[Bug libstdc++/114742] New: invalid use of '__ieee128' in <ext/numeric_traits.h> and <format>

[Bug libstdc++/114742] New: invalid use of '__ieee128' in <ext/numeric_traits.h> and <format>

[mkretz at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114742

            Bug ID: 114742
           Summary: invalid use of '__ieee128' in <ext/numeric_traits.h>
                    and <format>
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mkretz at gcc dot gnu.org
  Target Milestone: ---

https://compiler-explorer.com/z/K3GYY7Msc

Testcase:

#include <bit>
#include <format>

Compile on power64le with -O2 -std=c++23 -mcpu=power6 and optionally with 
-mlong-double-64

[Bug libstdc++/114742] invalid use of '__ieee128' in <ext/numeric_traits.h> and <format>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114742

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2024-04-16
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think we need something like this in <bits/c++config.h>

#ifndef __VSX__
# undef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
#endif

Otherwise we might set the ALT128_COMPAT macro during configure, but then it
becomes invalidated during compilation of user code if they "downgrade" to
something that doesn't support ieee128, using -mcpu=power6 and/or
-mlong-double-64

[Bug libstdc++/114742] invalid use of '__ieee128' in <ext/numeric_traits.h> and <format>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114742

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Mathias noted that still fails with -mcpu=power7

Checking for _ARCH_PWR8 or __POWER8_VECTOR__ instead works.

[Bug libstdc++/114742] invalid use of '__ieee128' in <ext/numeric_traits.h> and <format>

[bergner at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114742

Peter Bergner <bergner at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bergner at gcc dot gnu.org,
                   |                            |meissner at gcc dot gnu.org,
                   |                            |segher at gcc dot gnu.org

--- Comment #3 from Peter Bergner <bergner at gcc dot gnu.org> ---
Adding Mike and Segher to correct me if I'm wrong on anything... :-)

The canonical method for determining whether a specific cpu has VSX enabled or
not is checking for "#if defined(_ARCH_PWR*) && defined(__VSX__)".  I believe
we're trying to move away from testing things like "__POWER8_VECTOR__".

I seem to be blanking on the minimum cpu required for __ieee128, so I'll let
Mike or Segher comment on that.  I thought is was Power7, which would mean
using _ARCH_PWR7 above, but given -mcpu=power7 still fails, maybe Power8 is the
minimum needed for __ieee128 support?  Mike and Segher???

[Bug libstdc++/114742] invalid use of '__ieee128' in <ext/numeric_traits.h> and <format>

[meissner at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114742

--- Comment #4 from Michael Meissner <meissner at gcc dot gnu.org> ---
The minimum architecture for IEEE 128-bit support is power7, because it needs
the VSX registers to pass and return IEEE 128-bit values.

Now, in theory, IEEE 128-bit support could have required only Altivec support
(i.e. power6 adding -maltivec), but it was decided that the minimum processor
to support IEEE 128-bit would be power7.

Note, that the minimum CPU support for little endian is power8.  So, using
-mcpu=power6 or -mcpu=power7 on a LE system is not supported.

On power8 systems, IEEE 128-bit is emulated in software.  On power9 systems,
IEEE 128-bit has hardware support.

The macro __LONG_DOUBLE_IEEE128__ is defined if long double uses the IEEE
128-bit format, and __LONG_DOUBLE_IBM128__ is defined if long double uses the
IBM 128-bit format that uses a pair of doubles.

[Bug libstdc++/114742] invalid use of '__ieee128' in <ext/numeric_traits.h> and <format>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114742

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Michael Meissner from comment #4)
> The macro __LONG_DOUBLE_IEEE128__ is defined if long double uses the IEEE
> 128-bit format, and __LONG_DOUBLE_IBM128__ is defined if long double uses
> the IBM 128-bit format that uses a pair of doubles.

Yes, and libstdc++ relies on those heavily. But they isn't useful here, because
those macros say what the *active* long double format is. The point of the
_GLIBCXX_LONG_DOUBLE_ALT128_COMPAT macro is to say that the library needs to
support two long double formats at the same time. That is independent of which
one happens to be active during a given compilation.

There are files in libstdc++ which use both __ibm128 and __ieee128 in the same
translation unit, but we need to not do that when __ieee128 is disabled by an
option like -mlong-double-64


(In reply to Michael Meissner from comment #4)
> Note, that the minimum CPU support for little endian is power8.  So, using
> -mcpu=power6 or -mcpu=power7 on a LE system is not supported.

Ah! OK, so maybe this isn't an issue at all? The Compiler Explorer link in
comment 0 is using power64le for both examples. If I switch the selected
compilers to power64 (i.e. BE) there are no errors when using -mcpu=power6 or
-mlong-double-64

But maybe that's because the compiler explorer VM for power64 BE has an old
glibc without __float128 support in libm, and so in theory somebody could be
running BE on a new glibc and need ALT128_COMPAT.

[Bug libstdc++/114742] invalid use of '__ieee128' in <ext/numeric_traits.h> and <format>

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114742

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
At least glibc on powerpc64 be I think doesn't have any __float128/_Float128
support I think.

[Bug libstdc++/114742] invalid use of '__ieee128' in <ext/numeric_traits.h> and <format>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114742

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
OK, then ALT128_COMPAT is effectively only for LE.

So I think I'll make whatever change is needed so that ALT128_COMPAT isn't
defined for power6 LE and power7 LE, so the errors Matthias found don't happen.
But I won't spend time trying to actually make it work well on the unsupported
power6 LE combo.