Re: [RFC] temporary Lisp_Strings

[Dmitry Antipov <dmantipov@yandex.ru>]

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