[Bug target/115934] [15 Regression] nvptx vs. ivopts: replace constant_multiple_of with aff_combination_constant_multiple_p [PR114932]

[tnfchris at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115934

--- Comment #4 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
This one looks a bit like costing,

before the patch IVopts had:

<Invariant Expressions>:
inv_expr 1:     -element_7(D)
inv_expr 2:     (signed int) rite_5(D) - (signed int) element_7(D)

and after the patch it generates a few more alternatives:


<Invariant Expressions>:
inv_expr 1:     -element_7(D)
inv_expr 2:     ((signed int) left_4(D) + (signed int) rite_5(D)) - (signed
int) element_7(D)
inv_expr 3:     (signed int) left_4(D) + (signed int) rite_5(D)
inv_expr 4:     (signed int) rite_5(D) - (signed int) element_7(D)
inv_expr 5:     ((signed int) rite_5(D) - (signed int) element_7(D)) + (signed
int) left_4(D)
inv_expr 6:     ((signed int) rite_5(D) + (signed int) element_7(D)) + (signed
int) left_4(D)
inv_expr 7:     ((signed int) left_4(D) - (signed int) element_7(D)) + (signed
int) rite_5(D)

Before it decided it needed two separate IVs to satisfy these invariants:

Initial set of candidates:
  cost: 122 (complexity 0)
  reg_cost: 114
  cand_cost: 8
  cand_group_cost: 0 (complexity 0)
  candidates: 7, 9
   group:0 --> iv_cand:7, cost=(0,0)
   group:1 --> iv_cand:9, cost=(0,0)
   group:2 --> iv_cand:9, cost=(0,0)
   group:3 --> iv_cand:7, cost=(0,0)
  invariant variables: 1
  invariant expressions: 1

Original cost 122 (complexity 0)

Final cost 122 (complexity 0)

Selected IV set for loop 1 at
../gcc-dsg/gcc/testsuite/gcc.dg/tree-ssa/pr43378.c:7, 10 avg niters, 2 IVs:
Candidate 7:
  Var befor: left_14
  Var after: left_10
  Incr POS: orig biv
  IV struct:
    Type:       int
    Base:       left_4(D)
    Step:       element_7(D)
    Biv:        N
    Overflowness wrto loop niter:       Overflow
Candidate 9:
  Depend on inv.exprs: 1
  Var befor: rite_15
  Var after: rite_8
  Incr POS: orig biv
  IV struct:
    Type:       int
    Base:       rite_5(D)
    Step:       -element_7(D)
    Biv:        N
    Overflowness wrto loop niter:       Overflow


with the patch it decided it only needed the one IV:

Initial set of candidates:
  cost: 109 (complexity 0)
  reg_cost: 97
  cand_cost: 4
  cand_group_cost: 8 (complexity 0)
  candidates: 9
   group:0 --> iv_cand:9, cost=(4,0)
   group:1 --> iv_cand:9, cost=(0,0)
   group:2 --> iv_cand:9, cost=(0,0)
   group:3 --> iv_cand:9, cost=(4,0)
  invariant variables: 1
  invariant expressions: 1, 3

Initial set of candidates:
  cost: 109 (complexity 0)
  reg_cost: 97
  cand_cost: 4
  cand_group_cost: 8 (complexity 0)
  candidates: 9
   group:0 --> iv_cand:9, cost=(4,0)
   group:1 --> iv_cand:9, cost=(0,0)
   group:2 --> iv_cand:9, cost=(0,0)
   group:3 --> iv_cand:9, cost=(4,0)
  invariant variables: 1
  invariant expressions: 1, 3

Original cost 109 (complexity 0)

Final cost 109 (complexity 0)

Selected IV set for loop 1 at
../gcc-dsg/gcc/testsuite/gcc.dg/tree-ssa/pr43378.c:7, 10 avg niters, 1 IVs:
Candidate 9:
  Depend on inv.exprs: 1
  Var befor: rite_15
  Var after: rite_8
  Incr POS: orig biv
  IV struct:
    Type:       int
    Base:       rite_5(D)
    Step:       -element_7(D)
    Biv:        N
    Overflowness wrto loop niter:       Overflow

It realizes it can satisfy both IVs using 1 candidate and picks it because it
thinks the costs are much lower.
I don't however see an implementation of TARGET_ADDRESS_COST for the target.

On AArch64 for instance this is rejected by costing because the combined IV
requires more registers:

Initial set of candidates:
  cost: 17 (complexity 0)
  reg_cost: 5
  cand_cost: 4
  cand_group_cost: 8 (complexity 0)
  candidates: 9
   group:0 --> iv_cand:9, cost=(4,0)
   group:1 --> iv_cand:9, cost=(0,0)
   group:2 --> iv_cand:9, cost=(0,0)
   group:3 --> iv_cand:9, cost=(4,0)
  invariant variables: 1
  invariant expressions: 1, 3

Improved to:
  cost: 14 (complexity 0)
  reg_cost: 6
  cand_cost: 8
  cand_group_cost: 0 (complexity 0)
  candidates: 7, 9
   group:0 --> iv_cand:7, cost=(0,0)
   group:1 --> iv_cand:9, cost=(0,0)
   group:2 --> iv_cand:9, cost=(0,0)
   group:3 --> iv_cand:7, cost=(0,0)
  invariant variables: 1
  invariant expressions: 1

Original cost 14 (complexity 0)

Final cost 14 (complexity 0)

Selected IV set for loop 1 at /app/example.c:4, 10 avg niters, 2 IVs:

I'll have to take a look at what happens when a target has no cost model for
IVopts.