Re: Implement -Wduplicated-branches (PR c/64279) (v3)

[Jakub Jelinek <jakub at redhat dot com>]

On Mon, Jan 09, 2017 at 10:21:47AM +0100, Marek Polacek wrote:
> +/* Callback function to determine whether an expression TP or one of its
> +   subexpressions comes from macro expansion.  Used to suppress bogus
> +   warnings.  */
> +
> +static tree
> +expr_from_macro_expansion_r (tree *tp, int *, void *)
> +{
> +  if (CAN_HAVE_LOCATION_P (*tp)
> +      && from_macro_expansion_at (EXPR_LOCATION (*tp)))
> +    return integer_zero_node;
> +
> +  return NULL_TREE;
> +}

I know this is hard issue, but won't it disable the warning way too often?

Perhaps it is good enough for the initial version (GCC 7), but doesn't it stop
whenever one uses NULL in the branches, or some other trivial macros like
that?  Perhaps we want to do the analysis if there is anything from macro
expansion side-by-side on both the expressions and if you find something
from a macro expansion, then still warn if both corresponding expressions
are from the same macro expansion (either only non-function like one, or
perhaps also function-like one with the same arguments, if it is possible
to figure out those somehow)?  And perhaps it would be nice to choose
warning level, whether you want to warn only under these rules (no macros
or something smarter if implemented) vs. some certainly non-default more
aggressive mode that will just warn no matter what macros there are.

	Jakub