Re: Default value of x87 registers' reserved bits

[Stathis Kamperis <ekamperi%gmail.com@localhost>]

2010/5/30 Joerg Sonnenberger <joerg%britannica.bec.de@localhost>:
> On Fri, May 28, 2010 at 04:54:50AM +0300, Stathis Kamperis wrote:
>> Control, status, etc x87 registers (which are part of the fp
>> environment) are 16 bit wide. In protected mode though, they are
>> accessed as 32 bit. The upper bits, 31-16, are marked as `reserved'
>> and are ought to be ignored. Their default value is _not_ mentioned in
>> the CPU specs. At the same time though, when we write to these
>> registers we must not tamper with the reserved bits.
>
> The code to extract the state should mask out the high level bits and
> code to set it should copy / keep them. The constant should not include
> them at all and it should not use a singleton.
>
> Joerg
>

Thanks Joerg for taking time to comment.

So you suggest that we provide a partial fp env representation with
FE_DFL_ENV, that gets completed every time in the level of its
consumers (feupdateenv, fesetenv).

There's a corner case though, that needs special treatment. For
example, what would happen if for some reason (?) the user did
something like:

fenv_t tmp;
memcpy(&tmp, FE_DFL_ENV, sizeof tmp);

and then at some point:

feupdateenv(&tmp);

For this to work, we should be doing a memcmp() inside feupdateenv()
and not just envp == FE_DFL_ENV, to decide if we should complete it or
not.

On the other hand, I'm not even sure that POSIX allows FE_DFL_ENV to
be used in any other case besides fenv.h related functions, e.g.
memcpy(). Here are the exact words from the specs:

"It (FE_DFL_ENV) can be used as an argument to the functions within
the <fenv.h> header that manage the floating-point environment."

I've looked up "can" in the POSIX base definitions, but it wasn't very
enlightening:
"The usage of can and may were selected to contrast optional
application behavior (can) against optional implementation behavior
(may)."

To cut a long story short, I tend to agree with you that we should go
this route. Also, if this any constellation, I checked glibc and they
do the same. If noone objects, I consider this case closed.


Thank you all.

Cheers,
Stathis