[RFC] x86_64: Add SSE sfp-exceptions

[Uros Bizjak]

> Can somebody of the x86 maintainers have a look at this patch, please?
>
> On 3/30/20 9:28 PM, Adhemerval Zanella via Libc-alpha wrote:
> > The exported x86_64 fenv.h functions operate on both i387 and SSE (since
> > they should work on both float, double, and long double) while the
> > internal libc_fe* set either SSE (float, double, and float128) or
> > i387 (long double).
> >
> > The libgcc __sfp_handle_exceptions (used on float128 implementation),
> > however, will set either SEE or i387 exception depending of the
> > exception to raise.  This broke the internal assumption of float128
> > where only SSE operations will be used.
> >
> > This patch reimplements the libgcc __sfp_handle_exceptions to use only
> > SSE operations and sets libgcc to use it instead of its own
> > implementation.
> >
> > And I think we should fix libgcc in a similar manner, since checking on
> > config/i386/64/sfp-machine.h it already only supports SSE rounding mode
> > and x86_64 ABI also expectes float128 to use SSE registers [1]
> > (although it is not clear on how future implementation might implement
> > it).
> >
> > Checked on x86_64-linux-gnu.

I don't think the new implementation is correct. Please consider
attached testcase that compares GCC implementation to the new
implementation. Specifically, PE, OF and UF exceptions do not work as
expected.

For the compiled testcase, GCC version returns:

$ ./a.out
 PE  .  .  .  .
  . UE  .  .  .
  .  . OF  .  .
  .  .  . ZE  .
  .  .  .  .  .
  .  .  .  . IE

where the new implementation (compile with -DNEW) returns:

$ ./a.out
  .  .  .  .  .
  .  .  .  .  .
 PE  . OF  .  .
  .  .  . ZE  .
  .  .  .  .  .
  .  .  .  . IE

For reference, with -m32 -DNEW (so, x87 math) , the new implementation return:

$ ./a.out
  .  .  .  .  .
  .  .  .  .  .
  .  .  .  .  .
  .  .  . ZE  .
  .  .  .  .  .
  .  .  .  . IE

which is again different from GCC version.

Uros.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: exc.c
Type: text/x-csrc
Size: 4674 bytes
Desc: not available
URL: <https://sourceware.org/pipermail/libc-alpha/attachments/20200419/961d8eaf/attachment.bin>