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/vm_core.h b/vm_core.h
index 9186ab3146..47455a6f7b 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -1,3 +1,5 @@
+#ifndef RUBY_VM_CORE_H
+#define RUBY_VM_CORE_H
 /**********************************************************************
 
   vm_core.h -
@@ -9,9 +11,6 @@
 
 **********************************************************************/
 
-#ifndef RUBY_VM_CORE_H
-#define RUBY_VM_CORE_H
-
 /*
  * Enable check mode.
  *   1: enable local assertions.