[libunwind] Make HIDDEN_DIRECTIVE a function-like macro. NFCI.

[mstorsjo]

Summary

This avoids a hack for making it a no-op for windows.

Also explicitly check for _WIN32 instead of assuming it.

[compnerd]

Thanks for cleaning this up. IIRC, I have similar behavior in compiler-rt for HIDDEN_SYMBOL.

src/assembly.h

 #if defined(__POWERPC__) || defined(__powerpc__) || defined(__ppc__)
 #define SEPARATOR @
 #elif defined(__arm64__)
 #define SEPARATOR %%
 #else
 #define SEPARATOR ;
 #endif
 
-#if defined(__APPLE__)
-#define HIDDEN_DIRECTIVE .private_extern
-#elif defined(_WIN32)
-// In the COFF object file format, there's no attributes for a global,
-// non-static symbol to make it somehow hidden. So on windows, we don't
-// want to set this at all. To avoid conditionals in
-// DEFINE_LIBUNWIND_PRIVATE_FUNCTION below, make it .globl (which it already
-// is, defined in the same DEFINE_LIBUNWIND_PRIVATE_FUNCTION macro; the
-// duplicate .globl directives are harmless).
-#define HIDDEN_DIRECTIVE .globl
-#else
-#define HIDDEN_DIRECTIVE .hidden
-#endif
-
 #define GLUE2(a, b) a ## b
 #define GLUE(a, b) GLUE2(a, b)
 #define SYMBOL_NAME(name) GLUE(__USER_LABEL_PREFIX__, name)
 
 #if defined(__APPLE__)
 
 #define SYMBOL_IS_FUNC(name)
+#define HIDDEN_SYMBOL(name) .private_extern name
 #define NO_EXEC_STACK_DIRECTIVE
 
 #elif defined(__ELF__)
 
 #if defined(__arm__)
 #define SYMBOL_IS_FUNC(name) .type name,%function
 #else
 #define SYMBOL_IS_FUNC(name) .type name,@function
 #endif
+#define HIDDEN_SYMBOL(name) .hidden name
 
 #if defined(__GNU__) || defined(__FreeBSD__) || defined(__Fuchsia__) || \
     defined(__linux__)
 #define NO_EXEC_STACK_DIRECTIVE .section .note.GNU-stack,"",%progbits
 #else
 #define NO_EXEC_STACK_DIRECTIVE
 #endif
 
-#else
+#elif defined(_WIN32)
 
 #define SYMBOL_IS_FUNC(name)                                                   \
   .def name SEPARATOR                                                          \
     .scl 2 SEPARATOR                                                           \
     .type 32 SEPARATOR                                                         \
   .endef
+#define HIDDEN_SYMBOL(name)
 
 #define NO_EXEC_STACK_DIRECTIVE
 
+#else
+
+#error Unsupported target
+
 #endif
 
 #define DEFINE_LIBUNWIND_FUNCTION(name)                   \
   .globl SYMBOL_NAME(name) SEPARATOR                      \
   SYMBOL_IS_FUNC(SYMBOL_NAME(name)) SEPARATOR             \
   SYMBOL_NAME(name):
 
 #define DEFINE_LIBUNWIND_PRIVATE_FUNCTION(name)           \
   .globl SYMBOL_NAME(name) SEPARATOR                      \
-  HIDDEN_DIRECTIVE SYMBOL_NAME(name) SEPARATOR            \
+  HIDDEN_SYMBOL(SYMBOL_NAME(name)) SEPARATOR              \
   SYMBOL_IS_FUNC(SYMBOL_NAME(name)) SEPARATOR             \
   SYMBOL_NAME(name):
 
 #if defined(__arm__)
 #if !defined(__ARM_ARCH)
 #define __ARM_ARCH 4
 #endif