linux-kernel - 2.6.28-rc6-tip fails to build with gcc 4.4.0

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Message-ID: <492AB9FD.5010602@gmail.com>
Date:	Mon, 24 Nov 2008 16:28:13 +0200
From:	Török Edwin <edwintorok@...il.com>
To:	James.Bottomley@...senPartnership.com
CC:	linux-scsi@...r.kernel.org,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: 2.6.28-rc6-tip fails to build with gcc 4.4.0

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
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