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Re: Builtin expansion versus headers optimization: Reductions


On Fri, 5 Jun 2015, Mikhail Maltsev wrote:

> There are other issues with macros in glibc headers (well, not as
> significant as performance-related concerns, but never the less).
> 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24016#c3
> 
> > #include <string.h>
> > int foo(void *x) {
> >     return strcmp(x + 1, "test");
> > }
> > 
> > does not cause warnings when compiled with -Wpointer-arith -O1
> > (glibc v. 2.17). It can be reduced to:
> > 
> > int foo(void *x) {
> >     return __extension__({ __builtin_strcmp(x + 1, "test"); });
> > }
> 
> In this case __extension__ inhibits -Wpointer-arith, because void*
> arithmetics is a valid GNU extension, but the user does not even know
> that he is using it.

In <https://gcc.gnu.org/ml/gcc/2001-04/msg00642.html> I suggested 
__unextension__ to allow macros to avoid suppressing such warnings inside 
the expansion of macro arguments (given that feature, lots of macro 
definitions would then need to change to use __unextension__ (ARG) instead 
of just (ARG)).

-- 
Joseph S. Myers
joseph@codesourcery.com


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