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diff --git a/include/ruby/internal/assume.h b/include/ruby/internal/assume.h
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+#ifndef RBIMPL_ASSUME_H /*-*-C++-*-vi:se ft=cpp:*/
+#define RBIMPL_ASSUME_H
+/**
+ * @file
+ * @author Ruby developers <ruby-core@ruby-lang.org>
+ * @copyright This file is a part of the programming language Ruby.
+ * Permission is hereby granted, to either redistribute and/or
+ * modify this file, provided that the conditions mentioned in the
+ * file COPYING are met. Consult the file for details.
+ * @warning Symbols prefixed with either `RBIMPL` or `rbimpl` are
+ * implementation details. Don't take them as canon. They could
+ * rapidly appear then vanish. The name (path) of this header file
+ * is also an implementation detail. Do not expect it to persist
+ * at the place it is now. Developers are free to move it anywhere
+ * anytime at will.
+ * @note To ruby-core: remember that this header can be possibly
+ * recursively included from extension libraries written in C++.
+ * Do not expect for instance `__VA_ARGS__` is always available.
+ * We assume C99 for ruby itself but we don't assume languages of
+ * extension libraries. They could be written in C++98.
+ * @brief Defines #RBIMPL_ASSUME / #RBIMPL_UNREACHABLE.
+ *
+ * These macros must be defined at once because:
+ *
+ * - #RBIMPL_ASSUME could fallback to #RBIMPL_UNREACHABLE.
+ * - #RBIMPL_UNREACHABLE could fallback to #RBIMPL_ASSUME.
+ */
+#include "ruby/impl/config.h"
+#include "ruby/impl/cast.h"
+#include "ruby/impl/compiler_since.h"
+#include "ruby/impl/has/builtin.h"
+#include "ruby/impl/warning_push.h"
+
+/** @cond INTERNAL_MACRO */
+#if RBIMPL_COMPILER_SINCE(MSVC, 13, 10, 0)
+# define RBIMPL_HAVE___ASSUME
+
+#elif RBIMPL_COMPILER_SINCE(Intel, 13, 0, 0)
+# define RBIMPL_HAVE___ASSUME
+#endif
+/** @endcond */
+
+/** Wraps (or simulates) `__builtin_unreachable`. */
+#if RBIMPL_HAS_BUILTIN(__builtin_unreachable)
+# define RBIMPL_UNREACHABLE_RETURN(_) __builtin_unreachable()
+
+#elif defined(RBIMPL_HAVE___ASSUME)
+# define RBIMPL_UNREACHABLE_RETURN(_) return (__assume(0), (_))
+
+#else
+# define RBIMPL_UNREACHABLE_RETURN(_) return (_)
+#endif
+
+/** Wraps (or simulates) `__builtin_unreachable`. */
+#if RBIMPL_HAS_BUILTIN(__builtin_unreachable)
+# define RBIMPL_UNREACHABLE __builtin_unreachable
+
+#elif defined(RBIMPL_HAVE___ASSUME)
+# define RBIMPL_UNREACHABLE() __assume(0)
+#endif
+
+/** Wraps (or simulates) `__assume`. */
+#if RBIMPL_COMPILER_SINCE(Intel, 13, 0, 0)
+# /* icc warnings are false positives. Ignore them. */
+# /* "warning #2261: __assume expression with side effects discarded" */
+# define RBIMPL_ASSUME(expr) \
+ RBIMPL_WARNING_PUSH() \
+ RBIMPL_WARNING_IGNORED(2261) \
+ __assume(expr) \
+ RBIMPL_WARNING_POP()
+
+#elif defined(RBIMPL_HAVE___ASSUME)
+# define RBIMPL_ASSUME __assume
+
+#elif RBIMPL_HAS_BUILTIN(__builtin_assume)
+# define RBIMPL_ASSUME __builtin_assume
+
+#elif ! defined(RBIMPL_UNREACHABLE)
+# define RBIMPL_ASSUME(_) RBIMPL_CAST((void)(_))
+
+#else
+# define RBIMPL_ASSUME(_) \
+ (RB_LIKELY(!!(_)) ? RBIMPL_CAST((void)0) : RBIMPL_UNREACHABLE())
+#endif
+
+#if ! defined(RBIMPL_UNREACHABLE)
+# define RBIMPL_UNREACHABLE() RBIMPL_ASSUME(0)
+#endif
+
+#endif /* RBIMPL_ASSUME_H */