Re: Re: [PATCH] RISC-V: Add 'prefer_agnostic' tune parameter for vector policies

[钟居哲]

I think it's better to make it "demand policy" for "agnostic" instead of 
changing the fusion rule.

Could you try this ?

In riscv-vsetvl.cc line: 1147 - 1150

if (!m_ta)
  dflags |= demand_flags::DEMAND_TAIL_POLICY_P;
if (!m_ma)
  dflags |= demand_flags::DEMAND_MASK_POLICY_P;

change it to:

if (!m_ta || riscv_prefer_agnostic_p ())
  dflags |= demand_flags::DEMAND_TAIL_POLICY_P;
if (!m_ma || riscv_prefer_agnostic_p ())
  dflags |= demand_flags::DEMAND_MASK_POLICY_P;

to see whether it can work ?


juzhe.zh...@rivai.ai
 
From: Zhongyao Chen
Date: 2025-09-26 15:17
To: 钟居哲
Subject: Re: Re: [PATCH] RISC-V: Add 'prefer_agnostic' tune parameter for 
vector policies
On Fri, Sep 26, 2025 at 2:53 PM 钟居哲 <juzhe.zh...@rivai.ai> wrote:
>
> I understand what you want.
> But my question is why this following changes don't satisfy your need and why 
> you need to change VSETVL PASS:
>
> >> -  /* TODO: By default, we choose to use TAIL_ANY which allows
> >> -     compiler pick up either agnostic or undisturbed. Maybe we
> >> -     will have a compile option like -mprefer=agnostic to set
> >> -     this value???.  */
> >> +  if (riscv_prefer_agnostic_p ())
> >> +    return TAIL_AGNOSTIC;
>
>
> >> -  /* TODO: By default, we choose to use MASK_ANY which allows
> >> -     compiler pick up either agnostic or undisturbed. Maybe we
> >> -     will have a compile option like -mprefer=agnostic to set
> >> -     this value???.  */
> >>+  if (riscv_prefer_agnostic_p ())
> >> +    return MASK_AGNOSTIC;
> >>   return MASK_ANY;
>
>
> The VSETVL PASS should be able to disable fusion if you explicit set it as 
> "agnostic".
>
> ________________________________
> juzhe.zh...@rivai.ai
 
testing shows that these changes alone do not prevent fusion. I
believe the issue lies in the policy rules:
 
DEF_POLICY_RULE (tail_policy_only, ignore_policy, tail_policy_only,
always_true, always_true, nop)
DEF_POLICY_RULE (ignore_policy, tail_policy_only, tail_policy_only,
always_true, always_false, use_tail_policy)
 
The always_true condition in these rules indicates that fusion is
always allowed.