On Sun, Jan 09, 2011 at 02:55:13PM -0800, Richard Henderson wrote:
> 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;
I have not been able to trigger this code path with a deposit
instruction.
> 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?
>
Do you have a way to trigger this problem? or a dump of the ops and asm
output?
--
Aurelien Jarno GPG: 1024D/F1BCDB73
aurel...@aurel32.net http://www.aurel32.net
