Re: [PATCH] RISC-V: Expand VLMAX scalar move in reduction

[Jeff Law <jeffreyalaw@gmail.com>]

On 2/1/24 18:56, Juzhe-Zhong wrote:
> This patch fixes the following:
> 
>          vsetvli a5,a1,e32,m1,tu,ma
>          slli    a4,a5,2
>          sub     a1,a1,a5
>          vle32.v v2,0(a0)
>          add     a0,a0,a4
>          vadd.vv v1,v2,v1
>          bne     a1,zero,.L3
>          vsetivli        zero,1,e32,m1,ta,ma
>          vmv.s.x v2,zero
>          vsetvli a5,zero,e32,m1,ta,ma              ---> Redundant vsetvl.
>          vredsum.vs      v1,v1,v2
>          vmv.x.s a0,v1
>          ret
> 
> VSETVL PASS is able to fuse avl = 1 of scalar move and VLMAX avl of reduction.
> 
> However, this following RTL blocks the fusion in dependence analysis in VSETVL PASS:
> 
> (insn 49 24 50 5 (set (reg:RVVM1SI 98 v2 [148])
>          (if_then_else:RVVM1SI (unspec:RVVMF32BI [
>                      (const_vector:RVVMF32BI [
>                              (const_int 1 [0x1])
>                              repeat [
>                                  (const_int 0 [0])
>                              ]
>                          ])
>                      (const_int 1 [0x1])
>                      (const_int 2 [0x2]) repeated x2
>                      (const_int 0 [0])
>                      (reg:SI 66 vl)
>                      (reg:SI 67 vtype)
>                  ] UNSPEC_VPREDICATE)
>              (const_vector:RVVM1SI repeat [
>                      (const_int 0 [0])
>                  ])
>              (unspec:RVVM1SI [
>                      (reg:DI 0 zero)
>                  ] UNSPEC_VUNDEF))) 3813 {*pred_broadcastrvvm1si_zero}
>       (nil))
> (insn 50 49 51 5 (set (reg:DI 15 a5 [151])                          ---->  It set a5, blocks the following VLMAX into the scalar move above.
>          (unspec:DI [
>                  (const_int 32 [0x20])
>              ] UNSPEC_VLMAX)) 2566 {vlmax_avldi}
>       (expr_list:REG_EQUIV (unspec:DI [
>                  (const_int 32 [0x20])
>              ] UNSPEC_VLMAX)
>          (nil)))
> (insn 51 50 52 5 (set (reg:RVVM1SI 97 v1 [150])
>          (unspec:RVVM1SI [
>                  (unspec:RVVMF32BI [
>                          (const_vector:RVVMF32BI repeat [
>                                  (const_int 1 [0x1])
>                              ])
>                          (reg:DI 15 a5 [151])
>                          (const_int 2 [0x2])
>                          (const_int 1 [0x1])
>                          (reg:SI 66 vl)
>                          (reg:SI 67 vtype)
>                      ] UNSPEC_VPREDICATE)
>                  (unspec:RVVM1SI [
>                          (reg:RVVM1SI 97 v1 [orig:134 vect_result_14.6 ] [134])
>                          (reg:RVVM1SI 98 v2 [148])
>                      ] UNSPEC_REDUC_SUM)
>                  (unspec:RVVM1SI [
>                          (reg:DI 0 zero)
>                      ] UNSPEC_VUNDEF)
>              ] UNSPEC_REDUC)) 17541 {pred_redsumrvvm1si}
>       (expr_list:REG_DEAD (reg:RVVM1SI 98 v2 [148])
>          (expr_list:REG_DEAD (reg:SI 66 vl)
>              (expr_list:REG_DEAD (reg:DI 15 a5 [151])
>                  (expr_list:REG_DEAD (reg:DI 0 zero)
>                      (nil))))))
> 
> Such situation can only happen on auto-vectorization, never happen on intrinsic codes.
> Since the reduction is passed VLMAX AVL, it should be more natural to pass VLMAX to the scalar move which initial the value of the reduction.
> 
> After this patch:
> 
> 	vsetvli	a5,a1,e32,m1,tu,ma
> 	slli	a4,a5,2
> 	sub	a1,a1,a5
> 	vle32.v	v2,0(a0)
> 	add	a0,a0,a4
> 	vadd.vv	v1,v2,v1
> 	bne	a1,zero,.L3
> 	vsetvli	a5,zero,e32,m1,ta,ma
> 	vmv.s.x	v2,zero
> 	vredsum.vs	v1,v1,v2
> 	vmv.x.s	a0,v1
>          ret
> 
> Tested on both RV32/RV64 no regression.
> 
> 	PR target/113697
> 
> gcc/ChangeLog:
> 
> 	* config/riscv/riscv-v.cc (expand_reduction): Pass VLMAX avl to scalar move.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* gcc.target/riscv/rvv/autovec/pr113697.c: New test.
I suspect this broke 502.gcc in spec2017.  Basically it's hanging during 
the build phase.  I'm not sure if I'm going to have time this week to 
dive into it.


Optimization options used:

> GCC Flags:  -Ofast -flto -fsched-pressure -fno-strict-aliasing -fgnu89-inline -fcommon -fno-finite-math-only -fno-unsafe-math-optimizations



Given this appears to be a minor optimization issue, I wouldn't lose any 
sleep if it was reverted and deferred to gcc-15.

Anyway, good luck.  Sorry I can't do more on the debugging/reduction front.

Jeff