Re: Re: [PATCH v5] RISC-V: Fix register overlap issue for some xtheadvector instructions

[钟居哲 <juzhe.zhong@rivai.ai>]

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>> For the other insns, I wonder if we could get away with not really
>>disabling the newly added early-clobber alternatives for RVV but
>>just disparaging ("?") them?  That way we could re-use "full" for
>>the thv-disabled alternatives and "none" for the newly added ones.
>>("none" will still be misleading then, though :/)

I prefer to disable those early-clobber alternatives added of theadvector for RVV,
since disparage still make RA possible reaches the early clobber alternatives.

>>If this doesn't work or others feel the separation is not strict
>>enough, I'd prefer a separate attribute rather than overloading
>>group_overlap.  Maybe something like "spec_restriction" or similar
>>with two values "rvv" and "thv"?

I like this idea, it makes more sense to me. So I think it's better to add an attribute to
disable alternative for theadvector or RVV1.0.



juzhe.zhong@rivai.ai
 
From: Robin Dapp
Date: 2024-01-10 21:36
To: Jun Sha (Joshua); gcc-patches
CC: rdapp.gcc; jim.wilson.gcc; palmer; andrew; philipp.tomsich; jeffreyalaw; christoph.muellner; juzhe.zhong; Jin Ma; Xianmiao Qu
Subject: Re: [PATCH v5] RISC-V: Fix register overlap issue for some xtheadvector instructions
Hi Joshua,
 
> For th.vmadc/th.vmsbc as well as narrowing arithmetic instructions
> and floating-point compare instructions, an illegal instruction
> exception will be raised if the destination vector register overlaps
> a source vector register group.
> 
> To handle this issue, we use "group_overlap" and "enabled" attribute
> to disable some alternatives for xtheadvector.
 
>  ;; Widening instructions have group-overlap constraints.  Those are only
>  ;; valid for certain register-group sizes.  This attribute marks the
>  ;; alternatives not matching the required register-group size as disabled.
> -(define_attr "group_overlap" "none,W21,W42,W84,W43,W86,W87,W0"
> +(define_attr "group_overlap" "none,W21,W42,W84,W43,W86,W87,W0,thv_disabled,rvv_disabled"
>    (const_string "none"))
 
I realize there have been some discussions before but I find the naming
misleading.  The group_overlap attribute is supposed to specify whether
groups overlap (and mark the respective alternatives accepting
only this overlap).
Then we check if the groups overlap and disable all non-matching
alternatives.  "none" i.e. "no overlap" always matches.
 
Your first goal seems to be to disable existing non-early-clobber
alternatives for thv.  For this, maybe "full", "same" (or "any"?) would
work?  Please also add a comment in group_overlap_valid then that we
need not actually check for register equality.
 
For the other insns, I wonder if we could get away with not really
disabling the newly added early-clobber alternatives for RVV but
just disparaging ("?") them?  That way we could re-use "full" for
the thv-disabled alternatives and "none" for the newly added ones.
("none" will still be misleading then, though :/)
 
If this doesn't work or others feel the separation is not strict
enough, I'd prefer a separate attribute rather than overloading
group_overlap.  Maybe something like "spec_restriction" or similar
with two values "rvv" and "thv"?
 
Regards
Robin