Re: [patch 1/12] misc fixes

Anton Altaparmakov (aia21@cantab.net)
Sat, 10 Aug 2002 13:05:59 +0100


At 02:24 10/08/02, David S. Miller wrote:
> From: Andrew Morton <akpm@zip.com.au>
> Date: Fri, 09 Aug 2002 17:55:24 -0700
>
> - `retval' should not be initialised to "~0UL" because that is
> 0x00000000ffffffff with 64-bit sector_t.
>
>'0' defaults to 'int' on 32-bit systems so ~0 isn't right either.
>
>What I think you really want is "~(sector_t) 0".

Actually, ~0 is fine. This is because of the associativity of events
happening. Breaking it down into individual ops as the compiler sees it:

1) constant 0 -> int type
2) ~0 -> (int)-1
3) assign to larger type sector_t -> sign extend smaller type to size of
larger type -> (sector_t)-1

Therefore not specifying anything after the initial constant definition
_always_ works because of sign extension and the order of evaluation which
is guaranteed by the C language.

OTOH if you hard code 0U or 0L or 0UL or whatever it breaks because it
forces the compiler not to sign extend and assume this to be a hardvalue,
which is zero extended.

Note I just tried all possible combinations with a little test program and
both assignments and binary operators work as expected (with sign
extension) when the constant is given without qualifiers and both break
horribly (no sign extension) when the constant is given with qualifiers.

I did this testing for another reason, and that is PAGE_MASK and
PAGE_CACHE_MASK which break when used for 64-bit values on 32-bit
architectures exactly for the above outlined reasons.

If no one beats me to it I will submit a patch to change PAGE_SIZE (and
thus automatically PAGE_CACHE_SIZE) on all architectures to no longer use
the "UL" type qualifier which is responsible for the breakage. This
supersedes my previous ugly patch introducing a PAGE_{CACHE_,}MASK_LL...

Best regards,

Anton

-- 
   "I've not lost my mind. It's backed up on tape somewhere." - Unknown
-- 
Anton Altaparmakov <aia21 at cantab.net> (replace at with @)
Linux NTFS Maintainer / IRC: #ntfs on irc.openprojects.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/

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