[lld] r365296 - [WebAssembly] Add static_assert(sizeof(SymbolUnion) <= 96)

[Fangrui Song via llvm-commits]

Author: maskray
Date: Mon Jul  8 00:30:07 2019
New Revision: 365296

URL: http://llvm.org/viewvc/llvm-project?rev=365296&view=rev
Log:
[WebAssembly] Add static_assert(sizeof(SymbolUnion) <= 96)

On Windows, the bitfield layout rule places `ussigned Referenced : 1` at
byte offset 40, instead of byte offset 37 on *NIX. The consequence is that
sizeof(SymbolUnion) == 104 on Windows while 96 on *NIX.

To eliminate this difference, change these unsigned bitfields to bool.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D64238

Modified:
    lld/trunk/wasm/Symbols.h

Modified: lld/trunk/wasm/Symbols.h
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/wasm/Symbols.h?rev=365296&r1=365295&r2=365296&view=diff
==============================================================================
--- lld/trunk/wasm/Symbols.h (original)
+++ lld/trunk/wasm/Symbols.h Mon Jul  8 00:30:07 2019
@@ -131,26 +131,26 @@ protected:
   uint32_t OutputSymbolIndex = INVALID_INDEX;
   uint32_t GOTIndex = INVALID_INDEX;
   Kind SymbolKind;
-  unsigned Referenced : 1;
+  bool Referenced : 1;
 
 public:
   // True if the symbol was used for linking and thus need to be added to the
   // output file's symbol table. This is true for all symbols except for
   // unreferenced DSO symbols, lazy (archive) symbols, and bitcode symbols that
   // are unreferenced except by other bitcode objects.
-  unsigned IsUsedInRegularObj : 1;
+  bool IsUsedInRegularObj : 1;
 
   // True if ths symbol is explicity marked for export (i.e. via the -e/--export
   // command line flag)
-  unsigned ForceExport : 1;
+  bool ForceExport : 1;
 
   // False if LTO shouldn't inline whatever this symbol points to. If a symbol
   // is overwritten after LTO, LTO shouldn't inline the symbol because it
   // doesn't know the final contents of the symbol.
-  unsigned CanInline : 1;
+  bool CanInline : 1;
 
   // True if this symbol is specified by --trace-symbol option.
-  unsigned Traced : 1;
+  bool Traced : 1;
 };
 
 class FunctionSymbol : public Symbol {
@@ -475,6 +475,11 @@ union SymbolUnion {
   alignas(SectionSymbol) char I[sizeof(SectionSymbol)];
 };
 
+// It is important to keep the size of SymbolUnion small for performance and
+// memory usage reasons. 96 bytes is a soft limit based on the size of
+// UndefinedFunction on a 64-bit system.
+static_assert(sizeof(SymbolUnion) <= 96, "SymbolUnion too large");
+
 void printTraceSymbol(Symbol *Sym);
 void printTraceSymbolUndefined(StringRef Name, const InputFile* File);