[Bug c/47781] warnings from custom printf format specifiers

[daniel.santos at pobox dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47781

--- Comment #19 from Daniel Santos <daniel.santos at pobox dot com> ---
(In reply to Martin Sebor from comment #18)
> The Linux kernel also has a bunch of printf format extensions that GCC
> doesn't know anything about:
> https://www.kernel.org/doc/Documentation/printk-formats.txt.

Further, the printf format extensions in the kernel are designed so as to not
create warnings and so are often two character combinations by using a standard
format specifier followed by a modifying character.  I think that I ran a
script once to count how much extra memory the two bytes vs a single byte take
and it ended up in the 10s of kilobytes.  While this may not sound like much,
remember that the kernel data is never paged out and on some embedded systems,
it actually does make a difference.

Should GCC begin supporting custom printf format specifiers, then I would
propose we begin changing them in the kernel to take advantage of that small
savings.