Hello,
Ian Lance Taylor <iant@xxxxxxxxxx> writes:
> ludo@xxxxxxx (Ludovic CourtÃs) writes:
>
>> 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").
>
> Whoops, sorry about that. That should really be fixed.
>
> Still, your asm simply can not change registers and not tell gcc about
> it. That is certain to break. I think you're going to have to force
> the use of an explicit register or use a regular asm rather than an asm
> goto.
Yes, thatâs why Iâm asking for ideas to work around this. :-)
Namely, clobbering a specific register doesnât sound wise here since
this asm snippet will be part of the bytecode interpreter, where GCCâs
register allocator can supposedly do a better job if I donât make its
life harder by clobbering an arbitrarily-chosen register.
Likewise, using âasmâ instead of âasm gotoâ doesnât seem doable, since
the cost would outweigh the benefits AFAICS (e.g., one of the output
variables would have to be a boolean indicating whether an overflow
occurred...).
Thanks,
Ludoâ.
