[FFmpeg-devel] [PATCH v2] configure: Explicitly check for static_assert

[FFmpeg-devel] [PATCH v2] configure: Explicitly check for static_assert

[Andreas Rheinhardt]

C11 provides static assertions via _Static_assert and
provides static_assert as a convenience define for this
in assert.h. MSVC 19.27 declares support for C11, but does
not support _Static_assert, but somehow supports
static_assert. That's therefore what we use.

But apparently there are some old GCC toolchains where
_Static_assert is supported, but assert.h does not provide
the fallback define. Some fate boxes are affected by this
[1].

This commit therefore checks whether static_assert works
with assert.h included; if not, it errors out. Users like
the above can still add -Dstatic_assert=_Static_assert
to cflags as a workaround.

[1]: https://fate.ffmpeg.org/report.cgi?time=20240321123620&slot=sh4-debian-qemu-gcc-4.7

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
---
This is what a test without fallback looks like.
Posted to gather opinions on what people prefer.

 configure | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/configure b/configure
index 6d7b33b0ff..c2d2c70c20 100755
--- a/configure
+++ b/configure
@@ -5589,6 +5589,19 @@ check_cxxflags_cc -std=$stdcxx ctype.h "__cplusplus >= 201103L" ||
 check_cflags_cc -std=$stdc ctype.h "__STDC_VERSION__ >= 201112L" ||
     { check_cflags_cc -std=c11 ctype.h "__STDC_VERSION__ >= 201112L" && stdc="c11" || die "Compiler lacks C11 support"; }
 
+test_cc <<EOF || die "Compiler lacks support for C11 static_assert"
+#include <assert.h>
+#include <stddef.h>
+struct Foo {
+    int a;
+    void *ptr;
+} obj;
+static_assert(offsetof(struct Foo, a) == 0,
+              "First element of struct does not have offset 0");
+static_assert(offsetof(struct Foo, ptr) >= offsetof(struct Foo, a) + sizeof(obj.a),
+              "elements not properly ordered in struct");
+EOF
+
 check_cppflags -D_FILE_OFFSET_BITS=64
 check_cppflags -D_LARGEFILE_SOURCE
 
-- 
2.40.1

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Re: [FFmpeg-devel] [PATCH v2] configure: Explicitly check for static_assert

[Andreas Rheinhardt]

Andreas Rheinhardt:
> C11 provides static assertions via _Static_assert and
> provides static_assert as a convenience define for this
> in assert.h. MSVC 19.27 declares support for C11, but does
> not support _Static_assert, but somehow supports
> static_assert. That's therefore what we use.
> 
> But apparently there are some old GCC toolchains where
> _Static_assert is supported, but assert.h does not provide
> the fallback define. Some fate boxes are affected by this
> [1].
> 
> This commit therefore checks whether static_assert works
> with assert.h included; if not, it errors out. Users like
> the above can still add -Dstatic_assert=_Static_assert
> to cflags as a workaround.
> 
> [1]: https://fate.ffmpeg.org/report.cgi?time=20240321123620&slot=sh4-debian-qemu-gcc-4.7
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> ---
> This is what a test without fallback looks like.
> Posted to gather opinions on what people prefer.
> 
>  configure | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/configure b/configure
> index 6d7b33b0ff..c2d2c70c20 100755
> --- a/configure
> +++ b/configure
> @@ -5589,6 +5589,19 @@ check_cxxflags_cc -std=$stdcxx ctype.h "__cplusplus >= 201103L" ||
>  check_cflags_cc -std=$stdc ctype.h "__STDC_VERSION__ >= 201112L" ||
>      { check_cflags_cc -std=c11 ctype.h "__STDC_VERSION__ >= 201112L" && stdc="c11" || die "Compiler lacks C11 support"; }
>  
> +test_cc <<EOF || die "Compiler lacks support for C11 static_assert"
> +#include <assert.h>
> +#include <stddef.h>
> +struct Foo {
> +    int a;
> +    void *ptr;
> +} obj;
> +static_assert(offsetof(struct Foo, a) == 0,
> +              "First element of struct does not have offset 0");
> +static_assert(offsetof(struct Foo, ptr) >= offsetof(struct Foo, a) + sizeof(obj.a),
> +              "elements not properly ordered in struct");
> +EOF
> +
>  check_cppflags -D_FILE_OFFSET_BITS=64
>  check_cppflags -D_LARGEFILE_SOURCE
>  

Jan has tested old toolchains and found out that his GCC 4.7 has proper
C11 headers; so this seems to be unique to Michael's setup. This makes
me prefer this patch instead of the version with the fallback. (Michael
can simply add -Dstatic_assert=_Static_assert to his cflags.)
Of course others are still invited to share their opinions.

- Andreas

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Re: [FFmpeg-devel] [PATCH v2] configure: Explicitly check for static_assert

[Martin Storsjö]

On Thu, 21 Mar 2024, Andreas Rheinhardt wrote:

> Andreas Rheinhardt:
>> C11 provides static assertions via _Static_assert and
>> provides static_assert as a convenience define for this
>> in assert.h. MSVC 19.27 declares support for C11, but does
>> not support _Static_assert, but somehow supports
>> static_assert. That's therefore what we use.
>>
>> But apparently there are some old GCC toolchains where
>> _Static_assert is supported, but assert.h does not provide
>> the fallback define. Some fate boxes are affected by this
>> [1].
>>
>> This commit therefore checks whether static_assert works
>> with assert.h included; if not, it errors out. Users like
>> the above can still add -Dstatic_assert=_Static_assert
>> to cflags as a workaround.
>>
>> [1]: https://fate.ffmpeg.org/report.cgi?time=20240321123620&slot=sh4-debian-qemu-gcc-4.7
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
>> ---
>> This is what a test without fallback looks like.
>> Posted to gather opinions on what people prefer.
>>
>>  configure | 13 +++++++++++++
>>  1 file changed, 13 insertions(+)
>>
>> diff --git a/configure b/configure
>> index 6d7b33b0ff..c2d2c70c20 100755
>> --- a/configure
>> +++ b/configure
>> @@ -5589,6 +5589,19 @@ check_cxxflags_cc -std=$stdcxx ctype.h "__cplusplus >= 201103L" ||
>>  check_cflags_cc -std=$stdc ctype.h "__STDC_VERSION__ >= 201112L" ||
>>      { check_cflags_cc -std=c11 ctype.h "__STDC_VERSION__ >= 201112L" && stdc="c11" || die "Compiler lacks C11 support"; }
>>
>> +test_cc <<EOF || die "Compiler lacks support for C11 static_assert"
>> +#include <assert.h>
>> +#include <stddef.h>
>> +struct Foo {
>> +    int a;
>> +    void *ptr;
>> +} obj;
>> +static_assert(offsetof(struct Foo, a) == 0,
>> +              "First element of struct does not have offset 0");
>> +static_assert(offsetof(struct Foo, ptr) >= offsetof(struct Foo, a) + sizeof(obj.a),
>> +              "elements not properly ordered in struct");
>> +EOF
>> +
>>  check_cppflags -D_FILE_OFFSET_BITS=64
>>  check_cppflags -D_LARGEFILE_SOURCE
>>
>
> Jan has tested old toolchains and found out that his GCC 4.7 has proper
> C11 headers; so this seems to be unique to Michael's setup. This makes
> me prefer this patch instead of the version with the fallback. (Michael
> can simply add -Dstatic_assert=_Static_assert to his cflags.)
> Of course others are still invited to share their opinions.

Both patches seem to work fine with MSVC 19.27 - I vaguely prefer the v2 
version, which is simpler.


But to me, we could also just revert the change to 
libavcodec/ccaption_dec.c, and declare that we require MSVC 19.28 instead. 
MSVC 19.27, when executed with -std:c11 without -nologo, it prints this:

     /std:c11 is a preview implementation of the ISO C11 standard, and
     we're eager to hear about bugs and suggestions for improvements.
     However, note that these features are provided as-is without support.

And I don't have any specific reasons for wanting to use this compiler - I 
just tested the lowest version that was supposed to be supported earlier 
and noted that it had broken recently. So to me, reverting to requiring 
_Static_assert would be quite ok as well.

// Martin

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Re: [FFmpeg-devel] [PATCH v2] configure: Explicitly check for static_assert

[Andreas Rheinhardt]

Martin Storsjö:
> On Thu, 21 Mar 2024, Andreas Rheinhardt wrote:
> 
>> Andreas Rheinhardt:
>>> C11 provides static assertions via _Static_assert and
>>> provides static_assert as a convenience define for this
>>> in assert.h. MSVC 19.27 declares support for C11, but does
>>> not support _Static_assert, but somehow supports
>>> static_assert. That's therefore what we use.
>>>
>>> But apparently there are some old GCC toolchains where
>>> _Static_assert is supported, but assert.h does not provide
>>> the fallback define. Some fate boxes are affected by this
>>> [1].
>>>
>>> This commit therefore checks whether static_assert works
>>> with assert.h included; if not, it errors out. Users like
>>> the above can still add -Dstatic_assert=_Static_assert
>>> to cflags as a workaround.
>>>
>>> [1]:
>>> https://fate.ffmpeg.org/report.cgi?time=20240321123620&slot=sh4-debian-qemu-gcc-4.7
>>>
>>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
>>> ---
>>> This is what a test without fallback looks like.
>>> Posted to gather opinions on what people prefer.
>>>
>>>  configure | 13 +++++++++++++
>>>  1 file changed, 13 insertions(+)
>>>
>>> diff --git a/configure b/configure
>>> index 6d7b33b0ff..c2d2c70c20 100755
>>> --- a/configure
>>> +++ b/configure
>>> @@ -5589,6 +5589,19 @@ check_cxxflags_cc -std=$stdcxx ctype.h
>>> "__cplusplus >= 201103L" ||
>>>  check_cflags_cc -std=$stdc ctype.h "__STDC_VERSION__ >= 201112L" ||
>>>      { check_cflags_cc -std=c11 ctype.h "__STDC_VERSION__ >= 201112L"
>>> && stdc="c11" || die "Compiler lacks C11 support"; }
>>>
>>> +test_cc <<EOF || die "Compiler lacks support for C11 static_assert"
>>> +#include <assert.h>
>>> +#include <stddef.h>
>>> +struct Foo {
>>> +    int a;
>>> +    void *ptr;
>>> +} obj;
>>> +static_assert(offsetof(struct Foo, a) == 0,
>>> +              "First element of struct does not have offset 0");
>>> +static_assert(offsetof(struct Foo, ptr) >= offsetof(struct Foo, a) +
>>> sizeof(obj.a),
>>> +              "elements not properly ordered in struct");
>>> +EOF
>>> +
>>>  check_cppflags -D_FILE_OFFSET_BITS=64
>>>  check_cppflags -D_LARGEFILE_SOURCE
>>>
>>
>> Jan has tested old toolchains and found out that his GCC 4.7 has proper
>> C11 headers; so this seems to be unique to Michael's setup. This makes
>> me prefer this patch instead of the version with the fallback. (Michael
>> can simply add -Dstatic_assert=_Static_assert to his cflags.)
>> Of course others are still invited to share their opinions.
> 
> Both patches seem to work fine with MSVC 19.27 - I vaguely prefer the v2
> version, which is simpler.
> 
> 
> But to me, we could also just revert the change to
> libavcodec/ccaption_dec.c, and declare that we require MSVC 19.28
> instead. MSVC 19.27, when executed with -std:c11 without -nologo, it
> prints this:
> 
>     /std:c11 is a preview implementation of the ISO C11 standard, and
>     we're eager to hear about bugs and suggestions for improvements.
>     However, note that these features are provided as-is without support.
> 
> And I don't have any specific reasons for wanting to use this compiler -
> I just tested the lowest version that was supposed to be supported
> earlier and noted that it had broken recently. So to me, reverting to
> requiring _Static_assert would be quite ok as well.
> 

We can actually do both: Test for static_assert and for _Static_assert
(to exclude MSVC 19.27; is 19.28 still supposed to be a preview
implementation?).
The reason I prefer static_assert in the codebase is that _Static_assert
is actually deprecated with C23 (although I don't think it will be
removed any time).

- Andreas

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Re: [FFmpeg-devel] [PATCH v2] configure: Explicitly check for static_assert

[Martin Storsjö]

On Fri, 22 Mar 2024, Andreas Rheinhardt wrote:

> Martin Storsjö:
>> 
>> Both patches seem to work fine with MSVC 19.27 - I vaguely prefer the v2
>> version, which is simpler.
>> 
>> 
>> But to me, we could also just revert the change to
>> libavcodec/ccaption_dec.c, and declare that we require MSVC 19.28
>> instead. MSVC 19.27, when executed with -std:c11 without -nologo, it
>> prints this:
>> 
>>     /std:c11 is a preview implementation of the ISO C11 standard, and
>>     we're eager to hear about bugs and suggestions for improvements.
>>     However, note that these features are provided as-is without support.
>> 
>> And I don't have any specific reasons for wanting to use this compiler -
>> I just tested the lowest version that was supposed to be supported
>> earlier and noted that it had broken recently. So to me, reverting to
>> requiring _Static_assert would be quite ok as well.
>> 
>
> We can actually do both: Test for static_assert and for _Static_assert
> (to exclude MSVC 19.27; is 19.28 still supposed to be a preview
> implementation?).

19.28 no longer has that preview implementation banner, so from there on, 
it should be fine.

> The reason I prefer static_assert in the codebase is that _Static_assert
> is actually deprecated with C23 (although I don't think it will be
> removed any time).

Ah, I see. Right, with that in mind, unifying usage to static_assert 
sounds good.

No strong opinion either way about the configure checks still (or whether 
we should require _Static_assert to be supported), except that strictly 
requiring static_assert seems less kludgy than trying to define it 
ourselves.

// Martin
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