2.6.28-rc6-tip fails to build with gcc 4.4.0

2.6.28-rc6-tip fails to build with gcc 4.4.0

[Török Edwin]

Hi,

Using gcc version 4.4.0 20081120 (experimental) [trunk revision 142055]
I can't build latest tip/master (commit
b27fef5b222564b6e7ae71f7110c233b2fb45a38)

drivers/built-in.o: In function `sd_revalidate_disk':
sd.c:(.text+0x8bc76): undefined reference to `____ilog2_NaN'
make: *** [.tmp_vmlinux1] Error 1

Apparently gcc determines that sector_size can have an invalid constant
value as ilog2's parameter in sd_read_capacity.

I tried to narrow down, but all I found so far is that gcc 4.4.0 now
says that __builtin_constant_p() is true even in these cases:

a = .. constant value ...
if (cond) {
     a = ... constant value ...
}
__builtin_constant_p(a) ? ....

a = .. constant value ...
if (cond) {
     a = ... constant value ...
} else {
   a = ... another constant value ...
}
__builtin_constant_p(a) ? ....

I guess it clones the __builtin_constant_p for each branch, where a is
constant, gcc 4.3 doesn't seem to do that.

Did it find a real bug in sd.c, or is it a gcc bug?

Best regards,
--Edwin

Re: 2.6.28-rc6-tip fails to build with gcc 4.4.0

[Markus Trippelsdorf]

On Mon, Nov 24, 2008 at 04:28:13PM +0200, Török Edwin wrote:
> Hi,
> 
> Using gcc version 4.4.0 20081120 (experimental) [trunk revision 142055]
> I can't build latest tip/master (commit
> b27fef5b222564b6e7ae71f7110c233b2fb45a38)
> 
> drivers/built-in.o: In function `sd_revalidate_disk':
> sd.c:(.text+0x8bc76): undefined reference to `____ilog2_NaN'
> make: *** [.tmp_vmlinux1] Error 1
> 
> Apparently gcc determines that sector_size can have an invalid constant
> value as ilog2's parameter in sd_read_capacity.

> Did it find a real bug in sd.c, or is it a gcc bug?

See: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36359

Setting CONFIG_CC_OPTIMIZE_FOR_SIZE=y in your .config should fix it for
now.

-- 
Markus