[Bug c/115095] New: [missed optimization] fixed processing on constant string

[Bug c/115095] New: [missed optimization] fixed processing on constant string

[yann at droneaud dot fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115095

            Bug ID: 115095
           Summary: [missed optimization] fixed processing on constant
                    string
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: yann at droneaud dot fr
  Target Milestone: ---

Created attachment 58208
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58208&action=edit
source code of https://godbolt.org/z/Meqvbj8GG

I've found clang was able to compute the result of hashing a constant string at
compile time. I would have hope GCC -O3 would be able to optimize such
computation as well:

    static inline unsigned int hash(unsigned int h, const char *s)
    {
        while (*s) {
            h += *s;
            h *= *s++;
        }

        return h;
    }

    #define LOCATION() hash(hash(0, __FILE__), __func__)

    unsigned int location(void)
    {
        return LOCATION();
    }

is translated by clang to

    location:
        movl    $1418535820, %eax
        retq

but not by GCC, which doesn't compute the value of LOCATION() at compile time
but emit code that compute the value at runtime.

At first, I thought it was an issue with handling __FILE__ or __func__, but
trying with other string constants, GCC is not computing the value at compile
time.

See https://godbolt.org/z/Meqvbj8GG

[Bug middle-end/115095] [missed optimization] fixed processing on constant string

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115095

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
          Component|c                           |middle-end

[Bug middle-end/115095] [missed optimization] fixed processing on constant string

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115095

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
                 CC|                            |hubicka at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2024-05-15

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC isn't able to compute the number of iterations of a loop like the following
which would be required for us to peel it and do the required constant folding.
We're also not trying brute-force peeling for loops we cannot compute an
upper bound of its number of iterations (attempting to arrive at one when
evaluating the peeled stmts).

  <bb 3> [local count: 955630224]:
  # _26 = PHI <_9(8), 116(2)>
  # h_27 = PHI <h_15(8), 0(2)>
  # s_29 = PHI <s_14(8), "t.c"(2)>
  _10 = (unsigned int) _26;
  h_12 = _10 + h_27;
  s_14 = s_29 + 1;
  h_15 = _10 * h_12;
  _9 = *s_14;
  if (_9 != 0)
    goto <bb 8>; [89.00%]
  else
    goto <bb 4>; [11.00%]

  <bb 8> [local count: 850510900]:
  goto <bb 3>; [100.00%]