[Bug tree-optimization/98960] New: Failure to optimize accumulate loop to mul

[Bug tree-optimization/98960] New: Failure to optimize accumulate loop to mul

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98960

            Bug ID: 98960
           Summary: Failure to optimize accumulate loop to mul
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

int f(uint8_t max_value) {
    int sum = 0;
    for (int i = 0; i < max_value; i++) {
        sum += i;
    }
    return sum;
}

This can be optimized to `return (uint64_t)max_value * (max_value - 1) >> 1;`.
This transformation is done by LLVM, but not by GCC.

[Bug tree-optimization/98960] Failure to optimize accumulate loop to mul

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98960

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-02-04

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Final value replacement has

(set_scalar_evolution
  instantiated_below = 5
  (scalar = sum_7)
  (scalar_evolution = {0, +, {1, +, 1}_1}_1))
)
(chrec_apply 
  (varying_loop = 1 
) 
  (chrec = {0, +, {1, +, 1}_1}_1)
  (x = (unsigned int) max_value_4(D) + 4294967295)
  (res = scev_not_known))
not replacing:
  sum_1 = PHI <sum_7(3)>

because it doesn't know this (IIRC there's a duplicate bug where I tried some
things to handle this).

[Bug tree-optimization/98960] Failure to optimize accumulate loop to mul

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98960

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |llvm at rifkin dot dev

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 110203 has been marked as a duplicate of this bug. ***

[Bug tree-optimization/98960] Failure to optimize accumulate loop to mul

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98960

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|NEW                         |RESOLVED

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 65855.

*** This bug has been marked as a duplicate of bug 65855 ***