Re: [PATCH 0/5] Add support for operand-specific alignment requirements

[Richard Sandiford <richard.sandiford@arm.com>]

钟居哲 <juzhe.zhong@rivai.ai> writes:
> Hi, Richard.
>
> Current define_mode_attr can only map an attribute for a mode.
> I wonder whether we can map a mode to multiple attributes ?
>
> E.g. (define_mode_attr dest_constraint [(V16QI "&vr")])
>
> But I want it to be:
>
> (define_mode_attr dest_constraint [(V16QI (TARGET_MIN_VLEN <= 128 "vr") (TARGET_MIN_VLEN > 128 "&vr")) ])
>
> It seems that we can't achieve this for now. Would it be possible we exend it in GCC-15 ?

That construct doesn't seem particularly natural to me.  Could you explain
(conceptually) what kind of insn patterns you need?

Thanks,
Richard

>
>
> juzhe.zhong@rivai.ai
>  
> From: Richard Sandiford
> Date: 2023-11-22 18:08
> To: juzhe.zhong\@rivai.ai
> CC: gcc-patches; vmakarov\@redhat.com; kito.cheng
> Subject: Re: [PATCH 0/5] Add support for operand-specific alignment requirements
> "juzhe.zhong@rivai.ai" <juzhe.zhong@rivai.ai> writes:
>> Hi, Richard.
>>
>> Thanks for supporting register filter in IRA/LRA.
>> I found it is useful for RVV since we have a set of widen operations that allow source register overlap highpart of dest register group
>>
>> For example, if vsext.vf2 v0(dest consume reg v0 and reg v1), v1 (source consume v1 only)
>> I want to support the highpart overlap above. (Currently, we don't any overlap between source and dest in such instructions).
>>
>> So, I wonder whether we can pass "machine_mode" into register filter. Ok, I think it's too late since stage 1 closes. I wonder we can add it in GCC-15?
>  
> I think adding a mode would add too much overhead.  The mode would be
> the mode of the operand, but with subregs, the mode of the operand can
> be different from the mode of the RA allocno.  So it would no longer
> be enough for the RA to calculate a bitmask of filters.  It would need
> ro remember which modes are used with those filters.
>  
> We'd also need to turn the current HARD_REG_SETs into [MAX_MACHINE_MODE]
> arrays of HARD_REG_SETs.  (And there are now more than 256 machine modes
> for riscv.)
>  
> The pattern that uses the constraints should already "know" the mode.
> So if possible, I think it would be better to use different constraints
> for different modes, using define_mode_attrs.
>  
> Thanks,
> Richard
>