Followup on PR/109279: large constants on RISCV

[Vineet Gupta <vineetg@rivosinc.com>]

Hi Jeff,

I finally got around to collecting various observations on PR/109279 - 
more importantly the state of large constants in RV backend, apologies 
in advance for the long email.

It seems the various commits in area have improved the original test 
case of 0x1010101_01010101

   Before 2e886eef7f2b      |   With 2e886eef7f2b   | With 
0530254413f8     | With c104ef4b5eb1
     (const pool)           | define_insn_and_split | "splitter relaxed 
new |
                            | "*mvconst_internal"   | pseudos"             |
lui  a5,%hi(.LANCHOR0)     | li     a0,0x01010000  | li a5,0x01010000    
| li   a5,0x01010000
ld   a0,%lo(.LANCHOR0)(a5) | addi   a0,0x0101      | addi 
a5,a5,0x0101     | addi a5,a5,0x0101
ret                        | slli   a0,a0,16       | mv a0,a5            
| slli a0,a5,32
                            | addi   a0,a0,0x0101   | slli 
a5,a5,32         | add  a0,a0,a5
                            | slli   a0,a0,16       | add a0,a5,a0         |
                            | addi   a0,a0,0x0101   | 
ret                   |
                            | ret                   |

But same commits seem to have regressed Andrew's test from same PR 
(which is the theme of this email).
The seemingly contrived test turned out to be much more than I'd hoped for.

    long long f(void)
    {
      unsigned t = 0x101_0101;
      long long t1 = t;
      long long t2 = ((unsigned long long )t) << 32;
      asm("":"+r"(t1));
      return t1 | t2;
    }

   Before 2e886eef7f2b  |   With 2e886eef7f2b    | With 0530254413f8
     (ideal code)       | define_insn_and_split  | "splitter relaxed new
                        |                        |  pseudos"
    li   a0,0x1010000   |    li   a5,0x1010000   |    li a0,0x101_0000
    addi a0,a0,0x101    |    addi a5,a5,0x101    |    addi a0,a0,0x101
    slli a5,a0,32       |    mv   a0,a5          |    li a5,0x101_0000
    or   a0,a0,a5       |    slli a5,a5,32       |    slli a0,a0,32
    ret                 |    or   a0,a0,a5       |    addi a5,a5,0x101
                        |    ret                 |    or   a0,a5,a0
                                                 |    ret

As a baseline, RTL just before cse1 (in 260r.dfinit) in all of above is:

    # lower word

    (insn 6 2 7 2 (set (reg:DI 138)
         (const_int [0x1010000]))  {*movdi_64bit}

    (insn 7 6 8 2 (set (reg:DI 137)
         (plus:DI (reg:DI 138)
             (const_int [0x101]))) {adddi3}
      (expr_list:REG_EQUAL (const_int [0x1010101]) )

    (insn 5 8 9 2 (set (reg/v:DI 134 [ t1 ])
         (reg:DI 136 [ t1 ])) {*movdi_64bit}

    # upper word (created independently)

    (insn 9 5 10 2 (set (reg:DI 141)
         (const_int [0x1010000]))  {*movdi_64bit}

    (insn 10 9 11 2 (set (reg:DI 142)
         (plus:DI (reg:DI 141)
             (const_int [0x101]))) {adddi3}

    (insn 11 10 12 2 (set (reg:DI 140)
         (ashift:DI (reg:DI 142)
             (const_int 32 [0x20]))) {ashldi3}
        (expr_list:REG_EQUAL (const_int [0x101010100000000]))

    # stitch them
    (insn 12 11 13 2 (set (reg:DI 139)
         (ior:DI (reg/v:DI 134 [ t1 ])
             (reg:DI 140))) "const2.c":7:13 99 {iordi3}


Prior to 2e886eef7f2b, cse1 could do its job: finding oldest equivalent 
registers for the fragments of const and reusing the reg.

    (insn 7 6 8 2 (set (reg:DI 137)
         (plus:DI (reg:DI 138)
             (const_int [0x101]))) {adddi3}
         (expr_list:REG_EQUAL (const_int [0x1010101])))
    [...]

    (insn 11 10 12 2 (set (reg:DI 140)
         (ashift:DI (reg:DI 137)
                          ^^^^^   OLD EQUIV REG
             (const_int 32 [0x20]))) {ashldi3}
         (expr_list:REG_EQUAL (const_int [0x1010101_00000000])))


With 2e886eef7f2b, define_insn_and_split "*mvconst_internal" recog() 
kicks in during cse1, eliding insns for a const_int.

    (insn 7 6 8 2 (set (reg:DI 137)
         (const_int [0x1010101])) {*mvconst_internal}
         (expr_list:REG_EQUAL (const_int [0x1010101])))
    [...]

    (insn 11 10 12 2 (set (reg:DI 140)
         (const_int [0x1010101_00000000])) {*mvconst_internal}
         (expr_list:REG_EQUAL (const_int  [0x1010101_00000000]) ))

Eventually split1 breaks it up using same mvconst_internal splitter, but 
the cse opportunity has been lost.
*This is a now a baseline for large consts handling for RV backend which 
we all need to be aware of*.


(2) Now on to the nuances as to why things get progressively worse after 
commit 0530254413f8.

It all seems to get down to register allocation passes:

sched1 before 0530254413f8

    ;;     0--> b  0: i  22 r140=0x1010000    :alu
    ;;     1--> b  0: i  20 r137=0x1010000    :alu
    ;;     2--> b  0: i  23 r140=r140+0x101   :alu
    ;;     3--> b  0: i  21 r137=r137+0x101   :alu
    ;;     4--> b  0: i  24 r140=r140<<0x20   :alu
    ;;     5--> b  0: i  25 r136=r137         :alu
    ;;     6--> b  0: i   8 r136=asm_operands :nothing
    ;;     7--> b  0: i  17 a0=r136|r140      :alu
    ;;     8--> b  0: i  18 use a0            :nothing

sched1 with 0530254413f8

    ;;     0--> b  0: i  22 r144=0x1010000    :alu
    ;;     1--> b  0: i  20 r143=0x1010000    :alu
    ;;     2--> b  0: i  23 r145=r144+0x101   :alu
    ;;     3--> b  0: i  21 r137=r143+0x101   :alu
    ;;     4--> b  0: i  24 r140=r145<<0x20   :alu
    ;;     5--> b  0: i  25 r136=r137         :alu
    ;;     6--> b  0: i   8 r136=asm_operands :nothing
    ;;     7--> b  0: i  17 a0=r136|r140      :alu
    ;;     8--> b  0: i  18 use a0            :nothing

The insn stream is same, only differences being registers reuse (due to 
splitter restriction) vs. not.

Next IRA, for reasons I don't understand (and not brave enough yet to 
dive into) decides to regenerate const_int.
And my guess is, this being so late in the game that it gets 
rematerialized as is in the end, causing the duplicity.
FWIW, IRA for latter case only, emits additional REG_EQUIV notes which 
could also be playing a role.

    deleting insn with uid = 22.
    deleting insn with uid = 21.

    (insn 20 12 23 2 (set (reg:DI 143)
         (const_int  [0x1010000])) {*movdi_64bit}
      (expr_list:REG_EQUIV (const_int  [0x1010000])))

    (insn 23 20 24 2 (set (reg:DI 145)
         (const_int  [0x1010101])) *{*mvconst_internal})*
                ^^^^^^^^^^^
    (insn 24 23 25 2 (set (reg:DI 140)
         (ashift:DI (reg:DI 145)
             (const_int 32 [0x20]))) {ashldi3}
      (expr_list:REG_DEAD (reg:DI 145)
      (expr_list:REG_EQUIV (const_int [0x1010101_00000000]))))

    (insn 25 24 8 2 (set (reg:DI 136 [ t1 ])
         (plus:DI (reg:DI 143)
             (const_int [0x101]))) {adddi3}
      (expr_list:REG_DEAD (reg:DI 143)))

    (insn 17 8 18 2 (set (reg/i:DI 10 a0)
         (ior:DI (reg:DI 136 [ t1 ])
             (reg:DI 140)))  {iordi3}
      (expr_list:REG_DEAD (reg:DI 140)
      (expr_list:REG_DEAD (reg:DI 136 [ t1 ]) )))

I naively tried to gate mvconst_internal to !reload_completed, but that 
triggered some ICE.
It seems wrong anyways since post reload split2 also sometimes needs to 
invokes the mvconst_internal splitter (afaikr for the original 
0x01010... test)

Do you have any ideas/directions on how to tackle this ?

Thx,
-Vineet