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* [Bug libc/15720] New: stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__
@ 2013-07-06 4:10 bugdal at aerifal dot cx
2013-07-06 20:58 ` [Bug libc/15720] " joseph at codesourcery dot com
` (7 more replies)
0 siblings, 8 replies; 9+ messages in thread
From: bugdal at aerifal dot cx @ 2013-07-06 4:10 UTC (permalink / raw)
To: glibc-bugs
http://sourceware.org/bugzilla/show_bug.cgi?id=15720
Bug ID: 15720
Summary: stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__
Product: glibc
Version: unspecified
Status: NEW
Severity: normal
Priority: P2
Component: libc
Assignee: unassigned at sourceware dot org
Reporter: bugdal at aerifal dot cx
CC: drepper.fsp at gmail dot com
Based on the comment in stdc-predef.h, somebody apparently assumed
__STDC_IEC_559_COMPLEX__ means the implementation supports complex arithmetic.
This is not what it means. Complex arithmetic is mandatory in C99, and in C11
implementations that lack complex arithmetic must define __STDC_NO_COMPLEX__.
The meaning of __STDC_IEC_559_COMPLEX__ is that the implementation conforms to
Annex G, which among other things, requires the existence of the
otherwise-optional _Imaginary types which GCC does not support.
This is a fairly serious issue for applications, since on an implementation
conforming to Annex G, I*INFINITY is interpreted as intended, whereas on an
implementation lacking _Imaginary types and defining I as _Complex_I,
I*INFINITY results in a NAN due to the 0*INFINITY term.
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* [Bug libc/15720] stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__
2013-07-06 4:10 [Bug libc/15720] New: stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__ bugdal at aerifal dot cx
@ 2013-07-06 20:58 ` joseph at codesourcery dot com
2013-07-06 23:53 ` bugdal at aerifal dot cx
` (6 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: joseph at codesourcery dot com @ 2013-07-06 20:58 UTC (permalink / raw)
To: glibc-bugs
http://sourceware.org/bugzilla/show_bug.cgi?id=15720
--- Comment #1 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
I think this is no different from defining __STDC__ in GCC's default mode,
or defining __STDC_VERSION__ to indicate the selected standards mode
rather than to indicate full conformance to a particular standard version.
That is, it indicates intent rather than completeness (for example,
special cases of functions try to follow Annex G).
The main issue with implementing _Imaginary support in GCC would be
avoiding the ABI mess that arose from supporting complex numbers on all
targets without any thought about what the ABIs for argument passing and
return should be. That is, it would be appropriate to disable it except
for targets enabling it by a target hook that means any necessary
target-specific ABI work has been done including coordinating with anyone
responsible for the architecture ABI definition. (Saying that imaginary
types are passed like the corresponding real types is a tempting default -
except that float _Imaginary does *not* promote to double _Imaginary when
passed in variable arguments, so an unpromoted float _Imaginary could be
passed there where an unpromoted float never could.)
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* [Bug libc/15720] stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__
2013-07-06 4:10 [Bug libc/15720] New: stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__ bugdal at aerifal dot cx
2013-07-06 20:58 ` [Bug libc/15720] " joseph at codesourcery dot com
@ 2013-07-06 23:53 ` bugdal at aerifal dot cx
2013-07-07 16:21 ` joseph at codesourcery dot com
` (5 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: bugdal at aerifal dot cx @ 2013-07-06 23:53 UTC (permalink / raw)
To: glibc-bugs
http://sourceware.org/bugzilla/show_bug.cgi?id=15720
--- Comment #2 from Rich Felker <bugdal at aerifal dot cx> ---
On Sat, Jul 06, 2013 at 08:58:24PM +0000, joseph at codesourcery dot com wrote:
> I think this is no different from defining __STDC__ in GCC's default mode,
> or defining __STDC_VERSION__ to indicate the selected standards mode
> rather than to indicate full conformance to a particular standard version.
> That is, it indicates intent rather than completeness (for example,
> special cases of functions try to follow Annex G).
I disagree with this assessment. There's a big difference between
minor conformance errors (like excess ulps of error) and omitting
required types that make a valid Annex G program into a compile-time
constraint violation. If applications can't rely on the absence of
this macro to tell them _Imaginary is definitely not available, what
good is the macro?
> The main issue with implementing _Imaginary support in GCC would be
> avoiding the ABI mess that arose from supporting complex numbers on all
Complex numbers are already supported. Imaginary types have the exact
same representation as their corresponding real floating point types,
so they're much easier to support than complex types. Your explanation
for why GCC does not support them does not make sense. If GCC does add
imaginary type support, it should not be target-specific.
> responsible for the architecture ABI definition. (Saying that imaginary
> types are passed like the corresponding real types is a tempting default -
> except that float _Imaginary does *not* promote to double _Imaginary when
> passed in variable arguments, so an unpromoted float _Imaginary could be
> passed there where an unpromoted float never could.)
I wasn't aware of this issue, but I don't think it matters. There's no
reason from an ABI standpoint why you can't promote it, as long as
va_arg, when passed _Imaginary float as the type, knows to perform the
actual read as _Imaginary double instead.
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* [Bug libc/15720] stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__
2013-07-06 4:10 [Bug libc/15720] New: stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__ bugdal at aerifal dot cx
2013-07-06 20:58 ` [Bug libc/15720] " joseph at codesourcery dot com
2013-07-06 23:53 ` bugdal at aerifal dot cx
@ 2013-07-07 16:21 ` joseph at codesourcery dot com
2013-07-07 17:07 ` bugdal at aerifal dot cx
` (4 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: joseph at codesourcery dot com @ 2013-07-07 16:21 UTC (permalink / raw)
To: glibc-bugs
http://sourceware.org/bugzilla/show_bug.cgi?id=15720
--- Comment #3 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Sat, 6 Jul 2013, bugdal at aerifal dot cx wrote:
> > I think this is no different from defining __STDC__ in GCC's default mode,
> > or defining __STDC_VERSION__ to indicate the selected standards mode
> > rather than to indicate full conformance to a particular standard version.
> > That is, it indicates intent rather than completeness (for example,
> > special cases of functions try to follow Annex G).
>
> I disagree with this assessment. There's a big difference between
> minor conformance errors (like excess ulps of error) and omitting
> required types that make a valid Annex G program into a compile-time
> constraint violation. If applications can't rely on the absence of
> this macro to tell them _Imaginary is definitely not available, what
> good is the macro?
Not much good. You can't use __STDC_VERSION__ in practice to tell what
level of C11 features is supported, can't use __STDC__ to tell if
trigraphs are supported, and to the extent that you can use
__STDC_VERSION__ as an indication of availability of various C99 features
(albeit language features only, not library features even when defined
together with __STDC_HOSTED__), it took many years after compilers started
having C99 modes for there to be a reasonable consensus on a common
subsets of C99 features you could expect to be supported in such a mode
(which doesn't include anything from Annex F or Annex G, or UCNs in
identifiers, for example).
Such macros are only reliable for the standard-given semantics if the
application knows it is being built by an implementation with some
externally-given conformance statement covering the implementation as a
whole (compiler and library) (at which point they may be used to identify
which conformance options are in use). For normal compilations with
implementations documented not to implement everything in the standard,
the macros may be of heuristic value but may also need using together with
external information about particular implementations, or information
obtained from autoconf configure tests.
> > responsible for the architecture ABI definition. (Saying that imaginary
> > types are passed like the corresponding real types is a tempting default -
> > except that float _Imaginary does *not* promote to double _Imaginary when
> > passed in variable arguments, so an unpromoted float _Imaginary could be
> > passed there where an unpromoted float never could.)
>
> I wasn't aware of this issue, but I don't think it matters. There's no
> reason from an ABI standpoint why you can't promote it, as long as
> va_arg, when passed _Imaginary float as the type, knows to perform the
> actual read as _Imaginary double instead.
Of course the ABI could be defined to promote it, though I think it would
make more sense not to promote it - but the fact there is a reasonable
choice about the ABI in this area is sufficient reason why the "you"
deciding what choice to make needs to be the ABI maintainers (or in the
absence of such ABI maintainers, any de facto group that coordinates ABI
issues between implementations (the ia32-abi Google Group in the case of
32-bit x86 ELF, for example)) rather than one implementor.
Just look at the 32-bit Power Architecture ABI (unified version - "Power
Architecture(R) 32-bit Application Binary Interface Supplement 1.0 - Linux
& Embedded") for the complexities of how complex types are handled
(complete with two different ABI options for them in the EABI case,
matching different implementations) for what you get if you let the ABI
just be what happens by chance on a particular architecture for a
particular implementation without anything actually thinking about or
coordinating things on a per-architecture basis.
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* [Bug libc/15720] stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__
2013-07-06 4:10 [Bug libc/15720] New: stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__ bugdal at aerifal dot cx
` (2 preceding siblings ...)
2013-07-07 16:21 ` joseph at codesourcery dot com
@ 2013-07-07 17:07 ` bugdal at aerifal dot cx
2013-07-07 17:26 ` joseph at codesourcery dot com
` (3 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: bugdal at aerifal dot cx @ 2013-07-07 17:07 UTC (permalink / raw)
To: glibc-bugs
http://sourceware.org/bugzilla/show_bug.cgi?id=15720
--- Comment #4 from Rich Felker <bugdal at aerifal dot cx> ---
On Sun, Jul 07, 2013 at 04:21:04PM +0000, joseph at codesourcery dot com wrote:
> Not much good. You can't use __STDC_VERSION__ in practice to tell what
> level of C11 features is supported,
Combined with the other new __STDC_NO_*__ macros introduced in C11,
you should be able to determine which features are available.
> can't use __STDC__ to tell if
> trigraphs are supported,
This is a bug in GCC, but anybody using trigraphs intentionally or
otherwise should just be taken out back anyway...
Anyway, the definition of the __STDC_IEC_559_COMPLEX__ macro is just
misleading and wrong, so I think this should be fixed. Defining it
does not provide anything of value, and does cause problems for
applications.
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* [Bug libc/15720] stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__
2013-07-06 4:10 [Bug libc/15720] New: stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__ bugdal at aerifal dot cx
` (3 preceding siblings ...)
2013-07-07 17:07 ` bugdal at aerifal dot cx
@ 2013-07-07 17:26 ` joseph at codesourcery dot com
2013-11-04 15:52 ` jsm28 at gcc dot gnu.org
` (2 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: joseph at codesourcery dot com @ 2013-07-07 17:26 UTC (permalink / raw)
To: glibc-bugs
http://sourceware.org/bugzilla/show_bug.cgi?id=15720
--- Comment #5 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Sun, 7 Jul 2013, bugdal at aerifal dot cx wrote:
> > Not much good. You can't use __STDC_VERSION__ in practice to tell what
> > level of C11 features is supported,
>
> Combined with the other new __STDC_NO_*__ macros introduced in C11,
> you should be able to determine which features are available.
No, because the set of macros for optional features doesn't correspond at
all to the various subsets of C11 features that may or may not be
implemented in compilers' C11 modes. Wait five or ten years and C11 modes
may be more reliably more complete. Your "should" is only for
applications known to be compiled only with compilers and libraries
documented to conform in the relevant ways, not for real-world partial C11
support.
> > can't use __STDC__ to tell if trigraphs are supported,
>
> This is a bug in GCC, but anybody using trigraphs intentionally or
> otherwise should just be taken out back anyway...
It's a deliberate choice dating back (and documented) to GCC 2.0 at least
that __STDC__ does not reflect whether a strict conformance mode is
selected.
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* [Bug libc/15720] stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__
2013-07-06 4:10 [Bug libc/15720] New: stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__ bugdal at aerifal dot cx
` (4 preceding siblings ...)
2013-07-07 17:26 ` joseph at codesourcery dot com
@ 2013-11-04 15:52 ` jsm28 at gcc dot gnu.org
2014-06-13 13:25 ` fweimer at redhat dot com
2023-01-25 7:16 ` sam at gentoo dot org
7 siblings, 0 replies; 9+ messages in thread
From: jsm28 at gcc dot gnu.org @ 2013-11-04 15:52 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=15720
Joseph Myers <jsm28 at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution|--- |INVALID
--- Comment #6 from Joseph Myers <jsm28 at gcc dot gnu.org> ---
As discussed, this macro is being defined to indicate intent rather than
completeness, just like __STDC_VERSION__, and can only be interpreted as
referring to completeness in the presence of external conformance documentation
asserting conformance for an implementation as a whole (and if such an
implementation asserted conformance for the GCC/glibc combination in a mode
where this macro is defined, that would be a bug in that external
documentation). After my commit 1484e65736f4cab27e5051e0f06be8470e69af82, the
comment explicitly refers to intent.
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* [Bug libc/15720] stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__
2013-07-06 4:10 [Bug libc/15720] New: stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__ bugdal at aerifal dot cx
` (5 preceding siblings ...)
2013-11-04 15:52 ` jsm28 at gcc dot gnu.org
@ 2014-06-13 13:25 ` fweimer at redhat dot com
2023-01-25 7:16 ` sam at gentoo dot org
7 siblings, 0 replies; 9+ messages in thread
From: fweimer at redhat dot com @ 2014-06-13 13:25 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=15720
Florian Weimer <fweimer at redhat dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Flags| |security-
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* [Bug libc/15720] stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__
2013-07-06 4:10 [Bug libc/15720] New: stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__ bugdal at aerifal dot cx
` (6 preceding siblings ...)
2014-06-13 13:25 ` fweimer at redhat dot com
@ 2023-01-25 7:16 ` sam at gentoo dot org
7 siblings, 0 replies; 9+ messages in thread
From: sam at gentoo dot org @ 2023-01-25 7:16 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=15720
Sam James <sam at gentoo dot org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |sam at gentoo dot org
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2013-07-06 4:10 [Bug libc/15720] New: stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__ bugdal at aerifal dot cx
2013-07-06 20:58 ` [Bug libc/15720] " joseph at codesourcery dot com
2013-07-06 23:53 ` bugdal at aerifal dot cx
2013-07-07 16:21 ` joseph at codesourcery dot com
2013-07-07 17:07 ` bugdal at aerifal dot cx
2013-07-07 17:26 ` joseph at codesourcery dot com
2013-11-04 15:52 ` jsm28 at gcc dot gnu.org
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