Re: [PATCH 02/10] x86: Convert BUG() to use unreachable()
From: David Daney
Date: Fri Sep 11 2009 - 11:56:33 EST
Ingo Molnar wrote:
* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
On 2009年9月10日, H. Peter Anvin wrote:
On 09/10/2009 04:56 PM, David Daney wrote:
Use the new unreachable() macro instead of for(;;);. When
allyesconfig is built with a GCC-4.5 snapshot on i686 the size of the
text segment is reduced by 3987 bytes (from 6827019 to 6823032).
Signed-off-by: David Daney <ddaney@xxxxxxxxxxxxxxxxxx>
CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
CC: x86@xxxxxxxxxx
Acked-by: H. Peter Anvin <hpa@xxxxxxxxx>
... although of course this clashes with Roland McGrath's
patchset for the same thing which I applied earlier. I have to
say I like unreachable() in lower case better though...
I like David's version a bit better, since it takes care of more
architectures, and also because it avoids that butt-ugly special
case for gcc-4.4.1-RH-relase-10 backporting this feature.
I realize that the RH backport thing is good for testing now, but
at the same time, it really does look nasty. I wonder if we could
add some config-time compiler feature testing - so that you'd not
have a version test at all, but a CONFIG_BUILTIN_UNREACHABLE.
There are other cases where that kind of config-time testing could
be useful, and we could avoid doing various gcc checks dynamically
from within 'make' (along with checking for known-buggy versions
etc).
And yeah, it looks better in lower case. That said, I don't care
_that_ much, and people can fight it out.
Another detail that would be nice to be fixed is to propagate the
unreachable() call into the !CONFIG_BUG case as well in
asm-generic/bug.h.
Perhaps, although with a pre-GCC-4.5 compiler, it would end up adding
the endless loop code to BUG() sites for configurations where the user
has explicitly stated their preference for the code to be as small as
possible (by turning off CONFIG_BUG).
If that is acceptable, I will prepare another revision of the patch set.
Thanks,
David Daney
That would kill dozens of !CONFIG_BUG compiler warnings and would
make a dont-allow-warnings policy a possibility for random builds.
Ingo
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