Hello,
Iâm trying to use âasm gotoâ with GCC 4.5 to implement tagged integer
arithmetic for Guileâs VM. For instance, the addition of X and Y looks
like this:
{
SCM result = y;
asm volatile goto ("add %0, %[result]; jo %l[slow_add]; "
"sub %[tag], %[result]; "
"test %[tag], %[result]; "
"je %l[slow_add]; "
"mov %[result], (%[vsp])\n"
: /* no output */
: "r" (x), [result] "r" (result),
[vsp] "r" (sp), [tag] "i" (scm_tc2_int)
: "memory"
: slow_add);
NEXT; /* Jump to the next virtual IP. */
}
slow_add:
/* Add X and Y the slow way, using bignums if needed. */
However, this doesnât work because GCC doesnât allocate any new register
for âresultâ (which it is allowed to do given that âresultâ is an
input), so the âtestâ instruction modifies âyâ, and we may jump to
âslow_addâ with an erroneous value of âyâ.
I could add a clobber for a specific register and use that to store the
intermediate result, but Iâd rather let GCC choose a register for me.
Unfortunately, only specific register names are allowed in the clobber
list, so I canât, e.g., use the ârâ constraint to achieve this.
Any idea how I can work around this?
Thanks,
Ludoâ.
