[llvm] 1893caa - MCSymbol: Decrease the bitfield size of SymbolContents

[Fangrui Song via llvm-commits]

Author: Fangrui Song
Date: 2025-08-16T10:43:05-07:00
New Revision: 1893caa9bc9782eaef95d747658e070b132468a5

URL: https://github.com/llvm/llvm-project/commit/1893caa9bc9782eaef95d747658e070b132468a5
DIFF: https://github.com/llvm/llvm-project/commit/1893caa9bc9782eaef95d747658e070b132468a5.diff

LOG: MCSymbol: Decrease the bitfield size of SymbolContents

Follow-up to 57b0843f68f5f349c73d1bf54e321a1a6d1800bf

The size of MCSymbol has been reduced to 24 bytes on 64-bit systems.

Added: 
    

Modified: 
    llvm/include/llvm/MC/MCSymbol.h
    llvm/lib/MC/MCSymbol.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/MC/MCSymbol.h b/llvm/include/llvm/MC/MCSymbol.h
index ddc560ec52501..ce160bd2c3cc9 100644
--- a/llvm/include/llvm/MC/MCSymbol.h
+++ b/llvm/include/llvm/MC/MCSymbol.h
@@ -65,6 +65,10 @@ class MCSymbol {
   /// relative to, if any.
   mutable MCFragment *Fragment = nullptr;
 
+  /// This is actually a Contents enumerator, but is unsigned to avoid sign
+  /// extension and achieve better bitpacking with MSVC.
+  unsigned SymbolContents : 2;
+
   /// True if this symbol is named.  A named symbol will have a pointer to the
   /// name allocated in the bytes immediately prior to the MCSymbol.
   unsigned HasName : 1;
@@ -95,10 +99,6 @@ class MCSymbol {
   /// Used to detect cyclic dependency like `a = a + 1` and `a = b; b = a`.
   unsigned IsResolving : 1;
 
-  /// This is actually a Contents enumerator, but is unsigned to avoid sign
-  /// extension and achieve better bitpacking with MSVC.
-  unsigned SymbolContents : 3;
-
   /// The alignment of the symbol if it is 'common'.
   ///
   /// Internally, this is stored as log2(align) + 1.
@@ -145,10 +145,10 @@ class MCSymbol {
   };
 
   MCSymbol(const MCSymbolTableEntry *Name, bool isTemporary)
-      : IsTemporary(isTemporary), IsRedefinable(false), IsRegistered(false),
-        IsExternal(false), IsPrivateExtern(false), IsWeakExternal(false),
-        IsUsedInReloc(false), IsResolving(0), SymbolContents(SymContentsUnset),
-        CommonAlignLog2(0), Flags(0) {
+      : SymbolContents(SymContentsUnset), IsTemporary(isTemporary),
+        IsRedefinable(false), IsRegistered(false), IsExternal(false),
+        IsPrivateExtern(false), IsWeakExternal(false), IsUsedInReloc(false),
+        IsResolving(0), CommonAlignLog2(0), Flags(0) {
     Offset = 0;
     HasName = !!Name;
     if (Name)

diff  --git a/llvm/lib/MC/MCSymbol.cpp b/llvm/lib/MC/MCSymbol.cpp
index 8192896eeb6bb..b19842aae46ce 100644
--- a/llvm/lib/MC/MCSymbol.cpp
+++ b/llvm/lib/MC/MCSymbol.cpp
@@ -20,6 +20,10 @@
 
 using namespace llvm;
 
+// There are numerous MCSymbol objects, so keeping sizeof(MCSymbol) small is
+// crucial for minimizing peak memory usage.
+static_assert(sizeof(MCSymbol) <= 24, "Keep the base symbol small");
+
 // Only the address of this fragment is ever actually used.
 static MCFragment SentinelFragment;