Segment register support for the i386

[Linus Torvalds]

In article < 199912241641.RAA05275@delius.kettenis.local >,
Mark Kettenis  <kettenis@wins.uva.nl> wrote:
>
>Although I have a working example, I lot of work needs to be done to
>assure that the compiler can handle more complex cases correctly, and
>to make sure that the compiler signals invalid use of the segment
>registers correctly.

You might want to try talking to Steve Chamberlain: I know he tried to
do something similar and he may have suggestions (the suggestion may be
"forget it - it's too hairy", but it doesn't hurt to ask). 

I know there were horrible problems with disambiguating pointer
dereferences that looked the same: CSE needs to be _very_ careful.  If
you go ahead with this, would suggest adding a case like this to your
test-suite:

	int test_it(char *ptr)
	{
		char * gs_ptr __attribute("gsseg");

		gs_ptr = ptr;
		return *ptr == *gs_ptr;
	}

If done right, you should see (a) a warning for different types at the
gs_ptr assignment and (b) the compiler must NOT optimize away the
equality test, because while the pointers "look" the same to some degree
from a CSE standpoint, they obviously dereference potentially very
different values. 

I think pointer attributes could be very useful (alignment attributes,
calling convention attributes for function pointers etc), but there are
nasty issues with type checking etc.

		Linus