[Qemu-devel] [PATCH] configure: Force the C standard to gnu11

[Qemu-devel] [PATCH] configure: Force the C standard to gnu11

[Thomas Huth]

Different versions of GCC and Clang use different versions of the C standard.
This repeatedly caused problems already, e.g. with duplicated typedefs:

 https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html

or with for-loop variable initializers:

 https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html

To avoid these problems, we should enforce the C language version to the
same level for all compilers. Since our minimum compiler versions are
GCC v4.8 and Clang v3.4 now, and both basically support "gnu11" already,
this seems to be a good choice.

Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 configure | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/configure b/configure
index 79375af..07f8105 100755
--- a/configure
+++ b/configure
@@ -107,6 +107,9 @@ update_cxxflags() {
             -Wstrict-prototypes|-Wmissing-prototypes|-Wnested-externs|\
             -Wold-style-declaration|-Wold-style-definition|-Wredundant-decls)
                 ;;
+            -std=gnu11)
+                QEMU_CXXFLAGS=${QEMU_CXXFLAGS:+$QEMU_CXXFLAGS }-std=gnu++11
+                ;;
             *)
                 QEMU_CXXFLAGS=${QEMU_CXXFLAGS:+$QEMU_CXXFLAGS }$arg
                 ;;
@@ -585,7 +588,7 @@ ARFLAGS="${ARFLAGS-rv}"
 # left shift of signed integers is well defined and has the expected
 # 2s-complement style results. (Both clang and gcc agree that it
 # provides these semantics.)
-QEMU_CFLAGS="-fno-strict-aliasing -fno-common -fwrapv $QEMU_CFLAGS"
+QEMU_CFLAGS="-fno-strict-aliasing -fno-common -fwrapv -std=gnu11 $QEMU_CFLAGS"
 QEMU_CFLAGS="-Wall -Wundef -Wwrite-strings -Wmissing-prototypes $QEMU_CFLAGS"
 QEMU_CFLAGS="-Wstrict-prototypes -Wredundant-decls $QEMU_CFLAGS"
 QEMU_CFLAGS="-D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE $QEMU_CFLAGS"
-- 
1.8.3.1

Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11

[Daniel P. Berrangé]

On Mon, Jan 07, 2019 at 11:45:26AM +0100, Thomas Huth wrote:
> Different versions of GCC and Clang use different versions of the C standard.
> This repeatedly caused problems already, e.g. with duplicated typedefs:
> 
>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
> 
> or with for-loop variable initializers:
> 
>  https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html
> 
> To avoid these problems, we should enforce the C language version to the
> same level for all compilers. Since our minimum compiler versions are
> GCC v4.8 and Clang v3.4 now, and both basically support "gnu11" already,
> this seems to be a good choice.

In 4.x   gnu11 is marked as experimental. I'm not really comfortable
using experimental features - even if its warning free there's a risk
it would silently mis-compile something.

gnu99 is ok with 4.x - it is merely "incomplete".

> 
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  configure | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/configure b/configure
> index 79375af..07f8105 100755
> --- a/configure
> +++ b/configure
> @@ -107,6 +107,9 @@ update_cxxflags() {
>              -Wstrict-prototypes|-Wmissing-prototypes|-Wnested-externs|\
>              -Wold-style-declaration|-Wold-style-definition|-Wredundant-decls)
>                  ;;
> +            -std=gnu11)
> +                QEMU_CXXFLAGS=${QEMU_CXXFLAGS:+$QEMU_CXXFLAGS }-std=gnu++11
> +                ;;
>              *)
>                  QEMU_CXXFLAGS=${QEMU_CXXFLAGS:+$QEMU_CXXFLAGS }$arg
>                  ;;
> @@ -585,7 +588,7 @@ ARFLAGS="${ARFLAGS-rv}"
>  # left shift of signed integers is well defined and has the expected
>  # 2s-complement style results. (Both clang and gcc agree that it
>  # provides these semantics.)
> -QEMU_CFLAGS="-fno-strict-aliasing -fno-common -fwrapv $QEMU_CFLAGS"
> +QEMU_CFLAGS="-fno-strict-aliasing -fno-common -fwrapv -std=gnu11 $QEMU_CFLAGS"
>  QEMU_CFLAGS="-Wall -Wundef -Wwrite-strings -Wmissing-prototypes $QEMU_CFLAGS"
>  QEMU_CFLAGS="-Wstrict-prototypes -Wredundant-decls $QEMU_CFLAGS"
>  QEMU_CFLAGS="-D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE $QEMU_CFLAGS"
> -- 
> 1.8.3.1
> 

Regards,
Daniel
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Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11

[Thomas Huth]

On 2019-01-09 11:58, Daniel P. Berrangé wrote:
> On Mon, Jan 07, 2019 at 11:45:26AM +0100, Thomas Huth wrote:
>> Different versions of GCC and Clang use different versions of the C standard.
>> This repeatedly caused problems already, e.g. with duplicated typedefs:
>>
>>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
>>
>> or with for-loop variable initializers:
>>
>>  https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html
>>
>> To avoid these problems, we should enforce the C language version to the
>> same level for all compilers. Since our minimum compiler versions are
>> GCC v4.8 and Clang v3.4 now, and both basically support "gnu11" already,
>> this seems to be a good choice.
> 
> In 4.x   gnu11 is marked as experimental. I'm not really comfortable
> using experimental features - even if its warning free there's a risk
> it would silently mis-compile something.
> 
> gnu99 is ok with 4.x - it is merely "incomplete".

gnu11 has the big advantage that it also fixes the problem with
duplicated typedefs that are reported by older versions of Clang.

Are you sure about the experimental character in 4.x? I just looked at
https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Standards.html and it says:

"A fourth version of the C standard, known as C11, was published in 2011
as ISO/IEC 9899:2011. GCC has limited incomplete support for parts of
this standard, enabled with -std=c11 or -std=iso9899:2011."

It does not say anything about "experimental" there. The word
"experimental" is only used for the C++ support, but we hardly have C++
code in QEMU -- if you worry about that, I could simply drop the
"-std=gnu++11" part from my patch?

 Thomas

Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11

[Daniel P. Berrangé]

On Wed, Jan 09, 2019 at 12:25:43PM +0100, Thomas Huth wrote:
> On 2019-01-09 11:58, Daniel P. Berrangé wrote:
> > On Mon, Jan 07, 2019 at 11:45:26AM +0100, Thomas Huth wrote:
> >> Different versions of GCC and Clang use different versions of the C standard.
> >> This repeatedly caused problems already, e.g. with duplicated typedefs:
> >>
> >>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
> >>
> >> or with for-loop variable initializers:
> >>
> >>  https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html
> >>
> >> To avoid these problems, we should enforce the C language version to the
> >> same level for all compilers. Since our minimum compiler versions are
> >> GCC v4.8 and Clang v3.4 now, and both basically support "gnu11" already,
> >> this seems to be a good choice.
> > 
> > In 4.x   gnu11 is marked as experimental. I'm not really comfortable
> > using experimental features - even if its warning free there's a risk
> > it would silently mis-compile something.
> > 
> > gnu99 is ok with 4.x - it is merely "incomplete".
> 
> gnu11 has the big advantage that it also fixes the problem with
> duplicated typedefs that are reported by older versions of Clang.
> 
> Are you sure about the experimental character in 4.x? I just looked at
> https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Standards.html and it says:
> 
> "A fourth version of the C standard, known as C11, was published in 2011
> as ISO/IEC 9899:2011. GCC has limited incomplete support for parts of
> this standard, enabled with -std=c11 or -std=iso9899:2011."
> 
> It does not say anything about "experimental" there. The word
> "experimental" is only used for the C++ support, but we hardly have C++
> code in QEMU -- if you worry about that, I could simply drop the
> "-std=gnu++11" part from my patch?

I was looking at the "info gcc" docs on RHEL7, gcc-4.8.5-16.el7_4.1.x86_64:

  "3.4 Options Controlling C Dialect

   ....snip...

     'c11'
     'c1x'
     'iso9899:2011'
          ISO C11, the 2011 revision of the ISO C standard.  Support is
          incomplete and experimental.  The name 'c1x' is deprecated.

       ....snip...

     'gnu11'
     'gnu1x'
          GNU dialect of ISO C11.  Support is incomplete and
          experimental.  The name 'gnu1x' is deprecated."


Regards,
Daniel
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Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11

[Thomas Huth]

On 2019-01-09 12:44, Daniel P. Berrangé wrote:
> On Wed, Jan 09, 2019 at 12:25:43PM +0100, Thomas Huth wrote:
>> On 2019-01-09 11:58, Daniel P. Berrangé wrote:
>>> On Mon, Jan 07, 2019 at 11:45:26AM +0100, Thomas Huth wrote:
>>>> Different versions of GCC and Clang use different versions of the C standard.
>>>> This repeatedly caused problems already, e.g. with duplicated typedefs:
>>>>
>>>>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
>>>>
>>>> or with for-loop variable initializers:
>>>>
>>>>  https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html
>>>>
>>>> To avoid these problems, we should enforce the C language version to the
>>>> same level for all compilers. Since our minimum compiler versions are
>>>> GCC v4.8 and Clang v3.4 now, and both basically support "gnu11" already,
>>>> this seems to be a good choice.
>>>
>>> In 4.x   gnu11 is marked as experimental. I'm not really comfortable
>>> using experimental features - even if its warning free there's a risk
>>> it would silently mis-compile something.
>>>
>>> gnu99 is ok with 4.x - it is merely "incomplete".
>>
>> gnu11 has the big advantage that it also fixes the problem with
>> duplicated typedefs that are reported by older versions of Clang.
>>
>> Are you sure about the experimental character in 4.x? I just looked at
>> https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Standards.html and it says:
>>
>> "A fourth version of the C standard, known as C11, was published in 2011
>> as ISO/IEC 9899:2011. GCC has limited incomplete support for parts of
>> this standard, enabled with -std=c11 or -std=iso9899:2011."
>>
>> It does not say anything about "experimental" there. The word
>> "experimental" is only used for the C++ support, but we hardly have C++
>> code in QEMU -- if you worry about that, I could simply drop the
>> "-std=gnu++11" part from my patch?
> 
> I was looking at the "info gcc" docs on RHEL7, gcc-4.8.5-16.el7_4.1.x86_64:
> 
>   "3.4 Options Controlling C Dialect
> 
>        ....snip...
> 
>      'gnu11'
>      'gnu1x'
>           GNU dialect of ISO C11.  Support is incomplete and
>           experimental.  The name 'gnu1x' is deprecated."

Ok. Looks like the "Support is incomplete and experimental" sentence has
been removed with GCC 4.9.0 here. So GCC 4.8 is likely pretty close
already. IMHO we could give it a try and enable gnu11 for QEMU with GCC
v4.8, too. If we later find problems, we could still switch back to
gnu99 instead. Other opinions?

 Thomas

Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11

[Daniel P. Berrangé]

On Wed, Jan 09, 2019 at 01:52:02PM +0100, Thomas Huth wrote:
> On 2019-01-09 12:44, Daniel P. Berrangé wrote:
> > On Wed, Jan 09, 2019 at 12:25:43PM +0100, Thomas Huth wrote:
> >> On 2019-01-09 11:58, Daniel P. Berrangé wrote:
> >>> On Mon, Jan 07, 2019 at 11:45:26AM +0100, Thomas Huth wrote:
> >>>> Different versions of GCC and Clang use different versions of the C standard.
> >>>> This repeatedly caused problems already, e.g. with duplicated typedefs:
> >>>>
> >>>>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
> >>>>
> >>>> or with for-loop variable initializers:
> >>>>
> >>>>  https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html
> >>>>
> >>>> To avoid these problems, we should enforce the C language version to the
> >>>> same level for all compilers. Since our minimum compiler versions are
> >>>> GCC v4.8 and Clang v3.4 now, and both basically support "gnu11" already,
> >>>> this seems to be a good choice.
> >>>
> >>> In 4.x   gnu11 is marked as experimental. I'm not really comfortable
> >>> using experimental features - even if its warning free there's a risk
> >>> it would silently mis-compile something.
> >>>
> >>> gnu99 is ok with 4.x - it is merely "incomplete".
> >>
> >> gnu11 has the big advantage that it also fixes the problem with
> >> duplicated typedefs that are reported by older versions of Clang.
> >>
> >> Are you sure about the experimental character in 4.x? I just looked at
> >> https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Standards.html and it says:
> >>
> >> "A fourth version of the C standard, known as C11, was published in 2011
> >> as ISO/IEC 9899:2011. GCC has limited incomplete support for parts of
> >> this standard, enabled with -std=c11 or -std=iso9899:2011."
> >>
> >> It does not say anything about "experimental" there. The word
> >> "experimental" is only used for the C++ support, but we hardly have C++
> >> code in QEMU -- if you worry about that, I could simply drop the
> >> "-std=gnu++11" part from my patch?
> > 
> > I was looking at the "info gcc" docs on RHEL7, gcc-4.8.5-16.el7_4.1.x86_64:
> > 
> >   "3.4 Options Controlling C Dialect
> > 
> >        ....snip...
> > 
> >      'gnu11'
> >      'gnu1x'
> >           GNU dialect of ISO C11.  Support is incomplete and
> >           experimental.  The name 'gnu1x' is deprecated."
> 
> Ok. Looks like the "Support is incomplete and experimental" sentence has
> been removed with GCC 4.9.0 here. So GCC 4.8 is likely pretty close
> already. IMHO we could give it a try and enable gnu11 for QEMU with GCC
> v4.8, too. If we later find problems, we could still switch back to
> gnu99 instead. Other opinions?

Our code is already cleanly compiling with gnu99 standard - the problem
is merely that we sometimes introduce regressions due to not enforcing
that standard level. I don't think the features in gnu11 are compelling
enough to justify using something that's declared experimental. As long
as we always have a -std=gnu99 flag set, it will avoid the regressions
we've seen.

Regards,
Daniel
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Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11

[Markus Armbruster]

Thomas Huth <thuth@redhat.com> writes:

> On 2019-01-09 12:44, Daniel P. Berrangé wrote:
>> On Wed, Jan 09, 2019 at 12:25:43PM +0100, Thomas Huth wrote:
>>> On 2019-01-09 11:58, Daniel P. Berrangé wrote:
>>>> On Mon, Jan 07, 2019 at 11:45:26AM +0100, Thomas Huth wrote:
>>>>> Different versions of GCC and Clang use different versions of the C standard.
>>>>> This repeatedly caused problems already, e.g. with duplicated typedefs:
>>>>>
>>>>>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
>>>>>
>>>>> or with for-loop variable initializers:
>>>>>
>>>>>  https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html
>>>>>
>>>>> To avoid these problems, we should enforce the C language version to the
>>>>> same level for all compilers. Since our minimum compiler versions are
>>>>> GCC v4.8 and Clang v3.4 now, and both basically support "gnu11" already,
>>>>> this seems to be a good choice.
>>>>
>>>> In 4.x   gnu11 is marked as experimental. I'm not really comfortable
>>>> using experimental features - even if its warning free there's a risk
>>>> it would silently mis-compile something.
>>>>
>>>> gnu99 is ok with 4.x - it is merely "incomplete".
>>>
>>> gnu11 has the big advantage that it also fixes the problem with
>>> duplicated typedefs that are reported by older versions of Clang.
>>>
>>> Are you sure about the experimental character in 4.x? I just looked at
>>> https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Standards.html and it says:
>>>
>>> "A fourth version of the C standard, known as C11, was published in 2011
>>> as ISO/IEC 9899:2011. GCC has limited incomplete support for parts of
>>> this standard, enabled with -std=c11 or -std=iso9899:2011."
>>>
>>> It does not say anything about "experimental" there. The word
>>> "experimental" is only used for the C++ support, but we hardly have C++
>>> code in QEMU -- if you worry about that, I could simply drop the
>>> "-std=gnu++11" part from my patch?
>> 
>> I was looking at the "info gcc" docs on RHEL7, gcc-4.8.5-16.el7_4.1.x86_64:
>> 
>>   "3.4 Options Controlling C Dialect
>> 
>>        ....snip...
>> 
>>      'gnu11'
>>      'gnu1x'
>>           GNU dialect of ISO C11.  Support is incomplete and
>>           experimental.  The name 'gnu1x' is deprecated."
>
> Ok. Looks like the "Support is incomplete and experimental" sentence has
> been removed with GCC 4.9.0 here. So GCC 4.8 is likely pretty close
> already. IMHO we could give it a try and enable gnu11 for QEMU with GCC
> v4.8, too. If we later find problems, we could still switch back to
> gnu99 instead. Other opinions?

Switchinh back could be somewhat painful if we already started using C11
features.  And if we don't plan to, then what exactly will -std=gnu11
buy us?

Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11

[Thomas Huth]

On 2019-01-09 14:10, Markus Armbruster wrote:
> Thomas Huth <thuth@redhat.com> writes:
> 
>> On 2019-01-09 12:44, Daniel P. Berrangé wrote:
>>> On Wed, Jan 09, 2019 at 12:25:43PM +0100, Thomas Huth wrote:
>>>> On 2019-01-09 11:58, Daniel P. Berrangé wrote:
>>>>> On Mon, Jan 07, 2019 at 11:45:26AM +0100, Thomas Huth wrote:
>>>>>> Different versions of GCC and Clang use different versions of the C standard.
>>>>>> This repeatedly caused problems already, e.g. with duplicated typedefs:
>>>>>>
>>>>>>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
>>>>>>
>>>>>> or with for-loop variable initializers:
>>>>>>
>>>>>>  https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html
>>>>>>
>>>>>> To avoid these problems, we should enforce the C language version to the
>>>>>> same level for all compilers. Since our minimum compiler versions are
>>>>>> GCC v4.8 and Clang v3.4 now, and both basically support "gnu11" already,
>>>>>> this seems to be a good choice.
>>>>>
>>>>> In 4.x   gnu11 is marked as experimental. I'm not really comfortable
>>>>> using experimental features - even if its warning free there's a risk
>>>>> it would silently mis-compile something.
>>>>>
>>>>> gnu99 is ok with 4.x - it is merely "incomplete".
>>>>
>>>> gnu11 has the big advantage that it also fixes the problem with
>>>> duplicated typedefs that are reported by older versions of Clang.
>>>>
>>>> Are you sure about the experimental character in 4.x? I just looked at
>>>> https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Standards.html and it says:
>>>>
>>>> "A fourth version of the C standard, known as C11, was published in 2011
>>>> as ISO/IEC 9899:2011. GCC has limited incomplete support for parts of
>>>> this standard, enabled with -std=c11 or -std=iso9899:2011."
>>>>
>>>> It does not say anything about "experimental" there. The word
>>>> "experimental" is only used for the C++ support, but we hardly have C++
>>>> code in QEMU -- if you worry about that, I could simply drop the
>>>> "-std=gnu++11" part from my patch?
>>>
>>> I was looking at the "info gcc" docs on RHEL7, gcc-4.8.5-16.el7_4.1.x86_64:
>>>
>>>   "3.4 Options Controlling C Dialect
>>>
>>>        ....snip...
>>>
>>>      'gnu11'
>>>      'gnu1x'
>>>           GNU dialect of ISO C11.  Support is incomplete and
>>>           experimental.  The name 'gnu1x' is deprecated."
>>
>> Ok. Looks like the "Support is incomplete and experimental" sentence has
>> been removed with GCC 4.9.0 here. So GCC 4.8 is likely pretty close
>> already. IMHO we could give it a try and enable gnu11 for QEMU with GCC
>> v4.8, too. If we later find problems, we could still switch back to
>> gnu99 instead. Other opinions?
> 
> Switchinh back could be somewhat painful if we already started using C11
> features.  And if we don't plan to, then what exactly will -std=gnu11
> buy us?

With C11, we get safety for the "duplicated typedef" problem that we run
into regularly again and again, see e.g.:

https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html

 Thomas

Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11

[Thomas Huth]

On 2019-01-09 13:58, Daniel P. Berrangé wrote:
> On Wed, Jan 09, 2019 at 01:52:02PM +0100, Thomas Huth wrote:
>> On 2019-01-09 12:44, Daniel P. Berrangé wrote:
>>> On Wed, Jan 09, 2019 at 12:25:43PM +0100, Thomas Huth wrote:
>>>> On 2019-01-09 11:58, Daniel P. Berrangé wrote:
>>>>> On Mon, Jan 07, 2019 at 11:45:26AM +0100, Thomas Huth wrote:
>>>>>> Different versions of GCC and Clang use different versions of the C standard.
>>>>>> This repeatedly caused problems already, e.g. with duplicated typedefs:
>>>>>>
>>>>>>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
>>>>>>
>>>>>> or with for-loop variable initializers:
>>>>>>
>>>>>>  https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html
>>>>>>
>>>>>> To avoid these problems, we should enforce the C language version to the
>>>>>> same level for all compilers. Since our minimum compiler versions are
>>>>>> GCC v4.8 and Clang v3.4 now, and both basically support "gnu11" already,
>>>>>> this seems to be a good choice.
>>>>>
>>>>> In 4.x   gnu11 is marked as experimental. I'm not really comfortable
>>>>> using experimental features - even if its warning free there's a risk
>>>>> it would silently mis-compile something.
>>>>>
>>>>> gnu99 is ok with 4.x - it is merely "incomplete".
>>>>
>>>> gnu11 has the big advantage that it also fixes the problem with
>>>> duplicated typedefs that are reported by older versions of Clang.
>>>>
>>>> Are you sure about the experimental character in 4.x? I just looked at
>>>> https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Standards.html and it says:
>>>>
>>>> "A fourth version of the C standard, known as C11, was published in 2011
>>>> as ISO/IEC 9899:2011. GCC has limited incomplete support for parts of
>>>> this standard, enabled with -std=c11 or -std=iso9899:2011."
>>>>
>>>> It does not say anything about "experimental" there. The word
>>>> "experimental" is only used for the C++ support, but we hardly have C++
>>>> code in QEMU -- if you worry about that, I could simply drop the
>>>> "-std=gnu++11" part from my patch?
>>>
>>> I was looking at the "info gcc" docs on RHEL7, gcc-4.8.5-16.el7_4.1.x86_64:
>>>
>>>   "3.4 Options Controlling C Dialect
>>>
>>>        ....snip...
>>>
>>>      'gnu11'
>>>      'gnu1x'
>>>           GNU dialect of ISO C11.  Support is incomplete and
>>>           experimental.  The name 'gnu1x' is deprecated."
>>
>> Ok. Looks like the "Support is incomplete and experimental" sentence has
>> been removed with GCC 4.9.0 here. So GCC 4.8 is likely pretty close
>> already. IMHO we could give it a try and enable gnu11 for QEMU with GCC
>> v4.8, too. If we later find problems, we could still switch back to
>> gnu99 instead. Other opinions?
> 
> Our code is already cleanly compiling with gnu99 standard - the problem
> is merely that we sometimes introduce regressions due to not enforcing
> that standard level. I don't think the features in gnu11 are compelling
> enough to justify using something that's declared experimental.
What about the duplicated typedef problem? See:

 https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html

That one occured with Clang, but I think we've had plenty of these in
the past with GCC, too...

 Thomas

Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11

[Daniel P. Berrangé]

On Wed, Jan 09, 2019 at 02:20:02PM +0100, Thomas Huth wrote:
> On 2019-01-09 13:58, Daniel P. Berrangé wrote:
> > On Wed, Jan 09, 2019 at 01:52:02PM +0100, Thomas Huth wrote:
> >> On 2019-01-09 12:44, Daniel P. Berrangé wrote:
> >>> On Wed, Jan 09, 2019 at 12:25:43PM +0100, Thomas Huth wrote:
> >>>> On 2019-01-09 11:58, Daniel P. Berrangé wrote:
> >>>>> On Mon, Jan 07, 2019 at 11:45:26AM +0100, Thomas Huth wrote:
> >>>>>> Different versions of GCC and Clang use different versions of the C standard.
> >>>>>> This repeatedly caused problems already, e.g. with duplicated typedefs:
> >>>>>>
> >>>>>>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
> >>>>>>
> >>>>>> or with for-loop variable initializers:
> >>>>>>
> >>>>>>  https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html
> >>>>>>
> >>>>>> To avoid these problems, we should enforce the C language version to the
> >>>>>> same level for all compilers. Since our minimum compiler versions are
> >>>>>> GCC v4.8 and Clang v3.4 now, and both basically support "gnu11" already,
> >>>>>> this seems to be a good choice.
> >>>>>
> >>>>> In 4.x   gnu11 is marked as experimental. I'm not really comfortable
> >>>>> using experimental features - even if its warning free there's a risk
> >>>>> it would silently mis-compile something.
> >>>>>
> >>>>> gnu99 is ok with 4.x - it is merely "incomplete".
> >>>>
> >>>> gnu11 has the big advantage that it also fixes the problem with
> >>>> duplicated typedefs that are reported by older versions of Clang.
> >>>>
> >>>> Are you sure about the experimental character in 4.x? I just looked at
> >>>> https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Standards.html and it says:
> >>>>
> >>>> "A fourth version of the C standard, known as C11, was published in 2011
> >>>> as ISO/IEC 9899:2011. GCC has limited incomplete support for parts of
> >>>> this standard, enabled with -std=c11 or -std=iso9899:2011."
> >>>>
> >>>> It does not say anything about "experimental" there. The word
> >>>> "experimental" is only used for the C++ support, but we hardly have C++
> >>>> code in QEMU -- if you worry about that, I could simply drop the
> >>>> "-std=gnu++11" part from my patch?
> >>>
> >>> I was looking at the "info gcc" docs on RHEL7, gcc-4.8.5-16.el7_4.1.x86_64:
> >>>
> >>>   "3.4 Options Controlling C Dialect
> >>>
> >>>        ....snip...
> >>>
> >>>      'gnu11'
> >>>      'gnu1x'
> >>>           GNU dialect of ISO C11.  Support is incomplete and
> >>>           experimental.  The name 'gnu1x' is deprecated."
> >>
> >> Ok. Looks like the "Support is incomplete and experimental" sentence has
> >> been removed with GCC 4.9.0 here. So GCC 4.8 is likely pretty close
> >> already. IMHO we could give it a try and enable gnu11 for QEMU with GCC
> >> v4.8, too. If we later find problems, we could still switch back to
> >> gnu99 instead. Other opinions?
> > 
> > Our code is already cleanly compiling with gnu99 standard - the problem
> > is merely that we sometimes introduce regressions due to not enforcing
> > that standard level. I don't think the features in gnu11 are compelling
> > enough to justify using something that's declared experimental.
> What about the duplicated typedef problem? See:
> 
>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
> 
> That one occured with Clang, but I think we've had plenty of these in
> the past with GCC, too...

IIUC, That's only a problem because we don't pass any  -std  flag, and
so get the compilers default, which may be gnu11.  If we explicitly
set -std=gnu99, that problem will be reported by patchew, travis and
maintainers own build tests, and thus won't get anywhere near git master.

Regards,
Daniel
-- 
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Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11

[Thomas Huth]

On 2019-01-09 14:27, Daniel P. Berrangé wrote:
> On Wed, Jan 09, 2019 at 02:20:02PM +0100, Thomas Huth wrote:
>> On 2019-01-09 13:58, Daniel P. Berrangé wrote:
>>> On Wed, Jan 09, 2019 at 01:52:02PM +0100, Thomas Huth wrote:
>>>> On 2019-01-09 12:44, Daniel P. Berrangé wrote:
>>>>> On Wed, Jan 09, 2019 at 12:25:43PM +0100, Thomas Huth wrote:
>>>>>> On 2019-01-09 11:58, Daniel P. Berrangé wrote:
>>>>>>> On Mon, Jan 07, 2019 at 11:45:26AM +0100, Thomas Huth wrote:
>>>>>>>> Different versions of GCC and Clang use different versions of the C standard.
>>>>>>>> This repeatedly caused problems already, e.g. with duplicated typedefs:
>>>>>>>>
>>>>>>>>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
>>>>>>>>
>>>>>>>> or with for-loop variable initializers:
>>>>>>>>
>>>>>>>>  https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html
>>>>>>>>
>>>>>>>> To avoid these problems, we should enforce the C language version to the
>>>>>>>> same level for all compilers. Since our minimum compiler versions are
>>>>>>>> GCC v4.8 and Clang v3.4 now, and both basically support "gnu11" already,
>>>>>>>> this seems to be a good choice.
>>>>>>>
>>>>>>> In 4.x   gnu11 is marked as experimental. I'm not really comfortable
>>>>>>> using experimental features - even if its warning free there's a risk
>>>>>>> it would silently mis-compile something.
>>>>>>>
>>>>>>> gnu99 is ok with 4.x - it is merely "incomplete".
>>>>>>
>>>>>> gnu11 has the big advantage that it also fixes the problem with
>>>>>> duplicated typedefs that are reported by older versions of Clang.
>>>>>>
>>>>>> Are you sure about the experimental character in 4.x? I just looked at
>>>>>> https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Standards.html and it says:
>>>>>>
>>>>>> "A fourth version of the C standard, known as C11, was published in 2011
>>>>>> as ISO/IEC 9899:2011. GCC has limited incomplete support for parts of
>>>>>> this standard, enabled with -std=c11 or -std=iso9899:2011."
>>>>>>
>>>>>> It does not say anything about "experimental" there. The word
>>>>>> "experimental" is only used for the C++ support, but we hardly have C++
>>>>>> code in QEMU -- if you worry about that, I could simply drop the
>>>>>> "-std=gnu++11" part from my patch?
>>>>>
>>>>> I was looking at the "info gcc" docs on RHEL7, gcc-4.8.5-16.el7_4.1.x86_64:
>>>>>
>>>>>   "3.4 Options Controlling C Dialect
>>>>>
>>>>>        ....snip...
>>>>>
>>>>>      'gnu11'
>>>>>      'gnu1x'
>>>>>           GNU dialect of ISO C11.  Support is incomplete and
>>>>>           experimental.  The name 'gnu1x' is deprecated."
>>>>
>>>> Ok. Looks like the "Support is incomplete and experimental" sentence has
>>>> been removed with GCC 4.9.0 here. So GCC 4.8 is likely pretty close
>>>> already. IMHO we could give it a try and enable gnu11 for QEMU with GCC
>>>> v4.8, too. If we later find problems, we could still switch back to
>>>> gnu99 instead. Other opinions?
>>>
>>> Our code is already cleanly compiling with gnu99 standard - the problem
>>> is merely that we sometimes introduce regressions due to not enforcing
>>> that standard level. I don't think the features in gnu11 are compelling
>>> enough to justify using something that's declared experimental.
>> What about the duplicated typedef problem? See:
>>
>>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
>>
>> That one occured with Clang, but I think we've had plenty of these in
>> the past with GCC, too...
> 
> IIUC, That's only a problem because we don't pass any  -std  flag, and
> so get the compilers default, which may be gnu11.  If we explicitly
> set -std=gnu99, that problem will be reported by patchew, travis and
> maintainers own build tests, and thus won't get anywhere near git master.

Ah, right, I just tried it with a newer version of Clang, and indeed it
then prints out a warning if some code with a duplicated typedef is
compiled with "-std=gnu99" (while it remains silent without that
option). So in that case I'm fine with gnu99, too. I'll send a v2 of the
patch...

 Thomas

Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11

[Markus Armbruster]

Thomas Huth <thuth@redhat.com> writes:

> On 2019-01-09 14:10, Markus Armbruster wrote:
>> Thomas Huth <thuth@redhat.com> writes:
>> 
>>> On 2019-01-09 12:44, Daniel P. Berrangé wrote:
>>>> On Wed, Jan 09, 2019 at 12:25:43PM +0100, Thomas Huth wrote:
>>>>> On 2019-01-09 11:58, Daniel P. Berrangé wrote:
>>>>>> On Mon, Jan 07, 2019 at 11:45:26AM +0100, Thomas Huth wrote:
>>>>>>> Different versions of GCC and Clang use different versions of the C standard.
>>>>>>> This repeatedly caused problems already, e.g. with duplicated typedefs:
>>>>>>>
>>>>>>>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
>>>>>>>
>>>>>>> or with for-loop variable initializers:
>>>>>>>
>>>>>>>  https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html
>>>>>>>
>>>>>>> To avoid these problems, we should enforce the C language version to the
>>>>>>> same level for all compilers. Since our minimum compiler versions are
>>>>>>> GCC v4.8 and Clang v3.4 now, and both basically support "gnu11" already,
>>>>>>> this seems to be a good choice.
>>>>>>
>>>>>> In 4.x   gnu11 is marked as experimental. I'm not really comfortable
>>>>>> using experimental features - even if its warning free there's a risk
>>>>>> it would silently mis-compile something.
>>>>>>
>>>>>> gnu99 is ok with 4.x - it is merely "incomplete".
>>>>>
>>>>> gnu11 has the big advantage that it also fixes the problem with
>>>>> duplicated typedefs that are reported by older versions of Clang.
>>>>>
>>>>> Are you sure about the experimental character in 4.x? I just looked at
>>>>> https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Standards.html and it says:
>>>>>
>>>>> "A fourth version of the C standard, known as C11, was published in 2011
>>>>> as ISO/IEC 9899:2011. GCC has limited incomplete support for parts of
>>>>> this standard, enabled with -std=c11 or -std=iso9899:2011."
>>>>>
>>>>> It does not say anything about "experimental" there. The word
>>>>> "experimental" is only used for the C++ support, but we hardly have C++
>>>>> code in QEMU -- if you worry about that, I could simply drop the
>>>>> "-std=gnu++11" part from my patch?
>>>>
>>>> I was looking at the "info gcc" docs on RHEL7, gcc-4.8.5-16.el7_4.1.x86_64:
>>>>
>>>>   "3.4 Options Controlling C Dialect
>>>>
>>>>        ....snip...
>>>>
>>>>      'gnu11'
>>>>      'gnu1x'
>>>>           GNU dialect of ISO C11.  Support is incomplete and
>>>>           experimental.  The name 'gnu1x' is deprecated."
>>>
>>> Ok. Looks like the "Support is incomplete and experimental" sentence has
>>> been removed with GCC 4.9.0 here. So GCC 4.8 is likely pretty close
>>> already. IMHO we could give it a try and enable gnu11 for QEMU with GCC
>>> v4.8, too. If we later find problems, we could still switch back to
>>> gnu99 instead. Other opinions?
>> 
>> Switchinh back could be somewhat painful if we already started using C11
>> features.  And if we don't plan to, then what exactly will -std=gnu11
>> buy us?
>
> With C11, we get safety for the "duplicated typedef" problem that we run
> into regularly again and again, see e.g.:
>
> https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html

That's a compilation failure.  "Support is experimental" makes me afraid
of run time failures.

If we truly want C11, shouldn't we bump minimum required GCC to 4.9?

Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11

[Thomas Huth]

On 2019-01-09 15:20, Markus Armbruster wrote:
> Thomas Huth <thuth@redhat.com> writes:
> 
>> On 2019-01-09 14:10, Markus Armbruster wrote:
>>> Thomas Huth <thuth@redhat.com> writes:
>>>
>>>> On 2019-01-09 12:44, Daniel P. Berrangé wrote:
>>>>> On Wed, Jan 09, 2019 at 12:25:43PM +0100, Thomas Huth wrote:
>>>>>> On 2019-01-09 11:58, Daniel P. Berrangé wrote:
>>>>>>> On Mon, Jan 07, 2019 at 11:45:26AM +0100, Thomas Huth wrote:
>>>>>>>> Different versions of GCC and Clang use different versions of the C standard.
>>>>>>>> This repeatedly caused problems already, e.g. with duplicated typedefs:
>>>>>>>>
>>>>>>>>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
>>>>>>>>
>>>>>>>> or with for-loop variable initializers:
>>>>>>>>
>>>>>>>>  https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html
>>>>>>>>
>>>>>>>> To avoid these problems, we should enforce the C language version to the
>>>>>>>> same level for all compilers. Since our minimum compiler versions are
>>>>>>>> GCC v4.8 and Clang v3.4 now, and both basically support "gnu11" already,
>>>>>>>> this seems to be a good choice.
>>>>>>>
>>>>>>> In 4.x   gnu11 is marked as experimental. I'm not really comfortable
>>>>>>> using experimental features - even if its warning free there's a risk
>>>>>>> it would silently mis-compile something.
>>>>>>>
>>>>>>> gnu99 is ok with 4.x - it is merely "incomplete".
>>>>>>
>>>>>> gnu11 has the big advantage that it also fixes the problem with
>>>>>> duplicated typedefs that are reported by older versions of Clang.
>>>>>>
>>>>>> Are you sure about the experimental character in 4.x? I just looked at
>>>>>> https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Standards.html and it says:
>>>>>>
>>>>>> "A fourth version of the C standard, known as C11, was published in 2011
>>>>>> as ISO/IEC 9899:2011. GCC has limited incomplete support for parts of
>>>>>> this standard, enabled with -std=c11 or -std=iso9899:2011."
>>>>>>
>>>>>> It does not say anything about "experimental" there. The word
>>>>>> "experimental" is only used for the C++ support, but we hardly have C++
>>>>>> code in QEMU -- if you worry about that, I could simply drop the
>>>>>> "-std=gnu++11" part from my patch?
>>>>>
>>>>> I was looking at the "info gcc" docs on RHEL7, gcc-4.8.5-16.el7_4.1.x86_64:
>>>>>
>>>>>   "3.4 Options Controlling C Dialect
>>>>>
>>>>>        ....snip...
>>>>>
>>>>>      'gnu11'
>>>>>      'gnu1x'
>>>>>           GNU dialect of ISO C11.  Support is incomplete and
>>>>>           experimental.  The name 'gnu1x' is deprecated."
>>>>
>>>> Ok. Looks like the "Support is incomplete and experimental" sentence has
>>>> been removed with GCC 4.9.0 here. So GCC 4.8 is likely pretty close
>>>> already. IMHO we could give it a try and enable gnu11 for QEMU with GCC
>>>> v4.8, too. If we later find problems, we could still switch back to
>>>> gnu99 instead. Other opinions?
>>>
>>> Switchinh back could be somewhat painful if we already started using C11
>>> features.  And if we don't plan to, then what exactly will -std=gnu11
>>> buy us?
>>
>> With C11, we get safety for the "duplicated typedef" problem that we run
>> into regularly again and again, see e.g.:
>>
>> https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
> 
> That's a compilation failure.  "Support is experimental" makes me afraid
> of run time failures.
> 
> If we truly want C11, shouldn't we bump minimum required GCC to 4.9?

That's not possible, since we claim to support RHEL7 / CentOS7 that is
still using GCC v4.8.

 Thomas