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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
58 lines
3 KiB
C++
58 lines
3 KiB
C++
#ifndef RUBY3_ATTR_NOALIAS_H /*-*-C++-*-vi:se ft=cpp:*/
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#define RUBY3_ATTR_NOALIAS_H
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/**
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* @file
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* @author Ruby developers <ruby-core@ruby-lang.org>
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* @copyright This file is a part of the programming language Ruby.
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* Permission is hereby granted, to either redistribute and/or
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* modify this file, provided that the conditions mentioned in the
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* file COPYING are met. Consult the file for details.
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* @warning Symbols prefixed with either `RUBY3` or `ruby3` are
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* implementation details. Don't take them as canon. They could
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* rapidly appear then vanish. The name (path) of this header file
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* is also an implementation detail. Do not expect it to persist
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* at the place it is now. Developers are free to move it anywhere
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* anytime at will.
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* @note To ruby-core: remember that this header can be possibly
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* recursively included from extension libraries written in C++.
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* Do not expect for instance `__VA_ARGS__` is always available.
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* We assume C99 for ruby itself but we don't assume languages of
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* extension libraries. They could be written in C++98.
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* @brief Defines #RUBY3_ATTR_NOALIAS.
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*
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* ### Q&A ###
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*
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* - Q: There are seemingly similar attributes named #RUBY3_ATTR_CONST,
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* #RUBY3_ATTR_PURE, and #RUBY3_ATTR_NOALIAS. What are the difference?
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*
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* - A: Allowed operations are different.
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*
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* - #RUBY3_ATTR_CONST ... Functions attributed by this are not allowed to
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* read/write _any_ pointers at all (there are exceptional situations
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* when reading a pointer is possible but forget that; they are too
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* exceptional to be useful). Just remember that everything pointer-
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* related are NG.
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*
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* - #RUBY3_ATTR_PURE ... Functions attributed by this can read any
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* nonvolatile pointers, but no writes are allowed at all. The ability
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* to read _any_ nonvolatile pointers makes it possible to mark ::VALUE-
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* taking functions as being pure, as long as they are read-only.
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*
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* - #RUBY3_ATTR_NOALIAS ... Can both read/write, but only through pointers
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* passed to the function as parameters. This is a typical situation
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* when you create a C++ non-static member function which only concerns
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* `this`. No global variables are allowed to read/write. So this is
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* not a super-set of being pure. If you want to read something, that
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* has to be passed to the function as a pointer. ::VALUE -taking
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* functions thus cannot be attributed as such.
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*/
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#include "ruby/3/has/declspec_attribute.h"
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/** Wraps (or simulates) `__declspec((noalias))` */
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#if RUBY3_HAS_DECLSPEC_ATTRIBUTE(noalias)
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# define RUBY3_ATTR_NOALIAS() __declspec(noalias)
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#else
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# define RUBY3_ATTR_NOALIAS() /* void */
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#endif
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#endif /* RUBY3_ATTR_NOALIAS_H */
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