With the exception of built-ins with the ellipsis (like sprintf),
GCC silently accepts declarations of built-in functions without
prototypes as well as calls to such functions with any numbers
or types of arguments, compatible or otherwise.  Calls with
arguments whose number and types match exactly those of
the built-in are considered by the middle-end for optimization.
Other calls (compatible or not, irrespective of whether their
number matches the number expected by the function) are then
made to the library functions.

Attached is a small fix to -Wbuiltin-declaration-mismatch to
have it diagnose built-in declarations without a prototype.
The warning is enabled by default so it causes a fair number
of tests to fail because of declarations like 'void abort();'
The breakdown of the built-ins behind the test failures is
below.

Before I take the time to clean up the test suite let me post
what I have in case going this route is not acceptable.  As
an alternative, I could try to avoid some of these warnings,
e.g., by diagnosing incompatible calls instead but I think
it's even less worthwhile for built-ins than trying to do
it for ordinary functions with -Wstrict-prototypes.  There
is, in my view, no justification today for standard functions
to be declared without a prototype.  (I could also make
the warning depend on language mode and -Wpedantic if that
were more preferable.)

Martin

About 115 tests fail due to incompatible declarations of
the built-in functions below (the number shows the number
of warnings for each functions):

428   abort
  58   exit
  36   memcpy
  17   memmove
  15   realloc
  14   cabs
   5   strncpy
   4   strcmp
   3   alloca
   2   rindex
   1   aligned_alloc