[Bug c++/45265] GCC has an intermittent bug when computing the address of function parameters

["matz at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #36 from matz at gcc dot gnu dot org  2010-08-13 13:14 -------
> > If you include real segmentation
> > like on 80286, where there's no linear relationship between effective
> > address and segment+offset, subtraction would have been prohibitively
> > expensive to implement anyway.

> What you don't know is that when you subtracted far pointers the compiler
> generated the code to change seg16:ofs16 into abs20.

What in the words "real segmentation like on 286, where there's no linear
relationship between effective address and segment+offset" was unclear to
you to expect that abs20==seg16*16+ofs16?  The "prohibitive expensive"
referred to the necessary lookup of the base in the LDT/GDT that would have
been required for every far pointer subtraction.

> > And you still wouldn't get around the size limitation of ptrdiff_t that
> > was 16bit.

> What the hell are you talking about????? I personally disassembled code in
> the late 80's to see how the compiler implemented 32-bit arithmetic on a
> 286. It did, and it did it well. You weren't able to go beyond 16 bits in
> the 80's?

Did I say that?  Let's see: "size limitation of ptrdiff_t that was 16bit".
Nope.  I didn't.  The point being that if ptrdiff_t is only 16 bit, then
no matter how fantastically capable the compiler was in emitting 32bit
arithmetic, the result of subtracting to char pointers pointing more than
64KB (or in fact 32KB) apart would not fit into a ptrdiff_t.

> No, not me, I don't want to write nonsense on the web,

Maybe you don't necessarily want to.  But ... , well, there we are.

> I prefer to be clear headed. One never knows when stuff one wrote on the
> net will come back and bite one in the ass!!

I'm not sure you realize just how true that is.  But keep going, you're
by far one of the best trolls I've seen in GCC land.  Much better than
Pizarro.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45265