On Wed, 1 Feb 2023, juzhe.zh...@rivai.ai wrote:
> From: Ju-Zhe Zhong <juzhe.zh...@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 <rguent...@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)
