[Bug c/110660] New: conditional length reduction optimization

[juzhe.zhong at rivai dot ai via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110660

            Bug ID: 110660
           Summary: conditional length reduction optimization
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: juzhe.zhong at rivai dot ai
  Target Milestone: ---

Consider this following test:

#include <stdint.h>
int __attribute__((noipa))
add_loop (int32_t * __restrict x, int32_t n, int res, int * __restrict cond)
{
  for (int i = 0; i < n; ++i)
    if (cond[i])
      res += x[i];
  return res;
}


Current GCC can do vectorize reduction for RVV:

  <bb 4> [local count: 630715945]:
  ...

  _59 = .SELECT_VL (ivtmp_57, POLY_INT_CST [4, 4]);
  ivtmp_40 = _59 * 4;
  vect__4.10_43 = .LEN_MASK_LOAD (vectp_cond.8_41, 32B, _59, 0, { -1, ... });
  mask__18.11_45 = vect__4.10_43 != { 0, ... };
  vect__7.14_49 = .LEN_MASK_LOAD (vectp_x.12_47, 32B, _59, 0, mask__18.11_45);

  vect__ifc__33.15_51 = .VCOND_MASK (mask__18.11_45, vect__7.14_49, { 0, ...
});

  vect__34.16_52 = .COND_LEN_ADD ({ -1, ... }, vect_res_19.7_38, 
  vect__ifc__33.15_51, vect_res_19.7_38, _59, 0);

  ...

  <bb 5> [local count: 105119324]:
  _54 = .REDUC_PLUS (vect__34.16_52);
  _55 = res_11(D) + _54;


Actually, we can optmize "VCOND_MASK + COND_LEN_ADD" into single "COND_LEN_ADD"
with replacing the argument of "COND_LEN_ADD". 

Consider this following pattern:

dummy_mask = { -1, ... }
dummy_else_value = { 0, ... } ;; This is dummy for PLUS, since a + 0 = a

op1_2 = .VCOND_MASK (control_mask, op1_1, dummy_else_value);
result = .COND_LEN_ADD (dummy_mask, op0, op1_2, op2, loop_len, bias)

Since it is using dummy_mask and dummy_else_value, we can simplify this
operation into:

result = .COND_LEN_ADD (control_mask, op0, op1_1, op2, loop_len, bias)

To do this optimization, we can either do this optimization either in
middle-end
"match.pd" or in backend "combine pass" to handle this.

Which approach is better?

Thanks.