[ELF] Move SymbolUnion size assertion to source file

[smeenai]

Summary

Otherwise it fires for every single file which includes the header,
which is very noisy when building.

[peter.smith]

This looks sensible to me. If the size of SymbolUnion is changed then Symbols.cpp will be recompiled so we'll always get the assertion if it is only changed.

lld/ELF/Symbols.h

478

I think it will be worth moving the majority of the comment here as this is where someone will look when trying to modify SymbolUnion.

For example:

// It is important to keep the size of SymbolUnion small for performance and
// memory usage reasons. 64 bytes is a soft limit based on the size of Defined
// on a 64-bit system. This is enforced by a static_assert in Symbols.cpp.

Not a strong opinion, just a suggestion.

[smeenai]

Address review comment and move other asserts

[smeenai]

lld/ELF/Symbols.h

478

Good idea, thanks. I also belatedly noticed that the other assertions would have the same issue, so I moved them as well.

lld/ELF/Symbols.h

struct ElfSym {
 
   // _TLS_MODULE_BASE_ on targets that support TLSDESC.
   static Defined *tlsModuleBase;
 };
 
 // A buffer class that is large enough to hold any Symbol-derived
 // object. We allocate memory using this class and instantiate a symbol
 // using the placement new.
 union SymbolUnion {
   alignas(Defined) char a[sizeof(Defined)];
   alignas(CommonSymbol) char b[sizeof(CommonSymbol)];
   alignas(Undefined) char c[sizeof(Undefined)];
   alignas(SharedSymbol) char d[sizeof(SharedSymbol)];
   alignas(LazyObject) char e[sizeof(LazyObject)];
 };
 
-// It is important to keep the size of SymbolUnion small for performance and
-// memory usage reasons. 64 bytes is a soft limit based on the size of Defined
-// on a 64-bit system.
-static_assert(sizeof(SymbolUnion) <= 64, "SymbolUnion too large");
-
 template <typename T> struct AssertSymbol {
   static_assert(std::is_trivially_destructible<T>(),
                 "Symbol types must be trivially destructible");
   static_assert(sizeof(T) <= sizeof(SymbolUnion), "SymbolUnion too small");
   static_assert(alignof(T) <= alignof(SymbolUnion),
                 "SymbolUnion not aligned enough");
 };
 

lld/ELF/Symbols.cpp

 #include <cstring>
 
 using namespace llvm;
 using namespace llvm::object;
 using namespace llvm::ELF;
 using namespace lld;
 using namespace lld::elf;
 
+// It is important to keep the size of SymbolUnion small for performance and
+// memory usage reasons. 64 bytes is a soft limit based on the size of Defined
+// on a 64-bit system.
+static_assert(sizeof(SymbolUnion) <= 64, "SymbolUnion too large");
+
 std::string lld::toString(const elf::Symbol &sym) {
   StringRef name = sym.getName();
   std::string ret = demangle(name, config->demangle);
 
   const char *suffix = sym.getVersionSuffix();
   if (*suffix == '@')
     ret += suffix;
   return ret;