Re: somebody dropped a (warning) bomb

linux-os (Dick Johnson)
2007-02-09T13:16:11Z

On Fri, 9 Feb 2007, Jan Engelhardt wrote:

>
> On Feb 8 2007 16:42, Linus Torvalds wrote:
>>
>> Most C types don't, and some you can't even tell (do pointers generate
>> "signed" or "unsigned" comparisons?
>
> I'd say "neither", because both
>
> signed void *ptr; and
> unsigned void *xyz;
>
> are impossible (and dull at that). That's why you explicitly will
> have to cast one operand in a comparison to make it evident
> what sort of comparison is intended, i.e.
>
> if((unsigned long)ptr < PAGE_OFFSET)
>
> Further, giving again answer to the question whether they generate signed or
> unsigned comparisons: Have you ever seen a computer which addresses memory with
> negative numbers? Since the answer is most likely no, signed comparisons would

Yes, most all do. Indexed memory operands are signed displacements. See
page 2-16, Intel486 Microprocessor Programmer's Reference Manual. This
gets hidden by higher-level languages such as 'C'. It is also non-obvious
when using the AT&T (GNU) assembly language. However, the Intel documentation
shows it clearly:
MOV AL, [EBX+0xFFFFFFFF] ; -1

...will put the byte existing at the location just BEFORE the offset in
EBX, into register AL, while:

MOV AL, [EBX+0x00000001] ; +1

...will put the byte existing at the location just AFTER the offset in
EBX, into register AL.

Also, the documentation specifically states that sign extension is
provided for all effective address calculations.

> not make sense for me.
>
>> I'll argue that a compiler that
>> generates signed comparisons for them is broken, but it tends to be
>> something you can only see with a standards- conforming proghram if you
>> can allocate memory across the sign boundary, which may or may not be
>> true..)
>
> Jan
> --
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Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.24 on an i686 machine (5592.61 BogoMips).
New book: http://www.AbominableFirebug.com/
_
\x1a\x04

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