Re: TARGET_RTX_COSTS and pipeline latency vs. variable-latency instructions (was Re: [PATCH] RISC-V: Add XiangShan Nanhu microarchitecture.)

[Palmer Dabbelt <palmer@dabbelt.com>]

On Mon, 25 Mar 2024 13:49:18 PDT (-0700), jeffreyalaw@gmail.com wrote:
>
>
> On 3/25/24 2:31 PM, Palmer Dabbelt wrote:
>> On Mon, 25 Mar 2024 13:27:34 PDT (-0700), Jeff Law wrote:
>
>>> I'd doubt it's worth the complexity.  Picking some reasonable value gets
>>> you the vast majority of the benefit.   Something like
>>> COSTS_N_INSNS(6) is enough to get CSE to trigger.  So what's left is a
>>> reasonable cost, particularly for the division-by-constant case where we
>>> need a ceiling for synth_mult.
>>
>> Ya, makes sense.  I noticed our multi-word multiply costs are a bit odd
>> too (they really only work for 64-bit mul on 32-bit targets), but that's
>> probably not worth worrying about either.
> We do have a changes locally that adjust various costs.  One of which is
> highpart multiply.  One of the many things to start working through once
> gcc-15 opens for development.  Hence my desire to help keep gcc-14 on
> track for an on-time release.

Cool.  LMK if there's anything we can do to help on that front.

>
> Jeff