Re: Inline assembly and clobbered registers

[Ian Lance Taylor <iant at google dot com>]

ludo@gnu.org (Ludovic CourtÃs) writes:

> Ian Lance Taylor <iant@google.com> writes:
>
>> ludo@gnu.org (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.

Ian