[U-Boot] fdt performance

[Wolfgang Denk]

Dear Andre,

In message <CAPfzE3a2ne-xcjKUTK8WS78V0yxuSd50wSQvm1rSPgNUfwP4Ow at mail.gmail.com> you wrote:
> 
> >> Some of the checks in fdt_offset_ptr also look useless, such as if
> >> ((offset + len) < offset) which will always be false, or
> >> if (p + len < p)
> >
> > What happens if the "offset" or "p" point to addresses close to the
> > upper end of the address space, and adding "len" makes it wrap around?
> 
> I'm not sure how particular U-Boot is about this, but the C standard
> doesn't specify what to do in the situation of signed overflow, so

These are no signed numbers, right?

> it's possible that these checks could be simply optimised away. The

This is not hwat happens. 

> portable way to write it (I believe) is:
> if (INT_MAX - len < offset). I don't know what GCC does in this
> situation specifically though.

This has nothing to do with GCC.  It's a standard C question. Inmy
understanding, the expression "offset + len" (with "offset" being
"int", and "len" being "unsigned int"), will give an "unsigned int";
the comparison willt hen also be done using "unsigned int" data types.

So if you want to write a "portable" expression (though I have to
admit that I don't see how this would be more portable, or how the
current code is less portable), that would be:

	(UINT_MAX - len < offset)

At least that would give the same results - but to me the meaning
would be totally unclear when I read the code - while I think I
understand the current form.

Best regards,

Wolfgang Denk

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