Re: linux-next: build failure

[David Miller <davem@davemloft.net>]

From: Stephen Rothwell <sfr@canb.auug.org.au>
Date: Mon, 25 Aug 2008 21:28:33 +1000

> Hi Ingo,
> 
> Today's linux-next build (sparc64 defconfig) failed like this:
> 
> ERROR: "__BUILD_BUG_ON_non_constant" [drivers/net/sunvnet.ko] undefined!
> ERROR: "__BUILD_BUG_ON_non_constant" [drivers/block/sunvdc.ko] undefined!
> 
> Probably intorduced by commit f5b5d41dd51a31fe70e3a04fb80a3b90b84c6a4e
> ("debug: fix BUILD_BUG_ON() for non-constant expressions").

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$