G++ internals - Mangling

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## Function name mangling for C++ and Java

Both C++ and Jave provide overloaded function and methods, which are methods with the same types but different parameter lists. Selecting the correct version is done at compile time. Though the overloaded functions have the same name in the source code, they need to be translated into different assembler-level names, since typical assemblers and linkers cannot handle overloading. This process of encoding the parameter types with the method name into a unique name is called name mangling. The inverse process is called demangling.

It is convenient that C++ and Java use compatible mangling schemes, since the makes life easier for tools such as gdb, and it eases integration between C++ and Java.

Note there is also a standard "Jave Native Interface" (JNI) which implements a different calling convention, and uses a different mangling scheme. The JNI is a rather abstract ABI so Java can call methods written in C or C++; we are concerned here about a lower-level interface primarily intended for methods written in Java, but that can also be used for C++ (and less easily C).

### Method name mangling

C++ mangles a method by emitting the function name, followed by `__`, followed by encodings of any method qualifiers (such as `const`), followed by the mangling of the method's class, followed by the mangling of the parameters, in order.

For example `Foo::bar(int, long) const` is mangled as ``bar__C3Fooil'`.

For a constructor, the method name is left out. That is `Foo::Foo(int, long) const` is mangled as ``__C3Fooil'`.

GNU Java does the same.

### Primitive types

The C++ types `int`, `long`, `short`, `char`, and `long long` are mangled as ``i'`, ``l'`, ``s'`, ``c'`, and ``x'`, respectively. The corresponding unsigned types have ``U'` prefixed to the mangling. The type `signed char` is mangled ``Sc'`.

The C++ and Java floating-point types `float` and `double` are mangled as ``f'` and ``d'` respectively.

The C++ `bool` type and the Java `boolean` type are mangled as ``b'`.

The C++ `wchar_t` and the Java `char` types are mangled as ``w'`.

The Java integral types `byte`, `short`, `int` and `long` are mangled as ``c'`, ``s'`, ``i'`, and ``x'`, respectively.

C++ code that has included `javatypes.h` will mangle the typedefs `jbyte`, `jshort`, `jint` and `jlong` as respectively ``c'`, ``s'`, ``i'`, and ``x'`. (This has not been implemented yet.)

### Mangling of simple names

A simple class, package, template, or namespace name is encoded as the number of characters in the name, followed by the actual characters. Thus the class `Foo` is encoded as ``3Foo'`.

If any of the characters in the name are not alphanumeric (i.e not one of the standard ASCII letters, digits, or '_'), or the initial character is a digit, then the name is mangled as a sequence of encoded Unicode letters. A Unicode encoding starts with a ``U'` to indicate that Unicode escapes are used, followed by the number of bytes used by the Unicode encoding, followed by the bytes representing the encoding. ASSCI letters and non-initial digits are encoded without change. However, all other characters (including underscore and initial digits) are translated into a sequence starting with an underscore, followed by the big-endian 4-hex-digit lower-case encoding of the character.

If a method name contains Unicode-escaped characters, the entire mangled method name is followed by a ``U'`.

For example, the method `X\u0319::M\u002B(int)` is encoded as ``M_002b__U6X_0319iU'`.

### Pointer and reference types

A C++ pointer type is mangled as ``P'` followed by the mangling of the type pointed to.

A C++ reference type as mangled as ``R'` followed by the mangling of the type referenced.

A Java object reference type is equivalent to a C++ pointer parameter, so we mangle such an parameter type as ``P'` followed by the mangling of the class name.

### Qualified names

Both C++ and Java allow a class to be lexically nested inside another class. C++ also supports namespaces (not yet implemented by G++). Java also supports packages.

These are all mangled the same way: First the letter ``Q'` indicates that we are emitting a qualified name. That is followed by the number of parts in the qualified name. If that number is 9 or less, it is emitted with no delimiters. Otherwise, an underscore is written before and after the count. Then follows each part of the qualified name, as described above.

For example `Foo::\u0319::Bar` is encoded as ``Q33FooU5_03193Bar'`.

### Templates

A class template instantiation is encoded as the letter ``t'`, followed by the encoding of the template name, followed the number of template parameters, followed by encoding of the template parameters. If a template parameter is a type, it is written as a ``Z'` followed by the encoding of the type.

A function template specialization (either an instantiation or an explicit specialization) is encoded by an ``H'` followed by the encoding of the template parameters, as described above, followed by an ``_'`, the encoding of the argument types template function (not the specialization), another ``_'`, and the return type. (Like the argument types, the return type is the return type of the function template, not the specialization.) Template parameters in the argument and return types are encoded by an ``X'` for type parameters, or a ``Y'` for constant parameters, and an index indicating their position in the template parameter list declaration.

### Arrays

C++ array types are mangled by emitting ``A'`, followed by the length of the array, followed by an ``_'`, followed by the mangling of the element type. Of course, normally array parameter types decay into a pointer types, so you don't see this.

Java arrays are objects. A Java type `T[]` is mangled as if it were the C++ type `JArray<T>`. For example `java.lang.String[]` is encoded as ``Pt6JArray1ZPQ34java4lang6String'`.

### Table of demangling code characters

The following special characters are used in mangling:

``A'`
  Indicates a C++ array type.
``b'`
  Encodes the C++ `bool` type, and the Java `boolean` type.
``c'`
  Encodes the C++ `char` type, and the Java `byte` type.
``C'`
  A modifier to indicate a `const` type. Also used to indicate a `const` member function (in which cases it precedes the encoding of the method's class).
``d'`
  Encodes the C++ and Java `double` types.
``e'`
  Indicates extra unknown arguments `...`.
``f'`
  Encodes the C++ and Java `float` types.
``F'`
  Used to indicate a function type.
``H'`
  Used to indicate a template function.
``i'`
  Encodes the C++ and Java `int` types.
``J'`
  Indicates a complex type.
``l'`
  Encodes the C++ `long` type.
``P'`
  Indicates a pointer type. Followed by the type pointed to.
``Q'`
  Used to mangle qualified names, which arise from nested classes. Should also be used for namespaces (?). In Java used to mangle package-qualified names, and inner classes.
``r'`
  Encodes the GNU C++ `long double` type.
``R'`
  Indicates a reference type. Followed by the referenced type.
``s'`
  Encodes the C++ and java `short` types.
``S'`
  A modifier that indicates that the following integer type is signed. Only used with `char`. Also used as a modifier to indicate a static member function.
``t'`
  Indicates a template instantiation.
``T'`
  A back reference to a previously seen type.
``U'`
  A modifier that indicates that the following integer type is unsigned. Also used to indicate that the following class or namespace name is encoded using Unicode-mangling.
``v'`
  Encodes the C++ and Java `void` types.
``V'`
  A modified for a `const` type or method.
``w'`
  Encodes the C++ `wchar_t` type, and the Java `char` types.
``x'`
  Encodes the GNU C++ `long long` type, and the Java `long` type.
``X'`
  Encodes a template type parameter, when part of a function type.
``Y'`
  Encodes a template constant parameter, when part of a function type.
``Z'`
  Used for template type parameters.

The letters ``G'`, ``M'`, ``O'`, and ``p'` also seem to be used for obscure purposes ...

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