C--

C-- is a "portable assembly language", designed to ease the implementation of compilers that produce high-quality machine code. This is done by delegating low-level code-generation and program optimization to a C-- compiler. The language's syntax borrows heavily from C while omitting or changing standard C features such as variadic functions, pointer syntax, and aspects of C's type system, because they hamper essential features of C-- and ease of code-generation.

The name of the language is an in-joke, indicating that C-- is a reduced form of C, in the same way that "C++" was chosen to connote an improved version of C. (In C, -- and ++ mean "decrement" and "increment," respectively.)

Work on C-- began in the late 1990s. Since writing a custom code generator is a challenge in itself, and the compiler backends available to researchers at that time were complex and poorly documented, several projects had written compilers which generated C code (for instance, the original Modula-3 compiler). However, C is a poor choice for functional languages: it does not guarantee tail-call optimization, or support accurate garbage collection or efficient exception handling. C-- is a tightly-defined simpler alternative to C which supports all of these. Its most innovative feature is a run-time interface which allows writing of portable garbage collectors, exception handling systems and other run-time features which work with any C-- compiler.

The first version of C-- was released in April 1998 as a MSRA paper,[1] accompanied by a January 1999 paper on garbage collection.[2] A revised manual was posted in HTML form in May 1999.[6] Two sets of major changes proposed in 2000 by Norman Ramsey ("Proposed Changes") and Christian Lindig ("A New Grammar") led to C-- version 2, which was finalized around 2004 and officially released in 2005.[3]

The C-- type system is designed to reflect constraints imposed by hardware rather than conventions imposed by higher-level languages. A value stored in a register or memory may have only one type: bit-vector. However, bit-vector is a polymorphic type which comes in several widths, e.g. bits8, bits32, or bits64. A separate 32-or-64 bit family of floating-point types is supported. In addition to the bit-vector type, C-- provides a boolean type bool, which can be computed by expressions and used for control flow but cannot be stored in a register or memory. As in an assembly language, any higher type discipline, such as distinctions between signed, unsigned, float, and pointer, is imposed by the C-- operators or other syntactic constructs. C-- is not type-checked, nor does it enforce or check the calling convention.[3]^: 28

C-- version 2 removes the distinction between bit-vector and floating-point types. These types can be annotated with a string "kind" tag to distinguish, among other things, a variable's integer vs float typing and its storage behavior (global or local). The former is useful on targets that have separate registers for integer and floating-point values. Special types for pointers and the native word were introduced, although they are mapped to a bit-vector with a target-dependent length.[3]^: 10

The following C-- code calculates the sum and product of integers 1 through n[7] (n is received as an argument). It demonstrates two language features:

• Procedures can return multiple results.
• Tail recursion is explicitly requested with the "jump" keyword.

/* Tail recursion */
export sp2;
sp2( bits32 n ) {
  jump sp2_help( n, 1, 1 );
}

sp2_help( bits32 n, bits32 s, bits32 p ) {
  if n==1 {
      return( s, p );
  } else {
      jump sp2_help( n-1, s+n, p*n );
  }
}

The specification page of C-- lists a few implementations of C--. The "most actively developed" compiler, Quick C--, was abandoned in 2013.[8]

• BCPL
• LLVM

• Archive of old official website (cminusminus.org)
• Quick C-- code archive (the reference implementation)