add #include guard hack

According to MSVC manual (*1), cl.exe can skip including a header file
when that:

- contains #pragma once, or
- starts with #ifndef, or
- starts with #if ! defined.

GCC has a similar trick (*2), but it acts more stricter (e. g. there
must be _no tokens_ outside of #ifndef...#endif).

Sun C lacked #pragma once for a looong time.  Oracle Developer Studio
12.5 finally implemented it, but we cannot assume such recent version.

This changeset modifies header files so that each of them include
strictly one #ifndef...#endif.  I believe this is the most portable way
to trigger compiler optimizations. [Bug #16770]

*1: https://docs.microsoft.com/en-us/cpp/preprocessor/once
*2: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html

1 files changed, 2 insertions, 3 deletions

diff --git a/thread_win32.h b/thread_win32.h
index 60083da0a1..4fd5f8b10f 100644
--- a/thread_win32.h
+++ b/thread_win32.h
@@ -1,3 +1,5 @@
+#ifndef RUBY_THREAD_WIN32_H
+#define RUBY_THREAD_WIN32_H
 /**********************************************************************
 
   thread_win32.h -
@@ -9,8 +11,6 @@
 **********************************************************************/
 
 /* interface */
-#ifndef RUBY_THREAD_WIN32_H
-#define RUBY_THREAD_WIN32_H
 
 # ifdef __CYGWIN__
 # undef _WIN32
@@ -33,4 +33,3 @@ typedef struct rb_global_vm_lock_struct {
 } rb_global_vm_lock_t;
 
 #endif /* RUBY_THREAD_WIN32_H */
-