xref: /linux/tools/testing/selftests/bpf/usdt.h (revision e4dcbdff114e2c0a8059c396e233aa5d9637afce)

// SPDX-License-Identifier: BSD-2-Clause
/*
 *  This single-header library defines a collection of variadic macros for
 *  defining and triggering USDTs (User Statically-Defined Tracepoints):
 *
 *      - For USDTs without associated semaphore:
 *          USDT(group, name, args...)
 *
 *      - For USDTs with implicit (transparent to the user) semaphore:
 *          USDT_WITH_SEMA(group, name, args...)
 *          USDT_IS_ACTIVE(group, name)
 *
 *      - For USDTs with explicit (user-defined and provided) semaphore:
 *          USDT_WITH_EXPLICIT_SEMA(sema, group, name, args...)
 *          USDT_SEMA_IS_ACTIVE(sema)
 *
 *  all of which emit a NOP instruction into the instruction stream, and so
 *  have *zero* overhead for the surrounding code. USDTs are identified by
 *  a combination of `group` and `name` identifiers, which is used by external
 *  tracing tooling (tracers) for identifying exact USDTs of interest.
 *
 *  USDTs can have an associated (2-byte) activity counter (USDT semaphore),
 *  automatically maintained by Linux kernel whenever any correctly written
 *  BPF-based tracer is attached to the USDT. This USDT semaphore can be used
 *  to check whether there is a need to do any extra data collection and
 *  processing for a given USDT (if necessary), and otherwise avoid extra work
 *  for a common case of USDT not being traced ("active").
 *
 *  See documentation for USDT_WITH_SEMA()/USDT_IS_ACTIVE() or
 *  USDT_WITH_EXPLICIT_SEMA()/USDT_SEMA_IS_ACTIVE() APIs below for details on
 *  working with USDTs with implicitly or explicitly associated
 *  USDT semaphores, respectively.
 *
 *  There is also some additional data recorded into an auxiliary note
 *  section. The data in the note section describes the operands, in terms of
 *  size and location, used by tracing tooling to know where to find USDT
 *  arguments. Each location is encoded as an assembler operand string.
 *  Tracing tools (bpftrace and BPF-based tracers, systemtap, etc) insert
 *  breakpoints on top of the nop, and decode the location operand-strings,
 *  like an assembler, to find the values being passed.
 *
 *  The operand strings are selected by the compiler for each operand.
 *  They are constrained by inline-assembler codes.The default is:
 *
 *  #define USDT_ARG_CONSTRAINT nor
 *
 *  This is a good default if the operands tend to be integral and
 *  moderate in number (smaller than number of registers). In other
 *  cases, the compiler may report "'asm' requires impossible reload" or
 *  similar. In this case, consider simplifying the macro call (fewer
 *  and simpler operands), reduce optimization, or override the default
 *  constraints string via:
 *
 *  #define USDT_ARG_CONSTRAINT g
 *  #include <usdt.h>
 *
 * For some historical description of USDT v3 format (the one used by this
 * library and generally recognized and assumed by BPF-based tracing tools)
 * see [0]. The more formal specification can be found at [1]. Additional
 * argument constraints information can be found at [2].
 *
 * Original SystemTap's sys/sdt.h implementation ([3]) was used as a base for
 * this USDT library implementation. Current implementation differs *a lot* in
 * terms of exposed user API and general usability, which was the main goal
 * and focus of the reimplementation work. Nevertheless, underlying recorded
 * USDT definitions are fully binary compatible and any USDT-based tooling
 * should work equally well with USDTs defined by either SystemTap's or this
 * library's USDT implementation.
 *
 *   [0] https://ecos.sourceware.org/ml/systemtap/2010-q3/msg00145.html
 *   [1] https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation
 *   [2] https://gcc.gnu.org/onlinedocs/gcc/Constraints.html
 *   [3] https://sourceware.org/git/?p=systemtap.git;a=blob;f=includes/sys/sdt.h
 */
#ifndef __USDT_H
#define __USDT_H

/*
 * Changelog:
 *
 * 0.1.0
 * -----
 * - Initial release
 */
#define USDT_MAJOR_VERSION 0
#define USDT_MINOR_VERSION 1
#define USDT_PATCH_VERSION 0

/* C++20 and C23 added __VA_OPT__ as a standard replacement for non-standard `##__VA_ARGS__` extension */
#if (defined(__STDC_VERSION__) && __STDC_VERSION__ > 201710L) || (defined(__cplusplus) && __cplusplus > 201703L)
#define __usdt_va_opt 1
#define __usdt_va_args(...) __VA_OPT__(,) __VA_ARGS__
#else
#define __usdt_va_args(...) , ##__VA_ARGS__
#endif

/*
 * Trigger USDT with `group`:`name` identifier and pass through `args` as its
 * arguments. Zero arguments are acceptable as well. No USDT semaphore is
 * associated with this USDT.
 *
 * Such "semaphoreless" USDTs are commonly used when there is no extra data
 * collection or processing needed to collect and prepare USDT arguments and
 * they are just available in the surrounding code. USDT() macro will just
 * record their locations in CPU registers or in memory for tracing tooling to
 * be able to access them, if necessary.
 */
#ifdef __usdt_va_opt
#define USDT(group, name, ...)							\
	__usdt_probe(group, name, __usdt_sema_none, 0 __VA_OPT__(,) __VA_ARGS__)
#else
#define USDT(group, name, ...)							\
	__usdt_probe(group, name, __usdt_sema_none, 0, ##__VA_ARGS__)
#endif

/*
 * Trigger USDT with `group`:`name` identifier and pass through `args` as its
 * arguments. Zero arguments are acceptable as well. USDT also get an
 * implicitly-defined associated USDT semaphore, which will be "activated" by
 * tracing tooling and can be used to check whether USDT is being actively
 * observed.
 *
 * USDTs with semaphore are commonly used when there is a need to perform
 * additional data collection and processing to prepare USDT arguments, which
 * otherwise might not be necessary for the rest of application logic. In such
 * case, USDT semaphore can be used to avoid unnecessary extra work. If USDT
 * is not traced (which is presumed to be a common situation), the associated
 * USDT semaphore is "inactive", and so there is no need to waste resources to
 * prepare USDT arguments. Use USDT_IS_ACTIVE(group, name) to check whether
 * USDT is "active".
 *
 * N.B. There is an inherent (albeit short) gap between checking whether USDT
 * is active and triggering corresponding USDT, in which external tracer can
 * be attached to an USDT and activate USDT semaphore after the activity check.
 * If such a race occurs, tracers might miss one USDT execution. Tracers are
 * expected to accommodate such possibility and this is expected to not be
 * a problem for applications and tracers.
 *
 * N.B. Implicit USDT semaphore defined by USDT_WITH_SEMA() is contained
 * within a single executable or shared library and is not shared outside
 * them. I.e., if you use USDT_WITH_SEMA() with the same USDT group and name
 * identifier across executable and shared library, it will work and won't
 * conflict, per se, but will define independent USDT semaphores, one for each
 * shared library/executable in which USDT_WITH_SEMA(group, name) is used.
 * That is, if you attach to this USDT in one shared library (or executable),
 * then only USDT semaphore within that shared library (or executable) will be
 * updated by the kernel, while other libraries (or executable) will not see
 * activated USDT semaphore. In short, it's best to use unique USDT group:name
 * identifiers across different shared libraries (and, equivalently, between
 * executable and shared library). This is advanced consideration and is
 * rarely (if ever) seen in practice, but just to avoid surprises this is
 * called out here. (Static libraries become a part of final executable, once
 * linked by linker, so the above considerations don't apply to them.)
 */
#ifdef __usdt_va_opt
#define USDT_WITH_SEMA(group, name, ...)					\
	__usdt_probe(group, name,						\
		     __usdt_sema_implicit, __usdt_sema_name(group, name)	\
		     __VA_OPT__(,) __VA_ARGS__)
#else
#define USDT_WITH_SEMA(group, name, ...)					\
	__usdt_probe(group, name,						\
		     __usdt_sema_implicit, __usdt_sema_name(group, name),	\
		     ##__VA_ARGS__)
#endif

struct usdt_sema { volatile unsigned short active; };

/*
 * Check if USDT with `group`:`name` identifier is "active" (i.e., whether it
 * is attached to by external tracing tooling and is actively observed).
 *
 * This macro can be used to decide whether any additional and potentially
 * expensive data collection or processing should be done to pass extra
 * information into the given USDT. It is assumed that USDT is triggered with
 * USDT_WITH_SEMA() macro which will implicitly define associated USDT
 * semaphore. (If one needs more control over USDT semaphore, see
 * USDT_DEFINE_SEMA() and USDT_WITH_EXPLICIT_SEMA() macros below.)
 *
 * N.B. Such checks are necessarily racy and speculative. Between checking
 * whether USDT is active and triggering the USDT itself, tracer can be
 * detached with no notification. This race should be extremely rare and worst
 * case should result in one-time wasted extra data collection and processing.
 */
#define USDT_IS_ACTIVE(group, name) ({						\
	extern struct usdt_sema __usdt_sema_name(group, name)			\
		__usdt_asm_name(__usdt_sema_name(group, name));			\
	__usdt_sema_implicit(__usdt_sema_name(group, name));			\
	__usdt_sema_name(group, name).active > 0;				\
})

/*
 * APIs for working with user-defined explicit USDT semaphores.
 *
 * This is a less commonly used advanced API for use cases in which user needs
 * an explicit control over (potentially shared across multiple USDTs) USDT
 * semaphore instance. This can be used when there is a group of logically
 * related USDTs that all need extra data collection and processing whenever
 * any of a family of related USDTs are "activated" (i.e., traced). In such
 * a case, all such related USDTs will be associated with the same shared USDT
 * semaphore defined with USDT_DEFINE_SEMA() and the USDTs themselves will be
 * triggered with USDT_WITH_EXPLICIT_SEMA() macros, taking an explicit extra
 * USDT semaphore identifier as an extra parameter.
 */

/**
 * Underlying C global variable name for user-defined USDT semaphore with
 * `sema` identifier. Could be useful for debugging, but normally shouldn't be
 * used explicitly.
 */
#define USDT_SEMA(sema) __usdt_sema_##sema

/*
 * Define storage for user-defined USDT semaphore `sema`.
 *
 * Should be used only once in non-header source file to let compiler allocate
 * space for the semaphore variable. Just like with any other global variable.
 *
 * This macro can be used anywhere where global variable declaration is
 * allowed. Just like with global variable definitions, there should be only
 * one definition of user-defined USDT semaphore with given `sema` identifier,
 * otherwise compiler or linker will complain about duplicate variable
 * definition.
 *
 * For C++, it is allowed to use USDT_DEFINE_SEMA() both in global namespace
 * and inside namespaces (including nested namespaces). Just make sure that
 * USDT_DECLARE_SEMA() is placed within the namespace where this semaphore is
 * referenced, or any of its parent namespaces, so the C++ language-level
 * identifier is visible to the code that needs to reference the semaphore.
 * At the lowest layer, USDT semaphores have global naming and visibility
 * (they have a corresponding `__usdt_sema_<name>` symbol, which can be linked
 * against from C or C++ code, if necessary). To keep it simple, putting
 * USDT_DECLARE_SEMA() declarations into global namespaces is the simplest
 * no-brainer solution. All these aspects are irrelevant for plain C, because
 * C doesn't have namespaces and everything is always in the global namespace.
 *
 * N.B. Due to USDT metadata being recorded in non-allocatable ELF note
 * section, it has limitations when it comes to relocations, which, in
 * practice, means that it's not possible to correctly share USDT semaphores
 * between main executable and shared libraries, or even between multiple
 * shared libraries. USDT semaphore has to be contained to individual shared
 * library or executable to avoid unpleasant surprises with half-working USDT
 * semaphores. We enforce this by marking semaphore ELF symbols as having
 * a hidden visibility. This is quite an advanced use case and consideration
 * and for most users this should have no consequences whatsoever.
 */
#define USDT_DEFINE_SEMA(sema)							\
	struct usdt_sema __usdt_sema_sec USDT_SEMA(sema)			\
		__usdt_asm_name(USDT_SEMA(sema))				\
		__attribute__((visibility("hidden"))) = { 0 }

/*
 * Declare extern reference to user-defined USDT semaphore `sema`.
 *
 * Refers to a variable defined in another compilation unit by
 * USDT_DEFINE_SEMA() and allows to use the same USDT semaphore across
 * multiple compilation units (i.e., .c and .cpp files).
 *
 * See USDT_DEFINE_SEMA() notes above for C++ language usage peculiarities.
 */
#define USDT_DECLARE_SEMA(sema)							\
	extern struct usdt_sema USDT_SEMA(sema) __usdt_asm_name(USDT_SEMA(sema))

/*
 * Check if user-defined USDT semaphore `sema` is "active" (i.e., whether it
 * is attached to by external tracing tooling and is actively observed).
 *
 * This macro can be used to decide whether any additional and potentially
 * expensive data collection or processing should be done to pass extra
 * information into USDT(s) associated with USDT semaphore `sema`.
 *
 * N.B. Such checks are necessarily racy. Between checking the state of USDT
 * semaphore and triggering associated USDT(s), the active tracer might attach
 * or detach. This race should be extremely rare and worst case should result
 * in one-time missed USDT event or wasted extra data collection and
 * processing. USDT-using tracers should be written with this in mind and is
 * not a concern of the application defining USDTs with associated semaphore.
 */
#define USDT_SEMA_IS_ACTIVE(sema) (USDT_SEMA(sema).active > 0)

/*
 * Invoke USDT specified by `group` and `name` identifiers and associate
 * explicitly user-defined semaphore `sema` with it. Pass through `args` as
 * USDT arguments. `args` are optional and zero arguments are acceptable.
 *
 * Semaphore is defined with the help of USDT_DEFINE_SEMA() macro and can be
 * checked whether active with USDT_SEMA_IS_ACTIVE().
 */
#ifdef __usdt_va_opt
#define USDT_WITH_EXPLICIT_SEMA(sema, group, name, ...)				\
	__usdt_probe(group, name, __usdt_sema_explicit, USDT_SEMA(sema), ##__VA_ARGS__)
#else
#define USDT_WITH_EXPLICIT_SEMA(sema, group, name, ...)				\
	__usdt_probe(group, name, __usdt_sema_explicit, USDT_SEMA(sema) __VA_OPT__(,) __VA_ARGS__)
#endif

/*
 * Adjustable implementation aspects
 */
#ifndef USDT_ARG_CONSTRAINT
#if defined __powerpc__
#define USDT_ARG_CONSTRAINT		nZr
#elif defined __arm__
#define USDT_ARG_CONSTRAINT		g
#elif defined __loongarch__
#define USDT_ARG_CONSTRAINT		nmr
#else
#define USDT_ARG_CONSTRAINT		nor
#endif
#endif /* USDT_ARG_CONSTRAINT */

#ifndef USDT_NOP
#if defined(__ia64__) || defined(__s390__) || defined(__s390x__)
#define USDT_NOP			nop 0
#else
#define USDT_NOP			nop
#endif
#endif /* USDT_NOP */

/*
 * Implementation details
 */
/* USDT name for implicitly-defined USDT semaphore, derived from group:name */
#define __usdt_sema_name(group, name)	__usdt_sema_##group##__##name
/* ELF section into which USDT semaphores are put */
#define __usdt_sema_sec			__attribute__((section(".probes")))

#define __usdt_concat(a, b)		a ## b
#define __usdt_apply(fn, n)		__usdt_concat(fn, n)

#ifndef __usdt_nth
#define __usdt_nth(_, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, N, ...) N
#endif

#ifndef __usdt_narg
#ifdef __usdt_va_opt
#define __usdt_narg(...) __usdt_nth(_ __VA_OPT__(,) __VA_ARGS__, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
#else
#define __usdt_narg(...) __usdt_nth(_, ##__VA_ARGS__, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
#endif
#endif /* __usdt_narg */

#define __usdt_hash			#
#define __usdt_str_(x)			#x
#define __usdt_str(x)			__usdt_str_(x)

#ifndef __usdt_asm_name
#define __usdt_asm_name(name)		__asm__(__usdt_str(name))
#endif

#define __usdt_asm0()		"\n"
#define __usdt_asm1(x)		__usdt_str(x) "\n"
#define __usdt_asm2(x, ...)	__usdt_str(x) "," __usdt_asm1(__VA_ARGS__)
#define __usdt_asm3(x, ...)	__usdt_str(x) "," __usdt_asm2(__VA_ARGS__)
#define __usdt_asm4(x, ...)	__usdt_str(x) "," __usdt_asm3(__VA_ARGS__)
#define __usdt_asm5(x, ...)	__usdt_str(x) "," __usdt_asm4(__VA_ARGS__)
#define __usdt_asm6(x, ...)	__usdt_str(x) "," __usdt_asm5(__VA_ARGS__)
#define __usdt_asm7(x, ...)	__usdt_str(x) "," __usdt_asm6(__VA_ARGS__)
#define __usdt_asm8(x, ...)	__usdt_str(x) "," __usdt_asm7(__VA_ARGS__)
#define __usdt_asm9(x, ...)	__usdt_str(x) "," __usdt_asm8(__VA_ARGS__)
#define __usdt_asm10(x, ...)	__usdt_str(x) "," __usdt_asm9(__VA_ARGS__)
#define __usdt_asm11(x, ...)	__usdt_str(x) "," __usdt_asm10(__VA_ARGS__)
#define __usdt_asm12(x, ...)	__usdt_str(x) "," __usdt_asm11(__VA_ARGS__)
#define __usdt_asm(...)		__usdt_apply(__usdt_asm, __usdt_narg(__VA_ARGS__))(__VA_ARGS__)

#ifdef __LP64__
#define __usdt_asm_addr		.8byte
#else
#define __usdt_asm_addr		.4byte
#endif

#define __usdt_asm_strz_(x)	__usdt_asm1(.asciz #x)
#define __usdt_asm_strz(x)	__usdt_asm_strz_(x)
#define __usdt_asm_str_(x)	__usdt_asm1(.ascii #x)
#define __usdt_asm_str(x)	__usdt_asm_str_(x)

/* "semaphoreless" USDT case */
#ifndef __usdt_sema_none
#define __usdt_sema_none(sema)
#endif

/* implicitly defined __usdt_sema__group__name semaphore (using weak symbols) */
#ifndef __usdt_sema_implicit
#define __usdt_sema_implicit(sema)								\
	__asm__ __volatile__ (									\
	__usdt_asm1(.ifndef sema)								\
	__usdt_asm3(		.pushsection .probes, "aw", "progbits")				\
	__usdt_asm1(		.weak sema)							\
	__usdt_asm1(		.hidden sema)							\
	__usdt_asm1(		.align 2)							\
	__usdt_asm1(sema:)									\
	__usdt_asm1(		.zero 2)							\
	__usdt_asm2(		.type sema, @object)						\
	__usdt_asm2(		.size sema, 2)							\
	__usdt_asm1(		.popsection)							\
	__usdt_asm1(.endif)									\
	);
#endif

/* externally defined semaphore using USDT_DEFINE_SEMA() and passed explicitly by user */
#ifndef __usdt_sema_explicit
#define __usdt_sema_explicit(sema)								\
	__asm__ __volatile__ ("" :: "m" (sema));
#endif

/* main USDT definition (nop and .note.stapsdt metadata) */
#define __usdt_probe(group, name, sema_def, sema, ...) do {					\
	sema_def(sema)										\
	__asm__ __volatile__ (									\
	__usdt_asm( 990:	USDT_NOP)							\
	__usdt_asm3(		.pushsection .note.stapsdt, "", "note")				\
	__usdt_asm1(		.balign 4)							\
	__usdt_asm3(		.4byte 992f-991f,994f-993f,3)					\
	__usdt_asm1(991:	.asciz "stapsdt")						\
	__usdt_asm1(992:	.balign 4)							\
	__usdt_asm1(993:	__usdt_asm_addr 990b)						\
	__usdt_asm1(		__usdt_asm_addr _.stapsdt.base)					\
	__usdt_asm1(		__usdt_asm_addr sema)						\
	__usdt_asm_strz(group)									\
	__usdt_asm_strz(name)									\
	__usdt_asm_args(__VA_ARGS__)								\
	__usdt_asm1(		.ascii "\0")							\
	__usdt_asm1(994:	.balign 4)							\
	__usdt_asm1(		.popsection)							\
	__usdt_asm1(.ifndef _.stapsdt.base)							\
	__usdt_asm5(		.pushsection .stapsdt.base,"aG","progbits",.stapsdt.base,comdat)\
	__usdt_asm1(		.weak _.stapsdt.base)						\
	__usdt_asm1(		.hidden _.stapsdt.base)						\
	__usdt_asm1(_.stapsdt.base:)								\
	__usdt_asm1(		.space 1)							\
	__usdt_asm2(		.size _.stapsdt.base, 1)					\
	__usdt_asm1(		.popsection)							\
	__usdt_asm1(.endif)									\
	:: __usdt_asm_ops(__VA_ARGS__)								\
	);											\
} while (0)

/*
 * NB: gdb PR24541 highlighted an unspecified corner of the sdt.h
 * operand note format.
 *
 * The named register may be a longer or shorter (!) alias for the
 * storage where the value in question is found. For example, on
 * i386, 64-bit value may be put in register pairs, and a register
 * name stored would identify just one of them. Previously, gcc was
 * asked to emit the %w[id] (16-bit alias of some registers holding
 * operands), even when a wider 32-bit value was used.
 *
 * Bottom line: the byte-width given before the @ sign governs. If
 * there is a mismatch between that width and that of the named
 * register, then a sys/sdt.h note consumer may need to employ
 * architecture-specific heuristics to figure out where the compiler
 * has actually put the complete value.
 */
#if defined(__powerpc__) || defined(__powerpc64__)
#define __usdt_argref(id)	%I[id]%[id]
#elif defined(__i386__)
#define __usdt_argref(id)	%k[id]		/* gcc.gnu.org/PR80115 sourceware.org/PR24541 */
#else
#define __usdt_argref(id)	%[id]
#endif

#define __usdt_asm_arg(n)	__usdt_asm_str(%c[__usdt_asz##n])				\
				__usdt_asm1(.ascii "@")						\
				__usdt_asm_str(__usdt_argref(__usdt_aval##n))

#define __usdt_asm_args0	/* no arguments */
#define __usdt_asm_args1	__usdt_asm_arg(1)
#define __usdt_asm_args2	__usdt_asm_args1 __usdt_asm1(.ascii " ") __usdt_asm_arg(2)
#define __usdt_asm_args3	__usdt_asm_args2 __usdt_asm1(.ascii " ") __usdt_asm_arg(3)
#define __usdt_asm_args4	__usdt_asm_args3 __usdt_asm1(.ascii " ") __usdt_asm_arg(4)
#define __usdt_asm_args5	__usdt_asm_args4 __usdt_asm1(.ascii " ") __usdt_asm_arg(5)
#define __usdt_asm_args6	__usdt_asm_args5 __usdt_asm1(.ascii " ") __usdt_asm_arg(6)
#define __usdt_asm_args7	__usdt_asm_args6 __usdt_asm1(.ascii " ") __usdt_asm_arg(7)
#define __usdt_asm_args8	__usdt_asm_args7 __usdt_asm1(.ascii " ") __usdt_asm_arg(8)
#define __usdt_asm_args9	__usdt_asm_args8 __usdt_asm1(.ascii " ") __usdt_asm_arg(9)
#define __usdt_asm_args10	__usdt_asm_args9 __usdt_asm1(.ascii " ") __usdt_asm_arg(10)
#define __usdt_asm_args11	__usdt_asm_args10 __usdt_asm1(.ascii " ") __usdt_asm_arg(11)
#define __usdt_asm_args12	__usdt_asm_args11 __usdt_asm1(.ascii " ") __usdt_asm_arg(12)
#define __usdt_asm_args(...)	__usdt_apply(__usdt_asm_args, __usdt_narg(__VA_ARGS__))

#define __usdt_is_arr(x)	(__builtin_classify_type(x) == 14 || __builtin_classify_type(x) == 5)
#define __usdt_arg_size(x)	(__usdt_is_arr(x) ? sizeof(void *) : sizeof(x))

/*
 * We can't use __builtin_choose_expr() in C++, so fall back to table-based
 * signedness determination for known types, utilizing templates magic.
 */
#ifdef __cplusplus

#define __usdt_is_signed(x)	(!__usdt_is_arr(x) && __usdt_t<__typeof(x)>::is_signed)

#include <cstddef>

template<typename T> struct __usdt_t { static const bool is_signed = false; };
template<typename A> struct __usdt_t<A[]> : public __usdt_t<A *> {};
template<typename A, size_t N> struct __usdt_t<A[N]> : public __usdt_t<A *> {};

#define __usdt_def_signed(T)									\
template<> struct __usdt_t<T>		     { static const bool is_signed = true; };		\
template<> struct __usdt_t<const T>	     { static const bool is_signed = true; };		\
template<> struct __usdt_t<volatile T>	     { static const bool is_signed = true; };		\
template<> struct __usdt_t<const volatile T> { static const bool is_signed = true; }
#define __usdt_maybe_signed(T)									\
template<> struct __usdt_t<T>		     { static const bool is_signed = (T)-1 < (T)1; };	\
template<> struct __usdt_t<const T>	     { static const bool is_signed = (T)-1 < (T)1; };	\
template<> struct __usdt_t<volatile T>	     { static const bool is_signed = (T)-1 < (T)1; };	\
template<> struct __usdt_t<const volatile T> { static const bool is_signed = (T)-1 < (T)1; }

__usdt_def_signed(signed char);
__usdt_def_signed(short);
__usdt_def_signed(int);
__usdt_def_signed(long);
__usdt_def_signed(long long);
__usdt_maybe_signed(char);
__usdt_maybe_signed(wchar_t);

#else /* !__cplusplus */

#define __usdt_is_inttype(x)	(__builtin_classify_type(x) >= 1 && __builtin_classify_type(x) <= 4)
#define __usdt_inttype(x)	__typeof(__builtin_choose_expr(__usdt_is_inttype(x), (x), 0U))
#define __usdt_is_signed(x)	((__usdt_inttype(x))-1 < (__usdt_inttype(x))1)

#endif /* __cplusplus */

#define __usdt_asm_op(n, x)									\
	[__usdt_asz##n] "n" ((__usdt_is_signed(x) ? (int)-1 : 1) * (int)__usdt_arg_size(x)),	\
	[__usdt_aval##n] __usdt_str(USDT_ARG_CONSTRAINT)(x)

#define __usdt_asm_ops0()				[__usdt_dummy] "g" (0)
#define __usdt_asm_ops1(x)				__usdt_asm_op(1, x)
#define __usdt_asm_ops2(a,x)				__usdt_asm_ops1(a), __usdt_asm_op(2, x)
#define __usdt_asm_ops3(a,b,x)				__usdt_asm_ops2(a,b), __usdt_asm_op(3, x)
#define __usdt_asm_ops4(a,b,c,x)			__usdt_asm_ops3(a,b,c), __usdt_asm_op(4, x)
#define __usdt_asm_ops5(a,b,c,d,x)			__usdt_asm_ops4(a,b,c,d), __usdt_asm_op(5, x)
#define __usdt_asm_ops6(a,b,c,d,e,x)			__usdt_asm_ops5(a,b,c,d,e), __usdt_asm_op(6, x)
#define __usdt_asm_ops7(a,b,c,d,e,f,x)			__usdt_asm_ops6(a,b,c,d,e,f), __usdt_asm_op(7, x)
#define __usdt_asm_ops8(a,b,c,d,e,f,g,x)		__usdt_asm_ops7(a,b,c,d,e,f,g), __usdt_asm_op(8, x)
#define __usdt_asm_ops9(a,b,c,d,e,f,g,h,x)		__usdt_asm_ops8(a,b,c,d,e,f,g,h), __usdt_asm_op(9, x)
#define __usdt_asm_ops10(a,b,c,d,e,f,g,h,i,x)		__usdt_asm_ops9(a,b,c,d,e,f,g,h,i), __usdt_asm_op(10, x)
#define __usdt_asm_ops11(a,b,c,d,e,f,g,h,i,j,x)		__usdt_asm_ops10(a,b,c,d,e,f,g,h,i,j), __usdt_asm_op(11, x)
#define __usdt_asm_ops12(a,b,c,d,e,f,g,h,i,j,k,x)	__usdt_asm_ops11(a,b,c,d,e,f,g,h,i,j,k), __usdt_asm_op(12, x)
#define __usdt_asm_ops(...)				__usdt_apply(__usdt_asm_ops, __usdt_narg(__VA_ARGS__))(__VA_ARGS__)

#endif /* __USDT_H */