> On 16 Nov 2022, at 15:27, Richard Biener <richard.guenther@xxxxxxxxx> wrote: > > On Wed, Nov 16, 2022 at 4:02 PM Michael Matz via Gcc <gcc@xxxxxxxxxxx> wrote: >> >> Hey, >> >> On Wed, 16 Nov 2022, Alexander Monakov wrote: >> >>>> The idea is so obvious that I'm probably missing something, why autoconf >>>> can't use that idiom instead. But perhaps the (historic?) reasons why it >>>> couldn't be used are gone now? >>> >>> Ironically, modern GCC and LLVM optimize '&foobar != 0' to '1' even at -O0, >>> and thus no symbol reference remains in the resulting assembly. >> >> Err, right, *head-->table*. >> Playing with volatile should help: >> >> char foobar(void); >> char (* volatile ptr)(void); >> int main(void) { >> ptr = foobar; >> return ptr != 0; >> } > > using printf for foobar this works even with GCC 2.95.2 and with trunk > and -Wall diagnoses > > t.c:1:6: warning: conflicting types for built-in function 'printf'; > expected 'int(const char *, ...)' [-Wbuiltin-declaration-mismatch] > 1 | char printf(void); > | ^~~~~~ > t.c:1:1: note: 'printf' is declared in header '<stdio.h>' > +++ |+#include <stdio.h> > 1 | char printf(void); > > so without -Werror this should be fine. > Unrelated but I was a bit tempted to ask for throwing in -Wbuiltin-declaration-mismatch to default -Werror while Clang 16 was at it, but I suppose we don't want the world to burn too much, and it's got a very obvious usecase (this one) whereas implicit func decls are too hard to justify. > Richard.
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