e7be45abfef4 100644
--- a/llvm/include/llvm/DebugInfo/LogicalView/Core/LVElement.h
+++ b/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.