Re: Add new macro gl_WARN_ADD_MULTIPLE

[Bruno Haible]

Hi Alex,

> GNU Emacs spends about ~6s out of ~28s in its configuration process to
> determine supported C compiler warnings.

It's good that you tackle this problem. Thanks!

> This macro first checks if all the supplied warnings are
> supported, and then only checks them individually (via gl_WARN_ADD) if
> that check fails.

So, the speed improvement exists only for the newest compilers; people
who use a compiler that was released 1 or 2 years before the Emacs release
will see no improvement.

How about a modified algorithm?
  1) In a first pass, use the GCC's --help=warnings output to determine which
     warnings are likely supported. Unfortunately, for clang, I don't know
     of such a help option.
  2) When the first check (with all the options) fails, split that list into
     2 (or 3, or 4) chunks of nearly equal size, and perform the check
     on each sub-list. And so on, recurse.
     This way, you would still get some performance benefit if the compiler
     supports 80% of the requested warnings but not all of them.

Finally, since there is no semantic difference between gl_WARN_ADD and
gl_WARN_ADD_MULTIPLE, except that the latter allows multiple options, how
about extending the gl_WARN_ADD macro (to allow multiple options) instead
of defining a different macro (gl_WARN_ADD_MULTIPLE)?

Bruno