[Bug target/111720] RISC-V: Ugly codegen in RVV

["juzhe.zhong at rivai dot ai" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111720

--- Comment #21 from JuzheZhong <juzhe.zhong at rivai dot ai> ---
(In reply to rguenther@suse.de from comment #20)
> On Thu, 19 Oct 2023, juzhe.zhong at rivai dot ai wrote:
> 
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111720
> > 
> > --- Comment #19 from JuzheZhong <juzhe.zhong at rivai dot ai> ---
> > (In reply to Richard Biener from comment #18)
> > > With RVV you have intrinsic calls in GIMPLE so nothing to optimize:
> > > 
> > > vbool8_t fn ()
> > > {
> > >   vbool8_t vmask;
> > >   vuint8m1_t vand_m;
> > >   vuint8m1_t varr;
> > >   uint8_t arr[32];
> > > 
> > >   <bb 2> [local count: 1073741824]:
> > >   arr =
> > > "\x01\x02\x07\x01\x03\x04\x05\x03\x01\x00\x01\x02\x04\x04\t\t\x01\x02\x07\x01
> > > \x03\x04\x05\x03\x01\x00\x01\x02\x04\x04\t\t";
> > >   varr_3 = __riscv_vle8_v_u8m1 (&arr, 32); [return slot optimization]
> > >   vand_m_4 = __riscv_vand_vx_u8m1 (varr_3, 1, 32); [return slot optimization]
> > >   vmask_5 = __riscv_vreinterpret_v_u8m1_b8 (vand_m_4); [return slot
> > > optimization]
> > >   <retval> = vmask_5;
> > >   arr ={v} {CLOBBER(eol)};
> > >   return <retval>;
> > > 
> > > and on RTL I see lots of UNSPECs, RTL opts cannot do anything with those.
> > > 
> > > This is what Andrew said already.
> > 
> > Ok. I wonder why this issue is gone when I change it into:
> > 
> > arr as static
> > 
> > https://godbolt.org/z/Tdoshdfr6
> 
> Because the stacik initialization isn't required then.

I have experiment with a simplifed pattern:


(insn 14 13 15 2 (set (reg/v:RVVM1QI 134 [ varr ])
        (if_then_else:RVVM1QI (unspec:RVVMF8BI [
                    (const_vector:RVVMF8BI repeat [
                            (const_int 1 [0x1])
                        ])
                    (reg:DI 143)
                    (const_int 2 [0x2]) repeated x2
                    (const_int 0 [0])
                    (reg:SI 66 vl)
                    (reg:SI 67 vtype)
                ] UNSPEC_VPREDICATE)
            (mem:RVVM1QI (reg:DI 142) [0  S[16, 16] A8])
            (const_vector:RVVM1QI repeat [
                    (const_int 0 [0])
                ]))) "rvv.c":5:23 1476 {*pred_movrvvm1qi}
     (nil))
(insn 15 14 16 2 (set (reg:DI 144)
        (const_int 32 [0x20])) "rvv.c":6:5 206 {*movdi_64bit}
     (nil))
(insn 16 15 0 2 (set (mem:RVVM1QI (reg/v/f:DI 135 [ out ]) [0  S[16, 16] A8])
        (if_then_else:RVVM1QI (unspec:RVVMF8BI [
                    (const_vector:RVVMF8BI repeat [
                            (const_int 1 [0x1])
                        ])
                    (reg:DI 144)
                    (const_int 0 [0])
                    (reg:SI 66 vl)
                    (reg:SI 67 vtype)
                ] UNSPEC_VPREDICATE)
            (reg/v:RVVM1QI 134 [ varr ])
            (mem:RVVM1QI (reg/v/f:DI 135 [ out ]) [0  S[16, 16] A8])))
"rvv.c":6:5 1592 {pred_storervvm1qi}
     (nil))

You can see there is only one UNSPEC now. Still has redundant stack
transferring.

Is it because the pattern too complicated?