sdk/include/debug.hh - 3p/cheriot-rtos - Git at Google

 // Copyright Microsoft and CHERIoT Contributors.
 // SPDX-License-Identifier: MIT

 #pragma once
 #include "cdefs.h"
 #include <cheri.hh>
 #include <compartment.h>
 #include <concepts>
 #include <cstddef>
 #include <platform-uart.hh>
 #include <string.h>

 #include <array>
 #include <string_view>
 #include <type_traits>

 namespace DebugConcepts
 {
 	/// Helper concept for matching booleans
 	template<typename T>
 	concept IsBool = std::is_same_v<T, bool>;

 	/// Helper concept for matching enumerations.
 	template<typename T>
 	concept IsEnum = std::is_enum_v<T>;

 	/// Concept for something that can be lazily called to produce a bool.
 	template<typename T>
 	concept LazyAssertion = requires(T v)
 	{
 		{
 			v()
 			} -> IsBool;
 	};

 	template<typename T>
 	concept IsPointerButNotCString =
 	  std::is_pointer_v<T> && !std::is_same_v<T, const char *>;

 	template<typename T>
 	concept IsConvertibleToAddress =
 	  std::convertible_to<T, ptraddr_t> && !IsEnum<T>;

 } // namespace DebugConcepts

 /**
  * Abstract class for writing debug output.  This is used for custom output.
  *
  * This may be changed in the future to provide better support for custom
  * formatting.
  */
 struct DebugWriter
 {
 	/**
 	 * Write a single character.
 	 */
 	virtual void write(char) = 0;
 	/**
 	 * Write a C string.
 	 */
 	virtual void write(const char *) = 0;
 	/**
 	 * Write a string view.
 	 */
 	virtual void write(std::string_view) = 0;
 	/**
 	 * Write a 32-bit unsigned integer.
 	 */
 	virtual void write(uint32_t) = 0;
 	/**
 	 * Write a 32-bit signed integer.
 	 */
 	virtual void write(int32_t) = 0;
 	/**
 	 * Write a 64-bit unsigned integer.
 	 */
 	virtual void write(uint64_t) = 0;
 	/**
 	 * Write a 64-bit signed integer.
 	 */
 	virtual void write(int64_t) = 0;
 };

 /**
  * Helper function for writing enumerations.  Enumerations are written using
  * magic_enum to provide a string and then a numeric value.
  */
 template<typename T>
 void debug_enum_helper(uintptr_t    value,
                        DebugWriter &writer) requires DebugConcepts::IsEnum<T>
 {
 	writer.write(magic_enum::enum_name<T>(static_cast<T>(value)));
 	writer.write('(');
 	writer.write(uint32_t(value));
 	writer.write(')');
 }

 /**
  * Callback for custom types in debug output.  This should use the second
  * argument to write the first argument to the debug output.
  */
 using DebugCallback = void (*)(uintptr_t, DebugWriter &);

 struct DebugFormatArgument
 {
 	/**
 	 * The kind of value, for values that have special-cased handling.
 	 */
 	enum Kind : ptraddr_t
 	{
 		/// Boolean, printed as "true" or "false".
 		Bool,
 		/// Single character.
 		Character,
 		/// Signed 32-bit integer, printed as decimal.
 		SignedNumber32,
 		/// Unsigned 32-bit integer, printed as hexadecimal
 		UnsignedNumber32,
 		/// Signed 64-bit integer, printed as decimal.
 		SignedNumber64,
 		/// Unsigned 64-bit integer, printed as hexadecimal.
 		UnsignedNumber64,
 		/// Pointer, printed as a full capability.
 		Pointer,
 		/// Special case for permission sets, printed as in the capability
 		/// format.
 		PermissionSet,
 		/// C string, printed as-is.
 		CString,
 		/// String view, printed as-is.
 		StringView,
 	};

 	/**
 	 * The value that is being written.
 	 */
 	uintptr_t value;
 	/**
 	 * The kind of value that is being written.  This is either a pointer
 	 * to a `DebugCallback` or one of the `Kind` enumeration, depending on
 	 * whether the tag is set or not.
 	 */
 	uintptr_t kind;
 };

 /**
  * Adaptor that turns an argument of type `T` into a `DebugFormatArgument`.
  *
  * Users may specialise these to provide custom formatters.  See the
  * specialisation for enumerations for an example.
  */
 template<typename T>
 struct DebugFormatArgumentAdaptor;

 /**
  * Boolean specialisation, prints "true" or "false".
  */
 template<>
 struct DebugFormatArgumentAdaptor<bool>
 {
 	__always_inline static DebugFormatArgument construct(bool value)
 	{
 		return {static_cast<uintptr_t>(value), DebugFormatArgument::Bool};
 	}
 };

 /**
  * Character specialisation, prints the character.
  */
 template<>
 struct DebugFormatArgumentAdaptor<char>
 {
 	__always_inline static DebugFormatArgument construct(char value)
 	{
 		return {static_cast<uintptr_t>(value), DebugFormatArgument::Character};
 	}
 };

 /**
  * Unsigned character specialisation, prints the character as a hex number.
  */
 template<>
 struct DebugFormatArgumentAdaptor<uint8_t>
 {
 	__always_inline static DebugFormatArgument construct(uint8_t value)
 	{
 		return {static_cast<uintptr_t>(value),
 		        DebugFormatArgument::UnsignedNumber32};
 	}
 };

 /**
  * Unsigned 16-bit integer specialisation, prints the integer as a hex number.
  */
 template<>
 struct DebugFormatArgumentAdaptor<uint16_t>
 {
 	__always_inline static DebugFormatArgument construct(uint16_t value)
 	{
 		return {static_cast<uintptr_t>(value),
 		        DebugFormatArgument::UnsignedNumber32};
 	}
 };

 /**
  * Unsigned 32-bit integer specialisation, prints the integer as a hex number.
  */
 template<>
 struct DebugFormatArgumentAdaptor<uint32_t>
 {
 	__always_inline static DebugFormatArgument construct(uint32_t value)
 	{
 		return {static_cast<uintptr_t>(value),
 		        DebugFormatArgument::UnsignedNumber32};
 	}
 };

 /**
  * Unsigned 64-bit integer specialisation, prints the integer as a hex number.
  *
  * All smaller sizes are handled by zero extending to 32 bits.  We treat 64-bit
  * separately because it requires decomposing the two halves for printing and
  * that's redundant overhead for the majority of cases.
  */
 template<>
 struct DebugFormatArgumentAdaptor<uint64_t>
 {
 	__always_inline static DebugFormatArgument construct(uint64_t value)
 	{
 		uintptr_t fudgedValue;
 		memcpy(&fudgedValue, &value, sizeof(fudgedValue));
 		return {fudgedValue, DebugFormatArgument::UnsignedNumber64};
 	}
 };

 /**
  * Signed 8-bit integer specialisations, print the integer as a decimal number.
  */
 template<>
 struct DebugFormatArgumentAdaptor<int8_t>
 {
 	__always_inline static DebugFormatArgument construct(int8_t value)
 	{
 		return {static_cast<uintptr_t>(value),
 		        DebugFormatArgument::SignedNumber32};
 	}
 };

 /**
  * Signed 16-bit integer specialisations, print the integer as a decimal number.
  */
 template<>
 struct DebugFormatArgumentAdaptor<int16_t>
 {
 	__always_inline static DebugFormatArgument construct(int16_t value)
 	{
 		return {static_cast<uintptr_t>(value),
 		        DebugFormatArgument::SignedNumber32};
 	}
 };

 /**
  * Signed 32-bit integer specialisations, print the integer as a decimal number.
  */
 template<>
 struct DebugFormatArgumentAdaptor<int32_t>
 {
 	__always_inline static DebugFormatArgument construct(int32_t value)
 	{
 		return {static_cast<uintptr_t>(value),
 		        DebugFormatArgument::SignedNumber32};
 	}
 };

 /**
  * Signed 64-bit integer specialisations, print the integer as a decimal number.
  *
  * All smaller sizes are handled by sign extending to 32 bits.  We treat 64-bit
  * separately because it requires 64-bit division to convert a 64-bit integer
  * to a decimal and that, in turn, requires libcalls.
  */
 template<>
 struct DebugFormatArgumentAdaptor<int64_t>
 {
 	__always_inline static DebugFormatArgument construct(int64_t value)
 	{
 		static_assert(sizeof(uintptr_t) == sizeof(uint64_t));
 		uintptr_t fudgedValue;
 		memcpy(&fudgedValue, &value, sizeof(fudgedValue));
 		return {fudgedValue, DebugFormatArgument::SignedNumber64};
 	}
 };

 /**
  * C string specialisation, prints the string as-is.
  */
 template<>
 struct DebugFormatArgumentAdaptor<const char *>
 {
 	__always_inline static DebugFormatArgument construct(const char *value)
 	{
 		return {reinterpret_cast<uintptr_t>(value),
 		        DebugFormatArgument::CString};
 	}
 };

 /**
  * String view specialisation, prints the string as-is.
  *
  * Note that this relies on the string view persisting for the duration of the
  * call.  It passes a pointer to the string-view argument.
  */
 template<>
 struct DebugFormatArgumentAdaptor<std::string_view>
 {
 	__always_inline static DebugFormatArgument
 	construct(std::string_view &value)
 	{
 		return {reinterpret_cast<uintptr_t>(&value),
 		        DebugFormatArgument::StringView};
 	}
 };

 /**
  * Enum specialisation, prints the enum as a string and then the numeric value.
  *
  * This specialisation uses the generic printing facility in the library call
  * and passes a callback that will map the enumeration to a string.
  */
 template<DebugConcepts::IsEnum T>
 struct DebugFormatArgumentAdaptor<T>
 {
 	__always_inline static DebugFormatArgument construct(T value)
 	{
 #ifdef CHERIOT_AVOID_CAPRELOCS
 		return {static_cast<uintptr_t>(value),
 		        DebugFormatArgument::UnsignedNumber32};
 #else
 		return {static_cast<uintptr_t>(value),
 		        reinterpret_cast<uintptr_t>(&debug_enum_helper<T>)};
 #endif
 	}
 };

 /**
  * Permission set specialisation.
  */
 template<>
 struct DebugFormatArgumentAdaptor<CHERI::PermissionSet>
 {
 	__always_inline static DebugFormatArgument
 	construct(CHERI::PermissionSet value)
 	{
 		return {static_cast<uintptr_t>(value.as_raw()),
 		        DebugFormatArgument::PermissionSet};
 	}
 };

 /**
  * Pointer specialisation, prints the pointer as a full capability.
  */
 template<DebugConcepts::IsPointerButNotCString T>
 struct DebugFormatArgumentAdaptor<T>
 {
 	__always_inline static DebugFormatArgument construct(T value)
 	{
 		return {reinterpret_cast<uintptr_t>(
 		          static_cast<const volatile void *>(value)),
 		        DebugFormatArgument::Pointer};
 	}
 };

 /**
  * Specialisation for the CHERI capability wrapper class, prints the capability
  * in the same format as bare pointers.
  */
 template<typename T>
 struct DebugFormatArgumentAdaptor<CHERI::Capability<T>>
 {
 	__always_inline static DebugFormatArgument
 	construct(CHERI::Capability<T> value)
 	{
 		return {reinterpret_cast<uintptr_t>(
 		          static_cast<const volatile void *>(value)),
 		        DebugFormatArgument::Pointer};
 	}
 };

 /**
  * Specialisation for things that can be converted to addresses, prints as an
  * unsigned number.
  */
 template<DebugConcepts::IsConvertibleToAddress T>
 struct DebugFormatArgumentAdaptor<T>
 {
 	__always_inline static DebugFormatArgument construct(ptraddr_t value)
 	{
 		return {static_cast<uintptr_t>(value),

 		        DebugFormatArgument::UnsignedNumber32};
 	}
 };

 /**
  * Recursive helper that maps from a tuple representing the arguments into a
  * type-erased array.
  */
 template<size_t I>
 __always_inline inline void map_debug_argument(DebugFormatArgument *arguments,
                                                auto &&argumentTuple)
 {
 	arguments[I] =
 	  DebugFormatArgumentAdaptor<
 	    std::remove_cvref_t<decltype(std::get<I>(argumentTuple))>>{}
 	    .construct(std::get<I>(argumentTuple));
 	if constexpr (I > 0)
 	{
 		map_debug_argument<I - 1>(arguments, argumentTuple);
 	}
 }

 /**
  * Convert `args` into a type-erased array of `DebugFormatArgument`s in
  * `arguments`.
  */
 template<typename... Args>
 __always_inline inline void
 make_debug_arguments_list(DebugFormatArgument *arguments, Args... args)
 {
 	if constexpr (sizeof...(Args) > 0)
 	{
 		map_debug_argument<sizeof...(Args) - 1>(arguments,
 		                                        std::forward_as_tuple(args...));
 	}
 }

 /**
  * Library function that writes a debug message.  This runs with interrupts
  * disabled (to avoid interleaving) and prints an array of debug messages. This
  * is intended to allow a single call to print multiple format strings without
  * requiring the format strings to be copied, so that the debugging APIs can
  * wrap a user-provided format string.
  */
 __cheri_libcall void debug_log_message_write(const char          *context,
                                              const char          *format,
                                              DebugFormatArgument *messages,
                                              size_t               messageCount);

 __cheri_libcall void debug_report_failure(const char          *kind,
                                           const char          *file,
                                           const char          *function,
                                           int                  line,
                                           const char          *fmt,
                                           DebugFormatArgument *arguments,
                                           size_t               argumentCount);

 namespace
 {
 	/**
 	 * Helper class wrapping a string for use as a template argument.  This is
 	 * used to describe a context for conditional debugging that will be
 	 * prefixed to debug lines.
 	 */
 	template<size_t N>
 	struct DebugContext
 	{
 		/**
 		 * Constructor, captures the string argument.
 		 */
 		constexpr DebugContext(const char (&str)[N])
 		{
 			std::copy_n(str, N, value);
 		}

 		/**
 		 * Implicit conversion to a C string.
 		 */
 		constexpr operator const char *() const
 		{
 			return value;
 		}

 		/**
 		 * The captured string.  Must be public for this class to meet the
 		 * requirements of a structural type for use as a template argument.
 		 */
 		char value[N];
 	};

 	/**
 	 * Our libc++ does not currently provide source_location, but our clang
 	 * provides the necessary builtins for one.
 	 */
 	struct SourceLocation
 	{
 		/**
 		 * Explicitly construct a source location.
 		 */
 		constexpr SourceLocation(int         lineNumber,
 		                         int         columnNumber,
 		                         const char *fileName,
 		                         const char *functionName)
 		  : lineNumber(lineNumber),
 		    columnNumber(columnNumber),
 		    fileName(fileName),
 		    functionName(functionName)
 		{
 		}

 		/**
 		 * Construct a source location for the caller.
 		 */
 		static constexpr SourceLocation __always_inline
 		current(int         lineNumber   = __builtin_LINE(),
 		        int         columnNumber = __builtin_COLUMN(),
 		        const char *fileName     = __builtin_FILE(),
 		        const char *functionName = __builtin_FUNCTION()) noexcept
 		{
 			return {lineNumber, columnNumber, fileName, functionName};
 		}

 		/// Returns the line number for this source location.
 		[[nodiscard]] __always_inline constexpr int line() const noexcept
 		{
 			return lineNumber;
 		}
 		/// Returns the column number for this source location.
 		[[nodiscard]] __always_inline constexpr int column() const noexcept
 		{
 			return columnNumber;
 		}
 		/// Returns the file name for this source location.
 		[[nodiscard]] __always_inline constexpr const char *
 		file_name() const noexcept
 		{
 			return fileName;
 		}
 		/// Returns the function name for this source location.
 		[[nodiscard]] __always_inline constexpr const char *
 		function_name() const noexcept
 		{
 			return functionName;
 		}

 		private:
 		/// The line number of this source location.
 		int lineNumber;
 		/// The column number of this source location.
 		int columnNumber;
 		/// The file name of this source location.
 		const char *fileName;
 		/// The function name of this source location.
 		const char *functionName;
 	};

 	/**
 	 * Conditional debug class.  Used to control conditional output and
 	 * assertion checking.  Enables debug log messages and assertions if
 	 * `Enabled` is true.  Uses `Context` to print additional detail on debug
 	 * lines.  Each line is prefixed with the context string in magenta to make
 	 * it easy to see debug output from different subsystems in the same trace.
 	 *
 	 * This class is expected to be used as a type alias, something like:
 	 *
 	 * ```c++
 	 * constexpr bool DebugFoo = DEBUG_FOO;
 	 * using Debug = ConditionalDebug<DebugFoo, "Foo">;
 	 * ```
 	 */
 	template<bool Enabled, DebugContext Context>
 	class ConditionalDebug
 	{
 		public:
 		/**
 		 * Log a message.
 		 *
 		 * This function does nothing if the `Enabled` condition is false.
 		 */
 		template<typename... Args>
 		static void log(const char *fmt, Args... args)
 		{
 			if constexpr (Enabled)
 			{
 				asm volatile("" ::: "memory");
 				DebugFormatArgument arguments[sizeof...(Args)];
 				make_debug_arguments_list(arguments, args...);
 				const char *context = Context;
 				debug_log_message_write(
 				  context, fmt, arguments, sizeof...(Args));
 				asm volatile("" ::: "memory");
 			}
 		}

 		/**
 		 * Helper to report failure.
 		 *
 		 * This must not take the `SourceLocation` directly because doing so
 		 * prevents the compiler from decomposing and subsequently
 		 * constant-propagating its fields in the caller, resulting in 24 bytes
 		 * of stack space consumed for every assert or invariant.
 		 */
 		template<typename... Args>
 		static inline void report_failure(const char *kind,
 		                                  const char *file,
 		                                  const char *function,
 		                                  int         line,
 		                                  const char *fmt,
 		                                  Args... args)
 		{
 			// Ensure that the compiler does not reorder messages.
 			DebugFormatArgument arguments[sizeof...(Args)];
 			make_debug_arguments_list(arguments, args...);
 			const char *context = Context;
 			debug_report_failure(
 			  kind, file, function, line, fmt, arguments, sizeof...(Args));
 		}

 		/**
 		 * Function-like class for invariants that is expected to be used via
 		 * the deduction guide as:
 		 *
 		 * ConditionalDebug::Invariant(someCondition, "A message...", ...);
 		 *
 		 * Invariants are checked unconditionally but will log a verbose
 		 * message only if `Enabled` is true.
 		 */
 		template<typename... Args>
 		struct Invariant
 		{
 			/**
 			 * Constructor, performs the invariant check.
 			 */
 			__always_inline
 			Invariant(bool        condition,
 			          const char *fmt,
 			          Args... args,
 			          SourceLocation loc = SourceLocation::current())
 			{
 				if (__predict_false(!condition))
 				{
 					if constexpr (Enabled)
 					{
 						report_failure("Invariant",
 						               loc.file_name(),
 						               loc.function_name(),
 						               loc.line(),
 						               fmt,
 						               std::forward<Args>(args)...);
 					}
 					__builtin_trap();
 				}
 			}
 		};
 		/**
 		 * Function-like class for assertions that is expected to be used via
 		 * the deduction guide as:
 		 *
 		 * ConditionalDebug::Assert(someCondition, "A message...", ...);
 		 *
 		 * Assertions are checked only if `Enabled` is true.
 		 */
 		template<typename... Args>
 		struct Assert
 		{
 			/**
 			 * Constructor, performs the assertion check.
 			 */
 			template<typename T>
 			requires DebugConcepts::IsBool<T> __always_inline
 			Assert(T           condition,
 			       const char *fmt,
 			       Args... args,
 			       SourceLocation loc = SourceLocation::current())
 			{
 				if constexpr (Enabled)
 				{
 					if (__predict_false(!condition))
 					{
 						report_failure("Assertion",
 						               loc.file_name(),
 						               loc.function_name(),
 						               loc.line(),
 						               fmt,
 						               std::forward<Args>(args)...);
 						__builtin_trap();
 					}
 				}
 			}

 			/**
 			 * Constructor, performs an assertion check.
 			 *
 			 * This version is passed a lambda that returns a bool, rather than
 			 * a boolean condition.  This allows the compiler to completely
 			 * elide the contents of the lambda in builds where this component
 			 * is not being debugged.  This version should be used for places
 			 * where the assertion condition has side effects.
 			 */
 			template<typename T>
 			requires DebugConcepts::LazyAssertion<T> __always_inline
 			Assert(T         &&condition,
 			       const char *fmt,
 			       Args... args,
 			       SourceLocation loc = SourceLocation::current())
 			{
 				if constexpr (Enabled)
 				{
 					if (__predict_false(!condition()))
 					{
 						report_failure("Assertion",
 						               loc.file_name(),
 						               loc.function_name(),
 						               loc.line(),
 						               fmt,
 						               std::forward<Args>(args)...);
 						__builtin_trap();
 					}
 				}
 			}
 		};

 		/**
 		 * Deduction guide, allows `Invariant` to be used as if it were a
 		 * function.
 		 */
 		template<typename T, typename... Ts>
 		Invariant(T, const char *, Ts &&...) -> Invariant<Ts...>;

 		/**
 		 * Deduction guide, allows `Assert` to be used as if it were a
 		 * function.
 		 */
 		template<typename... Ts>
 		Assert(bool, const char *, Ts &&...) -> Assert<Ts...>;

 		/**
 		 * Deduction guide, allows `Assert` to be used as if it were a
 		 * function with a lambda argument.
 		 */
 		template<typename... Ts>
 		Assert(auto, const char *, Ts &&...) -> Assert<Ts...>;
 	};

 } // namespace
	// Copyright Microsoft and CHERIoT Contributors.
	// SPDX-License-Identifier: MIT

	#pragma once
	#include "cdefs.h"
	#include <cheri.hh>
	#include <compartment.h>
	#include <concepts>
	#include <cstddef>
	#include <platform-uart.hh>
	#include <string.h>

	#include <array>
	#include <string_view>
	#include <type_traits>

	namespace DebugConcepts
	{
	/// Helper concept for matching booleans
	template<typename T>
	concept IsBool = std::is_same_v<T, bool>;

	/// Helper concept for matching enumerations.
	template<typename T>
	concept IsEnum = std::is_enum_v<T>;

	/// Concept for something that can be lazily called to produce a bool.
	template<typename T>
	concept LazyAssertion = requires(T v)
	{
	{
	v()
	} -> IsBool;
	};

	template<typename T>
	concept IsPointerButNotCString =
	std::is_pointer_v<T> && !std::is_same_v<T, const char *>;

	template<typename T>
	concept IsConvertibleToAddress =
	std::convertible_to<T, ptraddr_t> && !IsEnum<T>;

	} // namespace DebugConcepts

	/**
	* Abstract class for writing debug output. This is used for custom output.
	*
	* This may be changed in the future to provide better support for custom
	* formatting.
	*/
	struct DebugWriter
	{
	/**
	* Write a single character.
	*/
	virtual void write(char) = 0;
	/**
	* Write a C string.
	*/
	virtual void write(const char *) = 0;
	/**
	* Write a string view.
	*/
	virtual void write(std::string_view) = 0;
	/**
	* Write a 32-bit unsigned integer.
	*/
	virtual void write(uint32_t) = 0;
	/**
	* Write a 32-bit signed integer.
	*/
	virtual void write(int32_t) = 0;
	/**
	* Write a 64-bit unsigned integer.
	*/
	virtual void write(uint64_t) = 0;
	/**
	* Write a 64-bit signed integer.
	*/
	virtual void write(int64_t) = 0;
	};

	/**
	* Helper function for writing enumerations. Enumerations are written using
	* magic_enum to provide a string and then a numeric value.
	*/
	template<typename T>
	void debug_enum_helper(uintptr_t value,
	DebugWriter &writer) requires DebugConcepts::IsEnum<T>
	{
	writer.write(magic_enum::enum_name<T>(static_cast<T>(value)));
	writer.write('(');
	writer.write(uint32_t(value));
	writer.write(')');
	}

	/**
	* Callback for custom types in debug output. This should use the second
	* argument to write the first argument to the debug output.
	*/
	using DebugCallback = void (*)(uintptr_t, DebugWriter &);

	struct DebugFormatArgument
	{
	/**
	* The kind of value, for values that have special-cased handling.
	*/
	enum Kind : ptraddr_t
	{
	/// Boolean, printed as "true" or "false".
	Bool,
	/// Single character.
	Character,
	/// Signed 32-bit integer, printed as decimal.
	SignedNumber32,
	/// Unsigned 32-bit integer, printed as hexadecimal
	UnsignedNumber32,
	/// Signed 64-bit integer, printed as decimal.
	SignedNumber64,
	/// Unsigned 64-bit integer, printed as hexadecimal.
	UnsignedNumber64,
	/// Pointer, printed as a full capability.
	Pointer,
	/// Special case for permission sets, printed as in the capability
	/// format.
	PermissionSet,
	/// C string, printed as-is.
	CString,
	/// String view, printed as-is.
	StringView,
	};

	/**
	* The value that is being written.
	*/
	uintptr_t value;
	/**
	* The kind of value that is being written. This is either a pointer
	* to a `DebugCallback` or one of the `Kind` enumeration, depending on
	* whether the tag is set or not.
	*/
	uintptr_t kind;
	};

	/**
	* Adaptor that turns an argument of type `T` into a `DebugFormatArgument`.
	*
	* Users may specialise these to provide custom formatters. See the
	* specialisation for enumerations for an example.
	*/
	template<typename T>
	struct DebugFormatArgumentAdaptor;

	/**
	* Boolean specialisation, prints "true" or "false".
	*/
	template<>
	struct DebugFormatArgumentAdaptor<bool>
	{
	__always_inline static DebugFormatArgument construct(bool value)
	{
	return {static_cast<uintptr_t>(value), DebugFormatArgument::Bool};
	}
	};

	/**
	* Character specialisation, prints the character.
	*/
	template<>
	struct DebugFormatArgumentAdaptor<char>
	{
	__always_inline static DebugFormatArgument construct(char value)
	{
	return {static_cast<uintptr_t>(value), DebugFormatArgument::Character};
	}
	};

	/**
	* Unsigned character specialisation, prints the character as a hex number.
	*/
	template<>
	struct DebugFormatArgumentAdaptor<uint8_t>
	{
	__always_inline static DebugFormatArgument construct(uint8_t value)
	{
	return {static_cast<uintptr_t>(value),
	DebugFormatArgument::UnsignedNumber32};
	}
	};

	/**
	* Unsigned 16-bit integer specialisation, prints the integer as a hex number.
	*/
	template<>
	struct DebugFormatArgumentAdaptor<uint16_t>
	{
	__always_inline static DebugFormatArgument construct(uint16_t value)
	{
	return {static_cast<uintptr_t>(value),
	DebugFormatArgument::UnsignedNumber32};
	}
	};

	/**
	* Unsigned 32-bit integer specialisation, prints the integer as a hex number.
	*/
	template<>
	struct DebugFormatArgumentAdaptor<uint32_t>
	{
	__always_inline static DebugFormatArgument construct(uint32_t value)
	{
	return {static_cast<uintptr_t>(value),
	DebugFormatArgument::UnsignedNumber32};
	}
	};

	/**
	* Unsigned 64-bit integer specialisation, prints the integer as a hex number.
	*
	* All smaller sizes are handled by zero extending to 32 bits. We treat 64-bit
	* separately because it requires decomposing the two halves for printing and
	* that's redundant overhead for the majority of cases.
	*/
	template<>
	struct DebugFormatArgumentAdaptor<uint64_t>
	{
	__always_inline static DebugFormatArgument construct(uint64_t value)
	{
	uintptr_t fudgedValue;
	memcpy(&fudgedValue, &value, sizeof(fudgedValue));
	return {fudgedValue, DebugFormatArgument::UnsignedNumber64};
	}
	};

	/**
	* Signed 8-bit integer specialisations, print the integer as a decimal number.
	*/
	template<>
	struct DebugFormatArgumentAdaptor<int8_t>
	{
	__always_inline static DebugFormatArgument construct(int8_t value)
	{
	return {static_cast<uintptr_t>(value),
	DebugFormatArgument::SignedNumber32};
	}
	};

	/**
	* Signed 16-bit integer specialisations, print the integer as a decimal number.
	*/
	template<>
	struct DebugFormatArgumentAdaptor<int16_t>
	{
	__always_inline static DebugFormatArgument construct(int16_t value)
	{
	return {static_cast<uintptr_t>(value),
	DebugFormatArgument::SignedNumber32};
	}
	};

	/**
	* Signed 32-bit integer specialisations, print the integer as a decimal number.
	*/
	template<>
	struct DebugFormatArgumentAdaptor<int32_t>
	{
	__always_inline static DebugFormatArgument construct(int32_t value)
	{
	return {static_cast<uintptr_t>(value),
	DebugFormatArgument::SignedNumber32};
	}
	};

	/**
	* Signed 64-bit integer specialisations, print the integer as a decimal number.
	*
	* All smaller sizes are handled by sign extending to 32 bits. We treat 64-bit
	* separately because it requires 64-bit division to convert a 64-bit integer
	* to a decimal and that, in turn, requires libcalls.
	*/
	template<>
	struct DebugFormatArgumentAdaptor<int64_t>
	{
	__always_inline static DebugFormatArgument construct(int64_t value)
	{
	static_assert(sizeof(uintptr_t) == sizeof(uint64_t));
	uintptr_t fudgedValue;
	memcpy(&fudgedValue, &value, sizeof(fudgedValue));
	return {fudgedValue, DebugFormatArgument::SignedNumber64};
	}
	};

	/**
	* C string specialisation, prints the string as-is.
	*/
	template<>
	struct DebugFormatArgumentAdaptor<const char *>
	{
	__always_inline static DebugFormatArgument construct(const char *value)
	{
	return {reinterpret_cast<uintptr_t>(value),
	DebugFormatArgument::CString};
	}
	};

	/**
	* String view specialisation, prints the string as-is.
	*
	* Note that this relies on the string view persisting for the duration of the
	* call. It passes a pointer to the string-view argument.
	*/
	template<>
	struct DebugFormatArgumentAdaptor<std::string_view>
	{
	__always_inline static DebugFormatArgument
	construct(std::string_view &value)
	{
	return {reinterpret_cast<uintptr_t>(&value),
	DebugFormatArgument::StringView};
	}
	};

	/**
	* Enum specialisation, prints the enum as a string and then the numeric value.
	*
	* This specialisation uses the generic printing facility in the library call
	* and passes a callback that will map the enumeration to a string.
	*/
	template<DebugConcepts::IsEnum T>
	struct DebugFormatArgumentAdaptor<T>
	{
	__always_inline static DebugFormatArgument construct(T value)
	{
	#ifdef CHERIOT_AVOID_CAPRELOCS
	return {static_cast<uintptr_t>(value),
	DebugFormatArgument::UnsignedNumber32};
	#else
	return {static_cast<uintptr_t>(value),
	reinterpret_cast<uintptr_t>(&debug_enum_helper<T>)};
	#endif
	}
	};

	/**
	* Permission set specialisation.
	*/
	template<>
	struct DebugFormatArgumentAdaptor<CHERI::PermissionSet>
	{
	__always_inline static DebugFormatArgument
	construct(CHERI::PermissionSet value)
	{
	return {static_cast<uintptr_t>(value.as_raw()),
	DebugFormatArgument::PermissionSet};
	}
	};

	/**
	* Pointer specialisation, prints the pointer as a full capability.
	*/
	template<DebugConcepts::IsPointerButNotCString T>
	struct DebugFormatArgumentAdaptor<T>
	{
	__always_inline static DebugFormatArgument construct(T value)
	{
	return {reinterpret_cast<uintptr_t>(
	static_cast<const volatile void *>(value)),
	DebugFormatArgument::Pointer};
	}
	};

	/**
	* Specialisation for the CHERI capability wrapper class, prints the capability
	* in the same format as bare pointers.
	*/
	template<typename T>
	struct DebugFormatArgumentAdaptor<CHERI::Capability<T>>
	{
	__always_inline static DebugFormatArgument
	construct(CHERI::Capability<T> value)
	{
	return {reinterpret_cast<uintptr_t>(
	static_cast<const volatile void *>(value)),
	DebugFormatArgument::Pointer};
	}
	};

	/**
	* Specialisation for things that can be converted to addresses, prints as an
	* unsigned number.
	*/
	template<DebugConcepts::IsConvertibleToAddress T>
	struct DebugFormatArgumentAdaptor<T>
	{
	__always_inline static DebugFormatArgument construct(ptraddr_t value)
	{
	return {static_cast<uintptr_t>(value),

	DebugFormatArgument::UnsignedNumber32};
	}
	};

	/**
	* Recursive helper that maps from a tuple representing the arguments into a
	* type-erased array.
	*/
	template<size_t I>
	__always_inline inline void map_debug_argument(DebugFormatArgument *arguments,
	auto &&argumentTuple)
	{
	arguments[I] =
	DebugFormatArgumentAdaptor<
	std::remove_cvref_t<decltype(std::get<I>(argumentTuple))>>{}
	.construct(std::get<I>(argumentTuple));
	if constexpr (I > 0)
	{
	map_debug_argument<I - 1>(arguments, argumentTuple);
	}
	}

	/**
	* Convert `args` into a type-erased array of `DebugFormatArgument`s in
	* `arguments`.
	*/
	template<typename... Args>
	__always_inline inline void
	make_debug_arguments_list(DebugFormatArgument *arguments, Args... args)
	{
	if constexpr (sizeof...(Args) > 0)
	{
	map_debug_argument<sizeof...(Args) - 1>(arguments,
	std::forward_as_tuple(args...));
	}
	}

	/**
	* Library function that writes a debug message. This runs with interrupts
	* disabled (to avoid interleaving) and prints an array of debug messages. This
	* is intended to allow a single call to print multiple format strings without
	* requiring the format strings to be copied, so that the debugging APIs can
	* wrap a user-provided format string.
	*/
	__cheri_libcall void debug_log_message_write(const char *context,
	const char *format,
	DebugFormatArgument *messages,
	size_t messageCount);

	__cheri_libcall void debug_report_failure(const char *kind,
	const char *file,
	const char *function,
	int line,
	const char *fmt,
	DebugFormatArgument *arguments,
	size_t argumentCount);

	namespace
	{
	/**
	* Helper class wrapping a string for use as a template argument. This is
	* used to describe a context for conditional debugging that will be
	* prefixed to debug lines.
	*/
	template<size_t N>
	struct DebugContext
	{
	/**
	* Constructor, captures the string argument.
	*/
	constexpr DebugContext(const char (&str)[N])
	{
	std::copy_n(str, N, value);
	}

	/**
	* Implicit conversion to a C string.
	*/
	constexpr operator const char *() const
	{
	return value;
	}

	/**
	* The captured string. Must be public for this class to meet the
	* requirements of a structural type for use as a template argument.
	*/
	char value[N];
	};

	/**
	* Our libc++ does not currently provide source_location, but our clang
	* provides the necessary builtins for one.
	*/
	struct SourceLocation
	{
	/**
	* Explicitly construct a source location.
	*/
	constexpr SourceLocation(int lineNumber,
	int columnNumber,
	const char *fileName,
	const char *functionName)
	: lineNumber(lineNumber),
	columnNumber(columnNumber),
	fileName(fileName),
	functionName(functionName)
	{
	}

	/**
	* Construct a source location for the caller.
	*/
	static constexpr SourceLocation __always_inline
	current(int lineNumber = __builtin_LINE(),
	int columnNumber = __builtin_COLUMN(),
	const char *fileName = __builtin_FILE(),
	const char *functionName = __builtin_FUNCTION()) noexcept
	{
	return {lineNumber, columnNumber, fileName, functionName};
	}

	/// Returns the line number for this source location.
	[[nodiscard]] __always_inline constexpr int line() const noexcept
	{
	return lineNumber;
	}
	/// Returns the column number for this source location.
	[[nodiscard]] __always_inline constexpr int column() const noexcept
	{
	return columnNumber;
	}
	/// Returns the file name for this source location.
	[[nodiscard]] __always_inline constexpr const char *
	file_name() const noexcept
	{
	return fileName;
	}
	/// Returns the function name for this source location.
	[[nodiscard]] __always_inline constexpr const char *
	function_name() const noexcept
	{
	return functionName;
	}

	private:
	/// The line number of this source location.
	int lineNumber;
	/// The column number of this source location.
	int columnNumber;
	/// The file name of this source location.
	const char *fileName;
	/// The function name of this source location.
	const char *functionName;
	};

	/**
	* Conditional debug class. Used to control conditional output and
	* assertion checking. Enables debug log messages and assertions if
	* `Enabled` is true. Uses `Context` to print additional detail on debug
	* lines. Each line is prefixed with the context string in magenta to make
	* it easy to see debug output from different subsystems in the same trace.
	*
	* This class is expected to be used as a type alias, something like:
	*
	* ```c++
	* constexpr bool DebugFoo = DEBUG_FOO;
	* using Debug = ConditionalDebug<DebugFoo, "Foo">;
	* ```
	*/
	template<bool Enabled, DebugContext Context>
	class ConditionalDebug
	{
	public:
	/**
	* Log a message.
	*
	* This function does nothing if the `Enabled` condition is false.
	*/
	template<typename... Args>
	static void log(const char *fmt, Args... args)
	{
	if constexpr (Enabled)
	{
	asm volatile("" ::: "memory");
	DebugFormatArgument arguments[sizeof...(Args)];
	make_debug_arguments_list(arguments, args...);
	const char *context = Context;
	debug_log_message_write(
	context, fmt, arguments, sizeof...(Args));
	asm volatile("" ::: "memory");
	}
	}

	/**
	* Helper to report failure.
	*
	* This must not take the `SourceLocation` directly because doing so
	* prevents the compiler from decomposing and subsequently
	* constant-propagating its fields in the caller, resulting in 24 bytes
	* of stack space consumed for every assert or invariant.
	*/
	template<typename... Args>
	static inline void report_failure(const char *kind,
	const char *file,
	const char *function,
	int line,
	const char *fmt,
	Args... args)
	{
	// Ensure that the compiler does not reorder messages.
	DebugFormatArgument arguments[sizeof...(Args)];
	make_debug_arguments_list(arguments, args...);
	const char *context = Context;
	debug_report_failure(
	kind, file, function, line, fmt, arguments, sizeof...(Args));
	}

	/**
	* Function-like class for invariants that is expected to be used via
	* the deduction guide as:
	*
	* ConditionalDebug::Invariant(someCondition, "A message...", ...);
	*
	* Invariants are checked unconditionally but will log a verbose
	* message only if `Enabled` is true.
	*/
	template<typename... Args>
	struct Invariant
	{
	/**
	* Constructor, performs the invariant check.
	*/
	__always_inline
	Invariant(bool condition,
	const char *fmt,
	Args... args,
	SourceLocation loc = SourceLocation::current())
	{
	if (__predict_false(!condition))
	{
	if constexpr (Enabled)
	{
	report_failure("Invariant",
	loc.file_name(),
	loc.function_name(),
	loc.line(),
	fmt,
	std::forward<Args>(args)...);
	}
	__builtin_trap();
	}
	}
	};
	/**
	* Function-like class for assertions that is expected to be used via
	* the deduction guide as:
	*
	* ConditionalDebug::Assert(someCondition, "A message...", ...);
	*
	* Assertions are checked only if `Enabled` is true.
	*/
	template<typename... Args>
	struct Assert
	{
	/**
	* Constructor, performs the assertion check.
	*/
	template<typename T>
	requires DebugConcepts::IsBool<T> __always_inline
	Assert(T condition,
	const char *fmt,
	Args... args,
	SourceLocation loc = SourceLocation::current())
	{
	if constexpr (Enabled)
	{
	if (__predict_false(!condition))
	{
	report_failure("Assertion",
	loc.file_name(),
	loc.function_name(),
	loc.line(),
	fmt,
	std::forward<Args>(args)...);
	__builtin_trap();
	}
	}
	}

	/**
	* Constructor, performs an assertion check.
	*
	* This version is passed a lambda that returns a bool, rather than
	* a boolean condition. This allows the compiler to completely
	* elide the contents of the lambda in builds where this component
	* is not being debugged. This version should be used for places
	* where the assertion condition has side effects.
	*/
	template<typename T>
	requires DebugConcepts::LazyAssertion<T> __always_inline
	Assert(T &&condition,
	const char *fmt,
	Args... args,
	SourceLocation loc = SourceLocation::current())
	{
	if constexpr (Enabled)
	{
	if (__predict_false(!condition()))
	{
	report_failure("Assertion",
	loc.file_name(),
	loc.function_name(),
	loc.line(),
	fmt,
	std::forward<Args>(args)...);
	__builtin_trap();
	}
	}
	}
	};

	/**
	* Deduction guide, allows `Invariant` to be used as if it were a
	* function.
	*/
	template<typename T, typename... Ts>
	Invariant(T, const char *, Ts &&...) -> Invariant<Ts...>;

	/**
	* Deduction guide, allows `Assert` to be used as if it were a
	* function.
	*/
	template<typename... Ts>
	Assert(bool, const char *, Ts &&...) -> Assert<Ts...>;

	/**
	* Deduction guide, allows `Assert` to be used as if it were a
	* function with a lambda argument.
	*/
	template<typename... Ts>
	Assert(auto, const char *, Ts &&...) -> Assert<Ts...>;
	};

	} // namespace