Re: immediate strings #2

[Paul Eggert <eggert@cs.ucla.edu>]

On 11/28/11 09:33, Stefan Monnier wrote:
> You need to move the immbit and gcmarkbit
> into the union :-(

Yes, something like this perhaps?  It also adds a bit of checking for
underlying assumptions about word size.

#include <verify.h>

#define IMMEDIATE_STRING_LENGTH_BITS 7

struct Data_Lisp_String
  {
    unsigned int immediate_bit : 1;
    signed int : IMMEDIATE_STRING_LENGTH_BITS; /* padding for immediate size */
    unsigned int gcmarkbit : 1;
    signed int : IMMEDIATE_STRING_LENGTH_BITS; /* and for immediate size_byte */
    INTERVAL intervals;
    ptrdiff_t size;
    ptrdiff_t size_byte;
    unsigned char *data;
  };

#define IMMEDIATE_STRING_SIZE \
  (sizeof (struct Data_Lisp_String) - offsetof (struct Data_Lisp_String, size))
verify (IMMEDIATE_STRING_SIZE <= 1 << (IMMEDIATE_STRING_LENGTH_BITS - 1));

struct Immediate_Lisp_String
  {
    unsigned int immediate_bit : 1;
    signed int size : IMMEDIATE_STRING_LENGTH_BITS;
    unsigned int gcmarkbit : 1;
    signed int size_byte : IMMEDIATE_STRING_LENGTH_BITS;
    INTERVAL intervals;
    unsigned char data[IMMEDIATE_STRING_SIZE];
  };

union Lisp_String
  {
    /* When IMMEDIATE.IMMEDIATE_BIT is 0.  */
    struct Data_Lisp_String data;

    /* When IMMEDIATE.IMMEDIATE_BIT is 1. */
    struct Immediate_Lisp_String immediate;
  };