Re: [PATCH] CPROP: Allow cprop optimization when the function has a single block

[Richard Biener <rguenther@suse.de>]

On Wed, 1 Feb 2023, juzhe.zhong@rivai.ai wrote:

> From: Ju-Zhe Zhong <juzhe.zhong@rivai.ai>
> 
> Hi, this patch is present for GCC 14 since I understand it's not appropriate
> to land it in GCC 13.
> 
> NUM_FIXED_BLOCKS = 2 since GCC define each function has aleast 2 blocks
> one is entry block, the other is exit block.
> So according this code, the function will not do cprop optimization when
> there is only exactly one single block.

cprop / GCSE are global dataflow problems, there's "nothing" to do for
a single block, the local problem plus its application isn't considered
but probably left for CSE.

Why does CSE not perform the desired transform?

> I am not sure whether it's correct to fix it like this.
> Can anyone tell me why forbid cprop optimization if the function only has s
> single block ?
> 
> Let's take a look at these 2 examples of RVV intrinsics:
> 1.    void f1 (void * in, void *out, int64_t x, int n)
>       {
>       vint64m1_t v = __riscv_vle64_v_i64m1 (in + 1, 4);
>       vint64m1_t v2 = __riscv_vle64_v_i64m1_tu (v, in + 2, 4);
>       vint64m1_t v3 = __riscv_vadd_vx_i64m1 (v2, x, 4);
>       vint64m1_t v4 = __riscv_vadd_vx_i64m1 (v3, x, 4);
>       __riscv_vse64_v_i64m1 (out + 2, v4, 4);
>       }
> 
> asm:
>      	addi	sp,sp,-16
> 	sw	a2,8(sp)
> 	sw	a3,12(sp)
> 	sw	a2,0(sp)
> 	sw	a3,4(sp)
> 	addi	a5,a0,1
> 	vsetivli	zero,4,e64,m1,ta,ma
> 	addi	a0,a0,2
> 	vle64.v	v24,0(a5)
> 	addi	a5,sp,8
> 	vlse64.v	v27,0(a5),zero
> 	addi	a1,a1,2
> 	vsetvli	zero,zero,e64,m1,tu,ma
> 	vle64.v	v24,0(a0)
> 	vsetvli	zero,zero,e64,m1,ta,ma
> 	vlse64.v	v25,0(sp),zero
> 	vadd.vv	v26,v24,v27
> 	vadd.vv	v24,v26,v25
> 	vse64.v	v24,0(a1)
> 	addi	sp,sp,16
> 	jr	ra
> you can see here there are 2 vlse64.v instructions that broadcast the scalar value "x"
> GCC fail to eliminate the second vlse64.v instruction since GCC doesn't do the cprop
> optimization (the function only has 1 single block). It can be optimized if we apply
> this patch.
> 
> 2. void f1 (void * in, void *out, int64_t x, int n)
> {
>     if (n) {
>       vint64m1_t v = __riscv_vle64_v_i64m1 (in + 1, 4);
>       vint64m1_t v2 = __riscv_vle64_v_i64m1_tu (v, in + 2, 4);
>       vint64m1_t v3 = __riscv_vadd_vx_i64m1 (v2, x, 4);
>       vint64m1_t v4 = __riscv_vadd_vx_i64m1 (v3, x, 4);
>       __riscv_vse64_v_i64m1 (out + 2, v4, 4);
>     }
> }
> 
> asm:
>   f1:
> 	vsetivli	zero,4,e64,m1,ta,ma
> 	beq	a4,zero,.L7
> 	addi	sp,sp,-16
> 	sw	a2,8(sp)
> 	sw	a3,12(sp)
> 	addi	a5,a0,1
> 	vle64.v	v24,0(a5)
> 	addi	a0,a0,2
> 	addi	a5,sp,8
> 	vlse64.v	v25,0(a5),zero
> 	addi	a1,a1,2
> 	vsetvli	zero,zero,e64,m1,tu,ma
> 	vle64.v	v24,0(a0)
> 	vadd.vv	v26,v24,v25
> 	vadd.vv	v24,v26,v25
> 	vse64.v	v24,0(a1)
> 	addi	sp,sp,16
> 	jr	ra
> .L7:
> 	ret
> 
> Here, if we add if (n) condition here, the program will end up with more than 1 block.
> So GCC will do the cprop optimization and the second vlse64.v instruction is eliminated.
> 
> I am not sure whether this patch is correct.
> Can anyone help me with that ?
> Thanks.
> 
> 
> gcc/ChangeLog:
> 
>         * cprop.cc (one_cprop_pass): Remove +1.
> 
> ---
>  gcc/cprop.cc | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/gcc/cprop.cc b/gcc/cprop.cc
> index 6ec0bda4a24..615bc5078b6 100644
> --- a/gcc/cprop.cc
> +++ b/gcc/cprop.cc
> @@ -1749,7 +1749,7 @@ one_cprop_pass (void)
>    int changed = 0;
>  
>    /* Return if there's nothing to do, or it is too expensive.  */
> -  if (n_basic_blocks_for_fn (cfun) <= NUM_FIXED_BLOCKS + 1
> +  if (n_basic_blocks_for_fn (cfun) <= NUM_FIXED_BLOCKS
>        || gcse_or_cprop_is_too_expensive (_ ("const/copy propagation disabled")))
>      return 0;
>  
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)