Richard Earnshaw <richard.earns...@foss.arm.com> writes:
> 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.
OK, that's good then. The patch is OK from my POV too FWIW.
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;
>>> }
>>>
