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

[MaskRay]

Summary

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.

[ruiu]

LGTM

[aykevl]

FYI: this change caused a build error when including the AVR backend:

[2780/3153] Building CXX object tools/lld/ELF/CMakeFiles/lldELF.dir/Arch/AVR.cpp.obj
FAILED: tools/lld/ELF/CMakeFiles/lldELF.dir/Arch/AVR.cpp.obj 
C:\ProgramData\chocolatey\bin\c++.exe  -DGTEST_HAS_RTTI=0 -D_FILE_OFFSET_BITS=64 -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -Itools/lld/ELF -ID:/a/1/s/llvm-project/lld/ELF -ID:/a/1/s/llvm-project/lld/include -Itools/lld/include -Iinclude -ID:/a/1/s/llvm-project/llvm/include -Wa,-mbig-obj -Werror=date-time -std=gnu++11 -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-maybe-uninitialized -Wno-class-memaccess -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wno-comment  -O2 -DNDEBUG    -fno-exceptions -fno-rtti -MD -MT tools/lld/ELF/CMakeFiles/lldELF.dir/Arch/AVR.cpp.obj -MF tools\lld\ELF\CMakeFiles\lldELF.dir\Arch\AVR.cpp.obj.d -o tools/lld/ELF/CMakeFiles/lldELF.dir/Arch/AVR.cpp.obj -c D:/a/1/s/llvm-project/lld/ELF/Arch/AVR.cpp
In file included from D:/a/1/s/llvm-project/lld/ELF/Arch/AVR.cpp:29:
D:/a/1/s/llvm-project/lld/ELF/Symbols.h:470:35: error: static assertion failed: SymbolUnion too large
 static_assert(sizeof(SymbolUnion) <= 80, "SymbolUnion too large");
               ~~~~~~~~~~~~~~~~~~~~^~~~~
[2781/3153] Building CXX object tools/lld/COFF/CMakeFiles/lldCOFF.dir/Symbols.cpp.obj
[2782/3153] Building CXX object tools/lld/COFF/CMakeFiles/lldCOFF.dir/Writer.cpp.obj
ninja: build stopped: subcommand failed.
mingw32-make: *** [Makefile:133: llvm-build] Error 1

I will see what can be done about it.

[aykevl]

Sorry, that was probably https://reviews.llvm.org/D64077 instead.

lld/trunk/wasm/Symbols.h

Symbol(StringRef Name, Kind K, uint32_t Flags, InputFile *F)
         ForceExport(false), CanInline(false), Traced(false) {}
 
   StringRef Name;
   InputFile *File;
   uint32_t Flags;
   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 {
 public:
   static bool classof(const Symbol *S) {
     return S->kind() == DefinedFunctionKind ||
            S->kind() == UndefinedFunctionKind;
   }
union SymbolUnion {
   alignas(DefinedEvent) char D[sizeof(DefinedEvent)];
   alignas(LazySymbol) char E[sizeof(LazySymbol)];
   alignas(UndefinedFunction) char F[sizeof(UndefinedFunction)];
   alignas(UndefinedData) char G[sizeof(UndefinedData)];
   alignas(UndefinedGlobal) char H[sizeof(UndefinedGlobal)];
   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);
 
 template <typename T, typename... ArgT>
 T *replaceSymbol(Symbol *S, ArgT &&... Arg) {
   static_assert(std::is_trivially_destructible<T>(),
                 "Symbol types must be trivially destructible");
   static_assert(sizeof(T) <= sizeof(SymbolUnion), "SymbolUnion too small");