On Mon, Jul 4, 2011 at 1:52 PM, H.J. Lu <hjl.to...@gmail.com> wrote:
> On Mon, Jul 4, 2011 at 12:57 PM, Richard Sandiford
> <rdsandif...@googlemail.com> wrote:
>> "H.J. Lu" <hongjiu...@intel.com> writes:
>>> RTL-based forward propagation pass shouldn't propagate hard register.
>>
>> That's seems a bit draconian. Many fixed hard registers ought to be OK.
>> E.g. there doesn't seem to be anything wrong with propagating uses of
>> the stack or frame pointers, subject to the usual availability checks.
>>
>> To play devil's advocate, an alternative might be to
>>
>> (a) make local_ref_killed_between_p return true for non-fixed hard
>> registers when a call or asm comes between the two instructions
>>
>> (b) make use_killed_between return true for non-fixed hard registers
>> when the instructions are in different basic blocks
>>
>> Thoughts?
>>
>
> There are a few problems with this suggestions:
>
> 1. The comments says:
>
> /* If USE is a subreg, see if it can be replaced by a pseudo. */
>
> static bool
> forward_propagate_subreg (df_ref use, rtx def_insn, rtx def_set)
> {
>
> It indicates this function is intended to work on pseudo registers.
>
> 2. propagate_rtx avoids hard registers:
>
> static rtx
> propagate_rtx (rtx x, enum machine_mode mode, rtx old_rtx, rtx new_rtx,
> bool speed)
> {
> rtx tem;
> bool collapsed;
> int flags;
>
> if (REG_P (new_rtx) && REGNO (new_rtx) < FIRST_PSEUDO_REGISTER)
> return NULL_RTX;
>
> It seems that fwprop is intended to deal with pseudo registers. If we
> want to extend it to hard registers, that should be a separate project.
>
> Thanks.
forward_propagate_subreg issue was introduced by
http://gcc.gnu.org/ml/gcc-patches/2009-08/msg01203.html
Before that, fwprop never tries to work on hard registers. Alan,
is your change to process hard registers intentional?
Thanks.
--
H.J.
