On 01/09/2011 01:53 PM, Aurelien Jarno wrote:
>> + if (inout == val) {
>> + TCGType type = rexw ? TCG_TYPE_I64 : TCG_TYPE_I32;
>> + TCGRegSet inuse = s->reserved_regs;
>> +
>> + tcg_regset_set_reg(inuse, inout);
>> + val = tcg_reg_alloc(s, tcg_target_available_regs[type], inuse);
>> +
>> + tcg_out_mov(s, type, val, inout);
>
> I am a bit worried by allocating a new register here, especially on the
> i386 target, where the number of free registers is quite low, and often
> 0. We already had to tweak some code to avoid calls to tcg_abort() due
> to missing registers.
Well, as I said, this case can't actually trigger due to a bug in the
register allocator. This can be seen in an insn like
mov %dl,%dh
where you would expect to see
deposit x,x,x,8,8
however, the matching constraint forces the destination and the matching
source into a new register:
/* if the input is aliased to an output and if it is
not dead after the instruction, we must allocate
a new register and move it */
if (!IS_DEAD_IARG(i - nb_oargs))
goto allocate_in_reg;
which means that we'll always see
mov y,x
deposit y,y,x,8,8
So I could simply put a tcg_abort there. It would be up to whoever
improves the register allocator to provide some mechanism for a
backend to allocate a scratch. What do you think?
r~
