From: Stephen Rothwell <sfr@canb.auug.org.au>
Date: Mon, 25 Aug 2008 21:28:33 +1000
I think __builtin_constant_p() is not seeing something it
should here.
It isn't accepting:
--------------------
static inline __attribute__((const))
bool is_power_of_2(unsigned long n)
{
return (n != 0 && ((n & (n - 1)) == 0));
}
--------------------
and thus is_power_of_2(256) as being a constant.
If a 'const' inline function being passed a const argument isn't
constant, what is! :-)
I bet the problem is the fact that is_power_of_2() is a function.
I did some tests and I can only trigger this problem with gcc-3.4 on
sparc, gcc-4.1 and gcc-4.2 worked fine.
It triggers with both -O2 and -Os for the following simple test case:
static inline __attribute__((const))
int is_power_of_2(unsigned long n)
{
return (n != 0 && ((n & (n - 1)) == 0));
}
extern int bar;
int main(void)
{
if (!__builtin_constant_p(is_power_of_2(256)))
bar++;
return 0;
}
davem@sunset:~/src/GIT/linux-2.6$ gcc-3.4 -Os -o x x.c
/tmp/ccy8FzD8.o: In function `main':
x.c:(.text+0x0): undefined reference to `bar'
x.c:(.text+0x4): undefined reference to `bar'
x.c:(.text+0xc): undefined reference to `bar'
collect2: ld returned 1 exit status
davem@sunset:~/src/GIT/linux-2.6$ gcc-3.4 -O2 -o x x.c
/tmp/ccQNzZuj.o: In function `main':
x.c:(.text+0x0): undefined reference to `bar'
x.c:(.text+0x4): undefined reference to `bar'
x.c:(.text+0xc): undefined reference to `bar'
collect2: ld returned 1 exit status
davem@sunset:~/src/GIT/linux-2.6$
--
