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, 6 insertions, 5 deletions

diff --git a/include/ruby/3/attr/restrict.h b/include/ruby/3/attr/restrict.h
index c42ad99a01..f6d7936b3a 100644
--- a/include/ruby/3/attr/restrict.h
+++ b/include/ruby/3/attr/restrict.h
@@ -1,4 +1,6 @@
-/**                                                     \noop-*-C++-*-vi:ft=cpp
+#ifndef RUBY3_ATTR_RESTRICT_H                        /*-*-C++-*-vi:se ft=cpp:*/
+#define RUBY3_ATTR_RESTRICT_H
+/**
  * @file
  * @author     Ruby developers <[email protected]>
  * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
@@ -24,10 +26,7 @@
  * `__has_declspec_attribute()` which involves macro substitution. */
 
 /** Wraps (or simulates) `__declspec(restrict)` */
-#if defined(RUBY3_ATTR_RESTRICT)
-# /* Take that. */
-
-#elif RUBY3_COMPILER_SINCE(MSVC, 14, 0, 0)
+#if RUBY3_COMPILER_SINCE(MSVC, 14, 0, 0)
 # define RUBY3_ATTR_RESTRICT() __declspec(RUBY3_TOKEN_PASTE(re, strict))
 
 #elif RUBY3_HAS_ATTRIBUTE(malloc)
@@ -39,3 +38,5 @@
 #else
 # define RUBY3_ATTR_RESTRICT() /* void */
 #endif
+
+#endif /* RUBY3_ATTR_RESTRICT_H */