Re: [RFC] temporary Lisp_Strings

[Dmitry Antipov]

On 09/02/2014 05:21 PM, Andreas Schwab wrote:

> There is none.  The stack could be in a completely different address
> region that may not be suitable in non-USE_LSB_TAG configurations.

IIUC non-USE_LSB_TAG configurations are not widely used, so this is
a tier-2 problem.

> > +#define alloca_string(str)                                     \
> > +  ({ Lisp_Object string;                                       \
> > +     struct Lisp_String *s;                                    \
> > +     ptrdiff_t nchars, nbytes, size = strlen (str);            \
> > +     parse_str_as_multibyte ((const unsigned char *) str,      \
> > +                            size, &nchars, &nbytes);           \
> > +     s = alloca (sizeof *s + nbytes + 1);                      \
> > +     s->data = (unsigned char *) s + sizeof (*s);           \
> > +     memcpy (s->data, str, nbytes);                         \
> > +     s->data[size] = '\0';                                  \
> > +     s->intervals = NULL;                                   \
> > +     if (nbytes == nchars || nbytes != size)                   \
> > +       s->size = size, s->size_byte = -1;                        \
> > +     else                                                      \
> > +       s->size = nchars, s->size_byte = nbytes;          \
> > +     XSETSTRING (string, s); string; })
>
> This will eventually fail with USE_LSB_TAG, since you don't enforce
> alignment.

What about alignment guaranteed by alloca?  IIUC compilers tends
to align it enough to US_LSB_TAG:

http://msdn.microsoft.com/en-us/library/x9sx5da1.aspx
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55945

Dmitry