Re: query about commit 666fdc46bc8489 ("RISC-V: Fix bad insn splits with paradoxical subregs")

[Vineet Gupta <vineetg@rivosinc.com>]

On 11/4/22 16:13, Jeff Law wrote:
>
> On 11/4/22 16:59, Vineet Gupta wrote:
>> Hi Jakub,
>>
>> I had a question about the aforementioned commit in RV backend.
>>
>> (define_split
>>   [(set (match_operand:GPR 0 "register_operand")
>>     (and:GPR (match_operand:GPR 1 "register_operand")
>>                (match_operand:GPR 2 "p2m1_shift_operand")))
>> +   (clobber (match_operand:GPR 3 "register_operand"))]
>>   ""
>> -  [(set (match_dup 0)
>> +  [(set (match_dup 3)
>>        (ashift:GPR (match_dup 1) (match_dup 2)))
>>
>> Is there something specific to this split which warrants this or so 
>> any split patterns involving shifts have this to avoid the shifting 
>> by more than SUBREG_REG problem.
>
> Not sure.  Note it was Jim Wilson's change, not Jakub's change AFAICT:
>
>
> commit 666fdc46bc848984ee7d2906f2dfe10e1ee5d535
> Author: Jim Wilson <jimw@sifive.com>
> Date:   Sat Jun 30 21:52:01 2018 +0000
>
>     RISC-V: Add patterns to convert AND mask to two shifts.
>
>             gcc/
>             * config/riscv/predicates.md (p2m1_shift_operand): New.
>             (high_mask_shift_operand): New.

Indeed Jim introduced the pattern with 666fdc46bc8, but the clobber was 
added later in 36ec3f57d305 ("RISC-V: Fix bad insn splits with 
paradoxical subregs"). He attributed this to Jakub, and with Jim not 
being super active these days, I tried reaching out to this cc list.

Sorry I pasted wrong sha-id in my orig msg, it needs to be 36ec3f57d305


>
> You might find further discussion in the gcc-patches archives.

I'll dig some more.

>
>>
>> Also could you please explain where the clobber itself is allocated ?
>
> I'd expect the combiner.   There's going to be 2 or more insns that 
> are combined to create this pattern where the output of one feeds a 
> subsequent insn and dies.  So it's effectively a temporary.  Combine 
> will re-use that temporary as the clobbered operand.

Make sense.

Thx,
-Vineet