Re: [PATCH 1/2]middle-end Fold BIT_FIELD_REF and Shifts into BIT_FIELD_REFs alone

[Jeff Law <jeffreyalaw@gmail.com>]

On 9/28/22 07:19, Tamar Christina wrote:
>> -----Original Message-----
>> From: Jeff Law <jeffreyalaw@gmail.com>
>> Sent: Saturday, September 24, 2022 8:38 PM
>> To: Tamar Christina <Tamar.Christina@arm.com>; gcc-patches@gcc.gnu.org
>> Cc: nd <nd@arm.com>; rguenther@suse.de
>> Subject: Re: [PATCH 1/2]middle-end Fold BIT_FIELD_REF and Shifts into
>> BIT_FIELD_REFs alone
>>
>>
>> On 9/23/22 05:42, Tamar Christina wrote:
>>> Hi All,
>>>
>>> This adds a match.pd rule that can fold right shifts and
>>> bit_field_refs of integers into just a bit_field_ref by adjusting the
>>> offset and the size of the extract and adds an extend to the previous size.
>>>
>>> Concretely turns:
>>>
>>> #include <arm_neon.h>
>>>
>>> unsigned int foor (uint32x4_t x)
>>> {
>>>       return x[1] >> 16;
>>> }
>>>
>>> which used to generate:
>>>
>>>     _1 = BIT_FIELD_REF <x_2(D), 32, 32>;
>>>     _3 = _1 >> 16;
>>>
>>> into
>>>
>>>     _4 = BIT_FIELD_REF <x_1(D), 16, 48>;
>>>     _2 = (unsigned int) _4;
>>>
>>> I currently limit the rewrite to only doing it if the resulting
>>> extract is in a mode the target supports. i.e. it won't rewrite it to
>>> extract say 13-bits because I worry that for targets that won't have a
>>> bitfield extract instruction this may be a de-optimization.
>>>
>>> Bootstrapped Regtested on aarch64-none-linux-gnu, x86_64-pc-linux-gnu
>>> and no issues.
>>>
>>> Testcase are added in patch 2/2.
>>>
>>> Ok for master?
>>>
>>> Thanks,
>>> Tamar
>>>
>>> gcc/ChangeLog:
>>>
>>> 	* match.pd: Add bitfield and shift folding.
>> Were you planning to handle left shifts as well?  It looks like it since you've
>> got iterations for the shift opcode and corresponding adjustment to the field,
>> but they currently only handle rshift/plus.
>>
> Hmm do left shifts work here? Since a left shift would increase the size of the
> resulting value by adding zeros to the end of the number, so you can't increase
> the size of the bitfield to do the same.

Dunno, I hadn't really thought about it.  It just looked like you were 
prepared to handle more cases with those iterators.


>
> I did however realize that truncating casts have the same effect as a right shift,
> so I have added that now.

ACK.

jeff