Re: [PATCH] ira: Skip some pseudos in move_unallocated_pseudos

[Jeff Law <law@redhat.com>]

On 12/22/20 11:40 PM, Kewen.Lin via Gcc-patches wrote:
> Hi Segher,
>
> on 2020/12/22 下午9:55, Segher Boessenkool wrote:
>> Hi!
>>
>> Just a dumb formatting comment:
>>
>> On Tue, Dec 22, 2020 at 04:05:39PM +0800, Kewen.Lin wrote:
>>> This patch is to make move_unallocated_pseudos consistent
>>> to what we have in function find_moveable_pseudos, where we
>>> record the original pseudo into pseudo_replaced_reg only if
>>> validate_change succeeds with newreg.  To ensure every
>>> unallocated pseudo in move_unallocated_pseudos has expected
>>> information, it's better to add a check and skip it if it's
>>> unexpected.  This avoids possible ICEs in future.
>>>
>>> btw, I happened to found this in the bootstrapping for one
>>> experimental local patch, which is considered as impractical.
>>> --- a/gcc/ira.c
>>> +++ b/gcc/ira.c
>>> @@ -5111,6 +5111,11 @@ move_unallocated_pseudos (void)
>>>        {
>>>  	int idx = i - first_moveable_pseudo;
>>>  	rtx other_reg = pseudo_replaced_reg[idx];
>>> +	/* If there is no appropriate pseudo in pseudo_replaced_reg, it
>>> +	   means validate_change fails for this new pseudo in function
>>> +	   find_moveable_pseudos, then bypass it here.*/
>> Dot space space.
> Good catch, thanks!  I forgot to reformat after polishing the comments.
> Will fix it with other potential comments.
>
>> The patch sounds fine to me.  Hard to tell without seeing the patch that
>> exposed the problem (for onlookers like me who do not know this code
>> well, anyway ;-) )
> The patch which made this issue exposed looks like:
>
> +; Like *rotl<mode>3_insert_3 but work with nonzero_bits rather than
> +; explicit AND.
> +(define_insn "*rotl<mode>3_insert_8"
> +  [(set (match_operand:GPR 0 "gpc_reg_operand" "=r")
> +        (ior:GPR (ashift:GPR (match_operand:GPR 1 "gpc_reg_operand" "r")
> +                             (match_operand:SI 2 "u6bit_cint_operand" "n"))
> +                 (match_operand:GPR 3 "gpc_reg_operand" "0")))]
> +  "HOST_WIDE_INT_1U << INTVAL (operands[2])
> +   > nonzero_bits (operands[3], <MODE>mode)"
> +{
> +  if (<MODE>mode == SImode)
> +    return "rlwimi %0,%1,%h2,0,31-%h2";
> +  else
> +    return "rldimi %0,%1,%H2,0";
> +}
> +  [(set_attr "type" "insert")])
>
> Some insn matches this pattern in combine, later ira tries to introduce
> one new pseudo since it meets the checks in find_moveable_pseudos, but
> it fails in the call to validate_change since the nonzero_bits is more
> rough and can't satisfy the pattern condition, leaving the unexpected
> entry in pseudo_replaced_reg.
But what doesn't make any sense to me is pseudo_replaced_reg[] is only
set when validation is successful in find_moveable_pseudos.   So I can't
see how this patch actually helps the problem you're describing.

jeff