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HISTORY.md

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The following is a summarized change history of the C compiler evolution as seen in the files here.

General Comments

  • Assignment operators are written as =+ instead of +=, etc.
  • No hexadecimal integers allowed in the earliest versions.
  • Maximum 8 bytes per symbol name.
  • Multiple ASCII characters can be combined into a single constant, e.g. 'xy'. This allows packing two 8-bit characters into one 16-bit word.
  • Machine-specific code is freely mixed by the machine independent code. Some knowledge of PDP-11 assembler is helpful.
  • Line numbers are tracked during scanning for use in error messages.
  • Keep in mind that early PDP-11s have a maximum of 56KB RAM for the kernel and user code. The earliest systems may have even less. Later, split I+D space would essentially double this.

V2 UNIX

  • Runs on the PDP-11/20 without memory mapping hardware.
  • Trees are allocated from the first 500 bytes of the program area, overwriting its initialization code.
  • Type keywords: char, int, float, double
  • Storage keywords: auto, extern, static
  • Control flow keywords: goto, return, if, while, else, switch, case, break, continue, do, default.

V3 UNIX

  • Trees are now allocated starting from address 0, indicating the availability of memory mapping hardware.
  • Fixed off-by-one error in detecting symbol table overflow.
  • Added default initialization to zero, when an initial value is not given. This saves a couple of bytes per data declaration that can be zero-initialized.
  • Changed the hashing logic for switch statements; pssiz increased from 8 to 9.
  • Added the struct keyword, replacing a commented-out slot for long. Added the . and -> operators. Note the compiler itself is not using struct yet, hence this is the "prestruct-c" branch. Struct sizes are always rounded up to an even/word boundary. Struct tags are in the same namespace as everything else; x->a is essentially just turned into x[a], replacing the name with an offset.
  • error() argument count expanded from 2 to 6, to allow more context to be written out.
  • Added many more calls to error().
  • Added debug flag that, when set, dumps the symbol table at the end of every declaration. This can be set at compile time.
  • Support signed decimal constants; e.g. 0177776 was changed to -2.
  • Removed the 8th bit from some characters before passing to putchar().
  • Added the bitwise complement operator ~.
  • Added the register keyword. A maximum of 3 register variables per function are supported (r2, r3, and r4). Exceeding that generates a compiler error. r0 and r1 are always reserved by the implementation for temporaries; if register variables are not defined, it may use r2-r4 also. r5-r7 are system defined, for the frame pointer, stack pointer, and program counter.
  • Use double within the compiler itself, when working with floating-point constants.
  • Code generator routine printn() split into two, printo() for octals and printd() for decimals. Each specialization is more optimized than the general version where the base is a variable, at the cost of slightly more code.
  • Added "%c" formatter to printf().
  • Moved scanning of numbers into an assembler routine, getnum().
  • Implicit function declaration allowed. It is no longer required to declare "extern foo;" for a function foo().

V5 UNIX

  • Note that the compiler itself is using struct now.
  • Moved tree allocation onto the program stack. (Memory mapping hardware will cause a trap when the stack needs to be expanded.)
  • Buffered I/O functions, similar to stdio, used instead of system calls to read/write files. For example, creat becomes fcreat, and open becomes fopen. The FILE object is not explicitly present; instead, 518 byte buffers are used. Presumably the first few words are equivalent to FILE, followed by a data buffer.
  • Signed octal constants supported, e.g. -0777.
  • Added sizeof keyword. This is evaluated early during parsing and replaced with an integer constant.
  • Added && and || operators, different from & and |.
  • Keyword initialization rewritten to use a keyword table instead of init() calls.
  • Return types added to many functions that return pointers.
  • Many constants replaced by preprocessor names. The preprocessor itself used at this point is not included, but it seems to only support simple substitutions via #define.
  • Support \0 for the NUL character; used by the compiler itself.
  • Support \xxx octal character constants.
  • Added the for() statement.
  • Defer code generation for switch statements until pass 2; add various strategies to evalulate them.
  • Utilize the PDP-11 "nand" instruction.
  • Code optimizer uses alloc() for heap allocation, using sbrk.
  • Added a third pass to optimize the code generated in pass 2. This re-reads the assembler code, one function at a time, and performs peephole optimization.

V7 UNIX

  • Added keywords: long, unsigned, union, short, typedef, enum. This essentially completes what would become known as K&R C. The rumored "entry" keyword is not present.
  • When register specified but no registers available, fall back to "auto" storage.
  • FILE pointers are used for writing to stdout and stderr. The buffering is now transparent.