[Bug c++/105748] New: gcc ignores the pure attribute

[yubowen15 at foxmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105748

            Bug ID: 105748
           Summary: gcc ignores the pure attribute
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: yubowen15 at foxmail dot com
  Target Milestone: ---

Snippet:
const int g_value() __attribute__((pure));
int bar(int n) {
    int s = 0;
    for (int i=0; i<n; ++i) {
        s += g_value() * i;
    }
    return s;
}

Expected: call g_value only once
Actual: call g_value n times

The problem can be reproduced in godbolt compiler explorer:
1. command line argument: -O3 -march=skylake-avx512 -std=c++2b
2. gcc failed to do loop-invariant code motion: https://godbolt.org/z/PvWc1xYvr
3. clang successfully performed loop-invariant code motion:
https://godbolt.org/z/aEhMT88jM