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

[jalopezg-git]

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.

This effectively reduces the sizes of key classes to

	sizeof(T) before this patch	sizeof(T) after this patch	% reduction
T = LVObject	48	40	20%
T = LVElement	104	80	30%
T = LVScope	176	144	22.22%
T = LVSymbol	176	144	22.22%
T = LVScopeCompileUnit	1016	960	5.83%

This, in turn, should have a very visible effect for parsing large debug information files.

[llvmbot]

@llvm/pr-subscribers-debuginfo

Author: Javier Lopez-Gomez (jalopezg-git)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/144399.diff

1 Files Affected:

• (modified) llvm/include/llvm/DebugInfo/LogicalView/Core/LVSupport.h (+22-4)

diff --git a/llvm/include/llvm/DebugInfo/LogicalView/Core/LVSupport.h b/llvm/include/llvm/DebugInfo/LogicalView/Core/LVSupport.h
index 01137f80c0f8b..4fb7d1d13e513 100644
--- a/llvm/include/llvm/DebugInfo/LogicalView/Core/LVSupport.h
+++ b/llvm/include/llvm/DebugInfo/LogicalView/Core/LVSupport.h
@@ -21,9 +21,11 @@
 #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 +40,30 @@ 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);
+  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.

[jalopezg-git]

@CarlosAlbertoEnciso FYI 🙂. Feel free to review / leave comments.

Also, could you measure peak memory usage (both, before and after this change) for the Chrome debug info case?

[jalopezg-git]

I have also made a couple of (very trivial) changes to LVScopeCompileUnit definition leading to some additional memory savings.

@CarlosAlbertoEnciso, @OCHyams, if you don't have further comments, I think this can be merged too 👍.