Re: [PATCH v3 1/3] rtl: directly handle MEM in gen_highpart [PR102125]

[Richard Earnshaw <Richard.Earnshaw@foss.arm.com>]

On 13/09/2021 10:38, Richard Sandiford via Gcc-patches wrote:
> Richard Earnshaw via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>> gen_lowpart_general handles forming a lowpart of a MEM by using
>> adjust_address to rework and validate a new version of the MEM.
>> Do the same for gen_highpart rather than calling simplify_gen_subreg
>> for this case.
> 
> Looks OK, but what went wrong with the existing code?  Did
> simplify_gen_subreg refuse to handle a MEM that you wanted
> it to handle, or did the validize_mem go wrong for some reason?

It refused to handle it and simply returned (subreg (mem)) - see the 
discussion on version 1 of the patch series.

R.

> 
> Thanks,
> Richard
> 
>> gcc/ChangeLog:
>>
>> 	PR target/102125
>> 	* emit-rtl.c (gen_highpart): Use adjust_address to handle
>> 	MEM rather than calling simplify_gen_subreg.
>> ---
>>   gcc/emit-rtl.c | 23 +++++++++++++----------
>>   1 file changed, 13 insertions(+), 10 deletions(-)
>>
>> diff --git a/gcc/emit-rtl.c b/gcc/emit-rtl.c
>> index 77ea8948ee8..0ba110879aa 100644
>> --- a/gcc/emit-rtl.c
>> +++ b/gcc/emit-rtl.c
>> @@ -1585,19 +1585,22 @@ gen_highpart (machine_mode mode, rtx x)
>>     gcc_assert (known_le (msize, (unsigned int) UNITS_PER_WORD)
>>   	      || known_eq (msize, GET_MODE_UNIT_SIZE (GET_MODE (x))));
>>   
>> -  result = simplify_gen_subreg (mode, x, GET_MODE (x),
>> -				subreg_highpart_offset (mode, GET_MODE (x)));
>> -  gcc_assert (result);
>> -
>> -  /* simplify_gen_subreg is not guaranteed to return a valid operand for
>> -     the target if we have a MEM.  gen_highpart must return a valid operand,
>> -     emitting code if necessary to do so.  */
>> -  if (MEM_P (result))
>> +  /* gen_lowpart_common handles a lot of special cases due to needing to handle
>> +     paradoxical subregs; it only calls simplify_gen_subreg when certain that
>> +     it will produce something meaningful.  The only case we need to handle
>> +     specially here is MEM.  */
>> +  if (MEM_P (x))
>>       {
>> -      result = validize_mem (result);
>> -      gcc_assert (result);
>> +      poly_int64 offset = subreg_highpart_offset (mode, GET_MODE (x));
>> +      return adjust_address (x, mode, offset);
>>       }
>>   
>> +  result = simplify_gen_subreg (mode, x, GET_MODE (x),
>> +				subreg_highpart_offset (mode, GET_MODE (x)));
>> +  /* Since we handle MEM directly above, we should never get a MEM back
>> +     from simplify_gen_subreg.  */
>> +  gcc_assert (result && !MEM_P (result));
>> +
>>     return result;
>>   }
>>