[librseq.git] / include / rseq / compiler.h

/* SPDX-License-Identifier: MIT */
/* SPDX-FileCopyrightText: 2021 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> */

/*
 * rseq/compiler.h
 *
 * Work-around asm goto compiler bugs.
 */

#ifndef _RSEQ_COMPILER_H
#define _RSEQ_COMPILER_H

#if defined __cplusplus
# include <type_traits>	/* for std::remove_cv */
#endif

#define rseq_likely(x)		__builtin_expect(!!(x), 1)
#define rseq_unlikely(x)	__builtin_expect(!!(x), 0)
#define rseq_barrier()		__asm__ __volatile__("" : : : "memory")

/*
 * Instruct the compiler to perform only a single access to a variable
 * (prohibits merging and refetching). The compiler is also forbidden to reorder
 * successive instances of RSEQ_ACCESS_ONCE(), but only when the compiler is aware of
 * particular ordering. Compiler ordering can be ensured, for example, by
 * putting two RSEQ_ACCESS_ONCE() in separate C statements.
 *
 * This macro does absolutely -nothing- to prevent the CPU from reordering,
 * merging, or refetching absolutely anything at any time.  Its main intended
 * use is to mediate communication between process-level code and irq/NMI
 * handlers, all running on the same CPU.
 */
#define RSEQ_ACCESS_ONCE(x)	(*(__volatile__  __typeof__(x) *)&(x))

#define RSEQ_WRITE_ONCE(x, v)	__extension__ ({ RSEQ_ACCESS_ONCE(x) = (v); })
#define RSEQ_READ_ONCE(x)	RSEQ_ACCESS_ONCE(x)

/*
 * gcc prior to 4.8.2 miscompiles asm goto.
 * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58670
 *
 * gcc prior to 8.1.0 miscompiles asm goto at O1.
 * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103908
 *
 * clang prior to version 13.0.1 miscompiles asm goto at O2.
 * https://github.com/llvm/llvm-project/issues/52735
 *
 * Work around these issues by adding a volatile inline asm with
 * memory clobber in the fallthrough after the asm goto and at each
 * label target.  Emit this for all compilers in case other similar
 * issues are found in the future.
 */
#define rseq_after_asm_goto()	__asm__ __volatile__ ("" : : : "memory")

/* Combine two tokens. */
#define RSEQ__COMBINE_TOKENS(_tokena, _tokenb)	\
	_tokena##_tokenb
#define RSEQ_COMBINE_TOKENS(_tokena, _tokenb)	\
	RSEQ__COMBINE_TOKENS(_tokena, _tokenb)

#if defined(__SIZEOF_LONG__)
#define RSEQ_BITS_PER_LONG	(__SIZEOF_LONG__ * 8)
#elif defined(_LP64)
#define RSEQ_BITS_PER_LONG	64
#else
#define RSEQ_BITS_PER_LONG	32
#endif

#ifdef __cplusplus
#define rseq_unqual_scalar_typeof(x)					\
	std::remove_cv<std::remove_reference<decltype(x)>::type>::type
#else
#define rseq_scalar_type_to_expr(type)					\
	unsigned type: (unsigned type)0,				\
	signed type: (signed type)0

/*
 * Use C11 _Generic to express unqualified type from expression. This removes
 * volatile qualifier from expression type.
 */
#define rseq_unqual_scalar_typeof(x)					\
	__typeof__(							\
		_Generic((x),						\
			char: (char)0,					\
			rseq_scalar_type_to_expr(char),			\
			rseq_scalar_type_to_expr(short),		\
			rseq_scalar_type_to_expr(int),			\
			rseq_scalar_type_to_expr(long),			\
			rseq_scalar_type_to_expr(long long),		\
			default: (x)					\
		)							\
	)
#endif

#endif  /* _RSEQ_COMPILER_H */
Commit	Line	Data
90702366	1	/* SPDX-License-Identifier: MIT */
f2d7b530 MJ	2	/* SPDX-FileCopyrightText: 2021 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> */
f2d7b530 MJ	3
dd76f2d6 MD	4	/*
	5	* rseq/compiler.h
	6	*
	7	* Work-around asm goto compiler bugs.
dd76f2d6 MD	8	*/
dd76f2d6 MD	9
44ec21eb MJ	10	#ifndef _RSEQ_COMPILER_H
44ec21eb MJ	11	#define _RSEQ_COMPILER_H
dd76f2d6	12
d292e9e1 MD	13	#if defined __cplusplus
	14	# include <type_traits> /* for std::remove_cv */
	15	#endif
	16
44ec21eb MJ	17	#define rseq_likely(x) __builtin_expect(!!(x), 1)
	18	#define rseq_unlikely(x) __builtin_expect(!!(x), 0)
	19	#define rseq_barrier() __asm__ __volatile__("" : : : "memory")
	20
	21	/*
	22	* Instruct the compiler to perform only a single access to a variable
	23	* (prohibits merging and refetching). The compiler is also forbidden to reorder
	24	* successive instances of RSEQ_ACCESS_ONCE(), but only when the compiler is aware of
	25	* particular ordering. Compiler ordering can be ensured, for example, by
	26	* putting two RSEQ_ACCESS_ONCE() in separate C statements.
	27	*
	28	* This macro does absolutely -nothing- to prevent the CPU from reordering,
	29	* merging, or refetching absolutely anything at any time. Its main intended
	30	* use is to mediate communication between process-level code and irq/NMI
	31	* handlers, all running on the same CPU.
	32	*/
	33	#define RSEQ_ACCESS_ONCE(x) ((__volatile__ __typeof__(x) )&(x))
	34
	35	#define RSEQ_WRITE_ONCE(x, v) __extension__ ({ RSEQ_ACCESS_ONCE(x) = (v); })
	36	#define RSEQ_READ_ONCE(x) RSEQ_ACCESS_ONCE(x)
	37
dd76f2d6 MD	38	/*
	39	* gcc prior to 4.8.2 miscompiles asm goto.
	40	* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58670
	41	*
	42	* gcc prior to 8.1.0 miscompiles asm goto at O1.
	43	* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103908
	44	*
	45	* clang prior to version 13.0.1 miscompiles asm goto at O2.
	46	* https://github.com/llvm/llvm-project/issues/52735
	47	*
	48	* Work around these issues by adding a volatile inline asm with
	49	* memory clobber in the fallthrough after the asm goto and at each
	50	* label target. Emit this for all compilers in case other similar
	51	* issues are found in the future.
	52	*/
6a5fa598	53	#define rseq_after_asm_goto() __asm__ __volatile__ ("" : : : "memory")
dd76f2d6	54
809f5ee3 MD	55	/* Combine two tokens. */
	56	#define RSEQ__COMBINE_TOKENS(_tokena, _tokenb) \
	57	_tokena##_tokenb
	58	#define RSEQ_COMBINE_TOKENS(_tokena, _tokenb) \
	59	RSEQ__COMBINE_TOKENS(_tokena, _tokenb)
	60
eb5d1cbe MD	61	#if defined(__SIZEOF_LONG__)
	62	#define RSEQ_BITS_PER_LONG (__SIZEOF_LONG__ * 8)
	63	#elif defined(_LP64)
	64	#define RSEQ_BITS_PER_LONG 64
	65	#else
	66	#define RSEQ_BITS_PER_LONG 32
	67	#endif
	68
d292e9e1 MD	69	#ifdef __cplusplus
d292e9e1 MD	70	#define rseq_unqual_scalar_typeof(x) \
96d67c89	71	std::remove_cv<std::remove_reference<decltype(x)>::type>::type
d292e9e1	72	#else
a95cabb3 MD	73	#define rseq_scalar_type_to_expr(type) \
	74	unsigned type: (unsigned type)0, \
	75	signed type: (signed type)0
	76
d292e9e1 MD	77	/*
	78	* Use C11 _Generic to express unqualified type from expression. This removes
	79	* volatile qualifier from expression type.
	80	*/
	81	#define rseq_unqual_scalar_typeof(x) \
	82	__typeof__( \
	83	_Generic((x), \
	84	char: (char)0, \
a95cabb3 MD	85	rseq_scalar_type_to_expr(char), \
	86	rseq_scalar_type_to_expr(short), \
	87	rseq_scalar_type_to_expr(int), \
	88	rseq_scalar_type_to_expr(long), \
	89	rseq_scalar_type_to_expr(long long), \
d292e9e1 MD	90	default: (x) \
	91	) \
	92	)
	93	#endif
	94
44ec21eb	95	#endif /* _RSEQ_COMPILER_H */