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List: linux-kernel
Subject: Re: [PATCH][TRIVIAL] Remove bogus "value 0x37ffffff truncated to
From: Davide Libenzi <davidel () xmailserver ! org>
Date: 2004-01-10 21:04:47
Message-ID: Pine.LNX.4.44.0401101243110.2210-100000 () bigblue ! dev ! mdolabs ! com
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On Sat, 10 Jan 2004, Bart Samwel wrote:
> > > --- page.h.orig 2004-01-10 18:15:17.000000000 +0100
> > > +++ page.h 2004-01-10 18:15:47.000000000 +0100
> > > @@ -123,7 +123,7 @@
> > >
> > > #define PAGE_OFFSET ((unsigned long)__PAGE_OFFSET)
> > > #define VMALLOC_RESERVE ((unsigned long)__VMALLOC_RESERVE)
> > > -#define MAXMEM (-__PAGE_OFFSET-__VMALLOC_RESERVE)
> > > +#define MAXMEM (0xFFFFFFFF-__PAGE_OFFSET-__VMALLOC_RESERVE+1)
> >
> >
> > Try:
> >
> > #define MAXMEM (~__PAGE_OFFSET + 1 - __VMALLOC_RESERVE)
>
> I tried that first, before I came up with the solution in the patch,
> because I didn't like the dependency of 0xFFFFFFFF being 32-bit. It was
> a nice idea, but it didn't work. Apparently, gas interprets ~ as a one's
> complement negation operator, not a bitwise or. Therefore,
> ~__PAGE_OFFSET is just as negative as -__PAGE_OFFSET as far as gas is
> concerned. It gives me the same warning.
That would mean a bug in as. __PAGE_OFFSET is unsigned and ~ is documented
(not a surprise) as "bitwise not". The bitwise not of __PAGE_OFFSET
(unsigned) is still unsigned. BTW 2.14 does not give warnings with both
the original statement and the ~ one. This:
\
PG=0xC0000000 \
VM=(128 << 20) \
\
mov (~PG + 1 - VM), %eax \
mov (-PG - VM), %eax \
\
generate this:
zzzzzzzz: file format elf32-i386
Disassembly of section .text:
00000000 <.text>:
0: a1 00 00 00 38 mov 0x38000000,%eax
5: a1 00 00 00 38 mov 0x38000000,%eax
w/out any warnings. And the result is obviously 0x38000000 and
not 0x37ffffff.
- Davide
-
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