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From: "yann at droneaud dot fr" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/115095] New: [missed optimization] fixed processing on
 constant string
Date: Tue, 14 May 2024 20:06:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115095

            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=3D58208&action=3Dedit
source code of https://godbolt.org/z/Meqvbj8GG

I've found clang was able to compute the result of hashing a constant strin=
g 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 +=3D *s;
            h *=3D *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 ti=
me
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 compi=
le
time.

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