https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105682
Bug ID: 105682 Summary: Both `-Wsuggest-attribute=pure` and `-Wsuggest-attribute=const` on same function Product: gcc Version: 12.1.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: sagebar at web dot de Target Milestone: --- The following example causes both a `-Wsuggest-attribute=pure` and `-Wsuggest-attribute=const` warning for the same function. Additionally, in this example, only `-Wsuggest-attribute=pure` would be correct (the `-Wsuggest-attribute=const` is completely bogus): Compile with `gcc -c -O2 -Wsuggest-attribute=const -Wsuggest-attribute=pure input.c`: ``` __attribute__((pure)) int callee(int x) { unsigned int a; __asm__ __volatile__(""); __asm__ __volatile__("" : "=X" (a)); x &= 1; if (a & 2) __asm__ __volatile__("" : "=m" (x)); return x & 4; } int caller(int x) { return callee(x); } ``` Output (gcc-12.1.0): >input.c: In function 'caller': >input.c:12:5: warning: function might be candidate for attribute 'pure' >[-Wsuggest-attribute=pure] > 12 | int caller(int x) { > | ^~~~~~ >input.c:12:5: warning: function might be candidate for attribute 'const' >[-Wsuggest-attribute=const] Output (gcc-11.2.0; expected behavior): >input.c: In function 'caller': >input.c:12:5: warning: function might be candidate for attribute 'pure' >[-Wsuggest-attribute=pure] > 12 | int caller(int x) { > | ^~~~~~ === After adding `__attribute__((pure))` to `caller` === ``` __attribute__((pure)) int caller(int x) { return callee(x); } ``` gcc-12.1.0 (bogus warning: `caller()` has no right to be const; it calls a pure function, and that function even contains inline assembly): >infile.c: In function 'caller': >infile.c:13:5: warning: function might be candidate for attribute 'const' >[-Wsuggest-attribute=const] > 13 | int caller(int x) { > | ^~~~~~ gcc-11.2.0 (expected behavior): > (No warnings) ======================= NOTES: - Not only does gcc suggest to add both pure & const to the same function, the later wouldn't even make any sense in this scenario (though suggesting `pure` is correct). - The strange-looking body of `callee()` is required to reproduce the bug. - Trying to further simplify it tends to make the warning go away, suggesting a problem with some kind of heuristic. - I know I also just reported https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105676, but that one is about a bogus `-Wsuggest-attribute=pure` warning, while this one is about a bogus `-Wsuggest-attribute=const` warning. The two bugs may be related, but simply suppressing `-Wsuggest-attribute=pure` on const-functions wouldn't be enough to fix the bug addressed in this report.