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

["Kewen.Lin" <linkw@linux.ibm.com>]

Hi Jeff,

on 2021/1/5 上午7:13, Jeff Law wrote:
> 
> 
> 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.
> 

Yeah, pseudo_replaced_reg[] is only set when validation is successful,
but we bump the max pseudo number in ira_create_new_reg as below
regardless of whether validation succeeds or not:

	  rtx newreg = ira_create_new_reg (def_reg);
	  if (validate_change (def_insn, DF_REF_REAL_LOC (def), newreg, 0))

Later in move_unallocated_pseudos, the iterating could cover those
pseudos which were created but not used due to failed validation.

  for (i = first_moveable_pseudo; i < last_moveable_pseudo; i++)
    if (reg_renumber[i] < 0)
      {
	int idx = i - first_moveable_pseudo;
	rtx other_reg = pseudo_replaced_reg[idx];                // (1)
	rtx_insn *def_insn = DF_REF_INSN (DF_REG_DEF_CHAIN (i));
	/* The use must follow all definitions of OTHER_REG, so we can
	   insert the new definition immediately after any of them.  */
	df_ref other_def = DF_REG_DEF_CHAIN (REGNO (other_reg))

Then we can get the NULL other_reg in (1), also have unexpected df info
which causes ICE.  The patch skips the handlings on those pseudos which
were intended to be used in validatation INSN but failed to.

BR,
Kewen