Re: Re: [PATCH] VECT: Change flow of decrement IV

["juzhe.zhong@rivai.ai" <juzhe.zhong@rivai.ai>]

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Before this patch:
foo:
ble a2,zero,.L5
csrr a3,vlenb
srli a4,a3,2
.L3:
minu a5,a2,a4
vsetvli zero,a5,e32,m1,ta,ma
vle32.v v2,0(a1)
vle32.v v1,0(a0)
vsetvli t1,zero,e32,m1,ta,ma
vadd.vv v1,v1,v2
vsetvli zero,a5,e32,m1,ta,ma
vse32.v v1,0(a0)
add a1,a1,a3
add a0,a0,a3
      sub   a2,a2,a5
bne a2,zero,.L3
.L5:
ret

After this patch:

foo:
ble a2,zero,.L5
csrr a3,vlenb
srli a4,a3,2
neg a7,a4   -->>>additional instruction
.L3:
minu a5,a2,a4
vsetvli zero,a5,e32,m1,ta,ma
vle32.v v2,0(a1)
vle32.v v1,0(a0)
vsetvli t1,zero,e32,m1,ta,ma
mv a6,a2  -->>>additional instruction
vadd.vv v1,v1,v2
vsetvli zero,a5,e32,m1,ta,ma
vse32.v v1,0(a0)
add a1,a1,a3
add a0,a0,a3
add a2,a2,a7
bgtu a6,a4,.L3
.L5:
ret

There is 1 more instruction in preheader and 1 more instruction in loop.
But I think it's OK for RVV since we will definitely be using SELECT_VL so this issue will gone.
As long as this flow is better to power (SCEV）。



juzhe.zhong@rivai.ai
 
From: Richard Sandiford
Date: 2023-05-30 19:31
To: juzhe.zhong
CC: gcc-patches; rguenther; linkw
Subject: Re: [PATCH] VECT: Change flow of decrement IV
juzhe.zhong@rivai.ai writes:
> From: Ju-Zhe Zhong <juzhe.zhong@rivai.ai>
>
> Follow Richi's suggestion, I change current decrement IV flow from:
>
> do {
>    remain -= MIN (vf, remain);
> } while (remain != 0);
>
> into:
>
> do {
>    old_remain = remain;
>    len = MIN (vf, remain);
>    remain -= vf;
> } while (old_remain >= vf);
>
> to enhance SCEV.
>
> ALL tests (decrement IV) of RVV are passed.
 
How does it affect RVV code quality?  I thought you specifically chose
the previous approach because code quality was better that way.
 
Richard