Hi,
Ian Lance Taylor <iant@xxxxxxxxxx> writes:
> ludo@xxxxxxx (Ludovic CourtÃs) writes:
>
>> 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?
>
> When your asm modifies a register, you need to tell gcc that that is
> happening. E.g., list the value in the list of outputs and give it a +
> constraint.
That doesnât work because âasm gotoâ doesnât support output variables as
of GCC 4.5 (info "(gcc) Extended Asm").
Thanks,
Ludoâ.
