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+=============================================
+C Dialect and Translation Assumptions for Xen
+=============================================
+
+This document specifies the C language dialect used by Xen and
+the assumptions Xen makes on the translation toolchain.
+It covers, in particular:
+
+1. the used language extensions;
+2. the translation limits that the translation toolchains must be able
+   to accommodate;
+3. the implementation-defined behaviors upon which Xen may depend.
+
+All points are of course relevant for portability.  In addition,
+programming in C is impossible without a detailed knowledge of the
+implementation-defined behaviors.  For this reason, it is recommended
+that Xen developers have familiarity with this document and the
+documentation referenced therein.
+
+This document needs maintenance and adaptation in the following
+circumstances:
+
+- whenever the compiler is changed or updated;
+- whenever the use of a certain language extension is added or removed;
+- whenever code modifications cause exceeding the stated translation limits.
+
+
+Applicable C Language Standard
+______________________________
+
+Xen is written in C99 with extensions.  The relevant ISO standard is
+
+    *ISO/IEC 9899:1999/Cor 3:2007*: Programming Languages - C,
+    Technical Corrigendum 3.
+    ISO/IEC, Geneva, Switzerland, 2007.
+
+
+Reference Documentation
+_______________________
+
+The following documents are referred to in the sequel:
+
+GCC_MANUAL:
+  https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc.pdf
+CPP_MANUAL:
+  https://gcc.gnu.org/onlinedocs/gcc-12.1.0/cpp.pdf
+ARM64_ABI_MANUAL:
+  https://github.com/ARM-software/abi-aa/blob/60a8eb8c55e999d74dac5e368fc9d7e36e38dda4/aapcs64/aapcs64.rst
+X86_64_ABI_MANUAL:
+  https://gitlab.com/x86-psABIs/x86-64-ABI/-/jobs/artifacts/master/raw/x86-64-ABI/abi.pdf?job=build
+
+
+C Language Extensions
+_____________________
+
+
+The following table lists the extensions currently used in Xen.
+The table columns are as follows:
+
+   Extension
+      a terse description of the extension;
+   Architectures
+      a set of Xen architectures making use of the extension;
+   References
+      when available, references to the documentation explaining
+      the syntax and semantics of (each instance of) the extension.
+
+
+.. list-table::
+   :widths: 30 15 55
+   :header-rows: 1
+
+   * - Extension
+     - Architectures
+     - References
+
+   * - Non-standard tokens
+     - ARM64, X86_64
+     - _Static_assert:
+          see Section "2.1 C Language" of GCC_MANUAL.
+       asm, __asm__:
+          see Sections "6.48 Alternate Keywords" and "6.47 How to Use Inline Assembly Language in C Code" of GCC_MANUAL.
+       __volatile__:
+          see Sections "6.48 Alternate Keywords" and "6.47.2.1 Volatile" of GCC_MANUAL.
+       __const__, __inline__, __inline:
+          see Section "6.48 Alternate Keywords" of GCC_MANUAL.
+       typeof, __typeof__:
+          see Section "6.7 Referring to a Type with typeof" of GCC_MANUAL.
+       __alignof__, __alignof:
+          see Sections "6.48 Alternate Keywords" and "6.44 Determining the Alignment of Functions, Types or Variables" of GCC_MANUAL.
+       __attribute__:
+          see Section "6.39 Attribute Syntax" of GCC_MANUAL.
+       __builtin_types_compatible_p:
+          see Section "6.59 Other Built-in Functions Provided by GCC" of GCC_MANUAL.
+       __builtin_va_arg:
+          non-documented GCC extension.
+       __builtin_offsetof:
+          see Section "6.53 Support for offsetof" of GCC_MANUAL.
+       __signed__:
+          non-documented GCC extension.
+
+   * - Empty initialization list
+     - ARM64, X86_64
+     - Non-documented GCC extension.
+
+   * - Arithmetic operator on void type
+     - ARM64, X86_64
+     - See Section "6.24 Arithmetic on void- and Function-Pointers" of GCC_MANUAL."
+
+   * - Statements and declarations in expressions
+     - ARM64, X86_64
+     - See Section "6.1 Statements and Declarations in Expressions" of GCC_MANUAL.
+
+   * - Structure or union definition with no members
+     - ARM64, X86_64
+     - See Section "6.19 Structures with No Members" of GCC_MANUAL.
+
+   * - Zero size array type
+     - ARM64, X86_64
+     - See Section "6.18 Arrays of Length Zero" of GCC_MANUAL.
+
+   * - Binary conditional expression
+     - ARM64, X86_64
+     - See Section "6.8 Conditionals with Omitted Operands" of GCC_MANUAL.
+
+   * - 'Case' label with upper/lower values
+     - ARM64, X86_64
+     - See Section "6.30 Case Ranges" of GCC_MANUAL.
+
+   * - Unnamed field that is not a bit-field
+     - ARM64, X86_64
+     - See Section "6.63 Unnamed Structure and Union Fields" of GCC_MANUAL.
+
+   * - Empty declaration
+     - ARM64, X86_64
+     - Non-documented GCC extension.
+       Note: an empty declaration is caused by a semicolon at file scope
+       with nothing before it (not to be confused with an empty statement).
+
+   * - Incomplete enum declaration
+     - ARM64
+     - See Section "6.49 Incomplete enum Types" of GCC_MANUAL.
+
+   * - Implicit conversion from a pointer to an incompatible pointer
+     - ARM64, X86_64
+     - Non-documented GCC extension.  The documentation for option
+       -Wincompatible-pointer-types in Section
+       "3.8 Options to Request or Suppress Warnings" of GCC_MANUAL
+       is possibly relevant.
+
+   * - Pointer to a function is converted to a pointer to an object or a pointer to an object is converted to a pointer to a function
+     - X86_64
+     - Non-documented GCC extension.  The information provided in
+       https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83584
+       is possibly relevant.
+
+   * - Token pasting of ',' and __VA_ARGS__
+     - ARM64, X86_64
+     - See Section "6.21 Macros with a Variable Number of Arguments" of GCC_MANUAL.
+
+   * - No arguments for '...' parameter of variadic macro
+     - ARM64, X86_64
+     - See Section "6.21 Macros with a Variable Number of Arguments" of GCC_MANUAL.
+
+   * - void function returning void expression
+     - ARM64, X86_64
+     - See the documentation for -Wreturn-type in Section "3.8 Options to Request or Suppress Warnings" of GCC_MANUAL.
+
+   * - GNU statement expressions from macro expansion
+     - ARM64, X86_64
+     - See Section "6.1 Statements and Declarations in Expressions" of GCC_MANUAL.
+
+   * - Invalid application of sizeof to a void type
+     - ARM64, X86_64
+     - See Section "6.24 Arithmetic on void- and Function-Pointers" of GCC_MANUAL.
+
+   * - Redeclaration of already-defined enum
+     - ARM64, X86_64
+     - See Section "6.49 Incomplete enum Types" of GCC_MANUAL.
+
+   * - static function is used in an inline function with external linkage
+     - ARM64, X86_64
+     - Non-documented GCC extension. An inline function with external linkage
+       can be inlined everywhere. If that calls a static functions, which is
+       not available everywhere, it is a constraint violation according to
+       C99 6.7.4p3: "An inline definition of a function with external linkage
+       shall not contain a definition of a modifiable object with static
+       storage duration, and shall not contain a reference to an identifier
+       with internal linkage."  A standard-compliant C compiler ought
+       to diagnose all constraint violations: when it does not, as is the
+       case for GCC, the behavior is implicitly undefined.
+
+   * - struct with flexible array member nested in a struct
+     - ARM64, X86_64
+     - See Section "6.18 Arrays of Length Zero" of GCC_MANUAL.
+
+   * - struct with flexible array member used as an array element
+     - ARM64, X86_64
+     - See Section "6.18 Arrays of Length Zero" of GCC_MANUAL.
+
+   * - enumerator value outside the range of int
+     - ARM64, X86_64
+     - Non-documented GCC extension.
+
+   * - Extended integer types
+     - X86_64
+     - See Section "6.9 128-bit Integers" of GCC_MANUAL.
+
+
+Translation Limits
+__________________
+
+The following table lists the translation limits that a toolchain has
+to satisfy in order to translate Xen.  The numbers given are a
+compromise: on the one hand, many modern compilers have very generous
+limits (in several cases, the only limitation is the amount of
+available memory); on the other hand we prefer setting limits that are
+not too high, because compilers do not have any obligation of
+diagnosing when a limit has been exceeded, and not too low, so as to
+avoid frequently updating this document.  In the table, only the
+limits that go beyond the minima specified by the relevant C Standard
+are listed.
+
+The table columns are as follows:
+
+   Limit
+      a terse description of the translation limit;
+   Architectures
+      a set relevant of Xen architectures;
+   Threshold
+      a value that the Xen project does not wish to exceed for that limit
+      (this is typically below, often much below what the translation
+      toolchain supports);
+   References
+      when available, references to the documentation providing evidence
+      that the translation toolchain honors the threshold (and more).
+
+.. list-table::
+   :widths: 30 15 10 45
+   :header-rows: 1
+
+   * - Limit
+     - Architectures
+     - Threshold
+     - References
+
+   * - Size of an object
+     - ARM64, X86_64
+     - 8388608
+     - The maximum size of an object is defined in the MAX_SIZE macro, and for a 32 bit architecture is 8MB.
+       The maximum size for an array is defined in the PTRDIFF_MAX and in a 32 bit architecture is 2^30-1.
+       See occurrences of these macros in GCC_MANUAL.
+
+   * - Characters in one logical source line
+     - ARM64
+     - 5000
+     - See Section "11.2 Implementation limits" of CPP_MANUAL.
+
+   * - Characters in one logical source line
+     - X86_64
+     - 12000
+     - See Section "11.2 Implementation limits" of CPP_MANUAL.
+
+   * - Nesting levels for #include files
+     - ARM64
+     - 24
+     - See Section "11.2 Implementation limits" of CPP_MANUAL.
+
+   * - Nesting levels for #include files
+     - X86_64
+     - 32
+     - See Section "11.2 Implementation limits" of CPP_MANUAL.
+
+   * - case labels for a switch statement (excluding those for any nested switch statements)
+     - X86_64
+     - 1500
+     - See Section "4.12 Statements" of GCC_MANUAL.
+
+   * - Number of significant initial characters in an external identifier
+     - ARM64, X86_64
+     - 63
+     - See Section "4.3 Identifiers" of GCC_MANUAL.
+
+
+Implementation-Defined Behaviors
+________________________________
+
+The following table lists the C language implementation-defined behaviors
+relevant for MISRA C:2012 Dir 1.1 upon which Xen may possibly depend.
+
+The table columns are as follows:
+
+   I.-D.B.
+      a terse description of the implementation-defined behavior;
+   Architectures
+      a set relevant of Xen architectures;
+   Value(s)
+      for i.-d.b.'s with values, the values allowed;
+   References
+      when available, references to the documentation providing details
+      about how the i.-d.b. is resolved by the translation toolchain.
+
+.. list-table::
+   :widths: 30 15 10 45
+   :header-rows: 1
+
+   * - I.-D.B.
+     - Architectures
+     - Value(s)
+     - References
+
+   * - Allowable bit-field types other than _Bool, signed int, and unsigned int
+     - ARM64, X86_64
+     - All explicitly signed integer types, all unsigned integer types,
+       and enumerations.
+     - See Section "4.9 Structures, Unions, Enumerations, and Bit-Fields".
+
+   * - #pragma preprocessing directive that is documented as causing translation failure or some other form of undefined behavior is encountered
+     - ARM64, X86_64
+     - pack, GCC visibility
+     - #pragma pack:
+          see Section "6.62.11 Structure-Layout Pragmas" of GCC_MANUAL.
+       #pragma GCC visibility:
+          see Section "6.62.14 Visibility Pragmas" of GCC_MANUAL.
+
+   * - The number of bits in a byte
+     - ARM64
+     - 8
+     - See Section "4.4 Characters" of GCC_MANUAL and Section "8.1 Data types" of ARM64_ABI_MANUAL.
+
+   * - The number of bits in a byte
+     - X86_64
+     - 8
+     - See Section "4.4 Characters" of GCC_MANUAL and Section "3.1.2 Data Representation" of X86_64_ABI_MANUAL.
+
+   * - Whether signed integer types are represented using sign and magnitude, two's complement, or one's complement, and whether the extraordinary value is a trap representation or an ordinary value
+     - ARM64, X86_64
+     - Two's complement
+     - See Section "4.5 Integers" of GCC_MANUAL.
+
+   * - Any extended integer types that exist in the implementation
+     - X86_64
+     - __uint128_t
+     - See Section "6.9 128-bit Integers" of GCC_MANUAL.
+
+   * - The number, order, and encoding of bytes in any object
+     - ARM64
+     -
+     - See Section "4.15 Architecture" of GCC_MANUAL and Chapter 5 "Data types and alignment" of ARM64_ABI_MANUAL.
+
+   * - The number, order, and encoding of bytes in any object
+     - X86_64
+     -
+     - See Section "4.15 Architecture" of GCC_MANUAL and Section "3.1.2 Data Representation" of X86_64_ABI_MANUAL.
+
+   * - Whether a bit-field can straddle a storage-unit boundary
+     - ARM64
+     -
+     - See Section "4.9 Structures, Unions, Enumerations, and Bit-Fields of GCC_MANUAL and Section "8.1.8 Bit-fields" of ARM64_ABI_MANUAL.
+
+   * - Whether a bit-field can straddle a storage-unit boundary
+     - X86_64
+     -
+     - See Section "4.9 Structures, Unions, Enumerations, and Bit-Fields" of GCC_MANUAL and Section "3.1.2 Data Representation" of X86_64_ABI_MANUAL.
+
+   * - The order of allocation of bit-fields within a unit
+     - ARM64
+     -
+     - See Section "4.9 Structures, Unions, Enumerations, and Bit-Fields of GCC_MANUAL and Section "8.1.8 Bit-fields" of ARM64_ABI_MANUAL.
+
+   * - The order of allocation of bit-fields within a unit
+     - X86_64
+     -
+     - See Section "4.9 Structures, Unions, Enumerations, and Bit-Fields" of GCC_MANUAL and Section "3.1.2 Data Representation" of X86_64_ABI_MANUAL.
+
+   * - What constitutes an access to an object that has volatile-qualified type
+     - ARM64, X86_64
+     -
+     - See Section "4.10 Qualifiers" of GCC_MANUAL.
+
+   * - The values or expressions assigned to the macros specified in the headers <float.h>, <limits.h>, and <stdint.h>
+     - ARM64
+     -
+     - See Section "4.15 Architecture" of GCC_MANUAL and Chapter 5 "Data types and alignment" of ARM64_ABI_MANUAL.
+
+   * - The values or expressions assigned to the macros specified in the headers <float.h>, <limits.h>, and <stdint.h>
+     - X86_64
+     -
+     - See Section "4.15 Architecture" of GCC_MANUAL and Section "3.1.2 Data Representation" of X86_64_ABI_MANUAL.
+
+   * - Character not in the basic source character set is encountered in a source file, except in an identifier, a character constant, a string literal, a header name, a comment, or a preprocessing token that is never converted to a token
+     - ARM64
+     - UTF-8
+     - See Section "1.1 Character sets" of CPP_MANUAL.
+       We assume the locale is not restricting any UTF-8 characters being part of the source character set.
+
+   * - The value of a char object into which has been stored any character other than a member of the basic execution character set
+     - ARM64
+     -
+     - See Section "4.4 Characters" of GCC_MANUAL and Section "8.1 Data types" of ARM64_ABI_MANUAL.
+
+   * - The value of a char object into which has been stored any character other than a member of the basic execution character set
+     - X86_64
+     -
+     - See Section "4.4 Characters" of GCC_MANUAL and Section "3.1.2 Data Representation" of X86_64_ABI_MANUAL.
+
+   * - The value of an integer character constant containing more than one character or containing a character or escape sequence that does not map to a single-byte execution character
+     - ARM64
+     -
+     - See Section "4.4 Characters" of GCC_MANUAL and Section "8.1 Data types" of ARM64_ABI_MANUAL.
+
+   * - The value of an integer character constant containing more than one character or containing a character or escape sequence that does not map to a single-byte execution character
+     - X86_64
+     -
+     - See Section "4.4 Characters" of GCC_MANUAL and Section "3.1.2 Data Representation" of X86_64_ABI_MANUAL.
+
+   * - The mapping of members of the source character set
+     - ARM64, X86_64
+     -
+     - See Section "4.4 Characters" of GCC_MANUAL and the documentation for -finput-charset=charset in the same manual.
+
+   * - The members of the source and execution character sets, except as explicitly specified in the Standard
+     - ARM64, X86_64
+     - UTF-8
+     - See Section "4.4 Characters" of GCC_MANUAL
+
+   * - The values of the members of the execution character set
+     - ARM64, X86_64
+     -
+     - See Section "4.4 Characters" of GCC_MANUAL and the documentation for -fexec-charset=charset in the same manual.
+
+   * - How a diagnostic is identified
+     - ARM64, X86_64
+     -
+     - See Section "4.1 Translation" of GCC_MANUAL.
+
+   * - The places that are searched for an included < > delimited header, and how the places are specified or the header is identified
+     - ARM64, X86_64
+     -
+     - See Chapter "2 Header Files" of CPP_MANUAL.
+
+   * - How the named source file is searched for in an included " " delimited header
+     - ARM64, X86_64
+     -
+     - See Chapter "2 Header Files" of CPP_MANUAL.
+
+   * - How sequences in both forms of header names are mapped to headers or external source file names
+     - ARM64, X86_64
+     -
+     - See Chapter "2 Header Files" of CPP_MANUAL.
+
+   * - Whether the # operator inserts a \ character before the \ character that begins a universal character name in a character constant or string literal
+     - ARM64, X86_64
+     -
+     - See Section "3.4 Stringizing" of CPP_MANUAL.
+
+   * - The current locale used to convert a wide string literal into corresponding wide character codes
+     - ARM64, X86_64
+     -
+     - See Section "4.4 Characters" of GCC_MANUAL and Section "11.1 Implementation-defined behavior" of CPP_MANUAL.
+
+   * - The value of a string literal containing a multibyte character or escape sequence not represented in the execution character set
+     - X86_64
+     -
+     - See Section "4.4 Characters" of GCC_MANUAL and Section "11.1 Implementation-defined behavior" of CPP_MANUAL.
+
+   * - The behavior on each recognized #pragma directive
+     - ARM64, X86_64
+     - pack, GCC visibility
+     - See Section "4.13 Preprocessing Directives" of GCC_MANUAL and Section "7 Pragmas" of CPP_MANUAL.
+
+   * - The method by which preprocessing tokens (possibly resulting from macro expansion) in a #include directive are combined into a header name
+     - X86_64
+     -
+     - See Section "4.13 Preprocessing Directives" of GCC_MANUAL and Section "11.1 Implementation-defined behavior" of CPP_MANUAL.
+
+
+END OF DOCUMENT.
