[FFmpeg-devel] [PATCH] configure: Enable -fno-common for Darwin targets, avoid linker warnings

[FFmpeg-devel] [PATCH] configure: Enable -fno-common for Darwin targets, avoid linker warnings

[Martin Storsjö]

Since GCC 10 and llvm.org Clang 11, -fno-common is the default.
However Apple's Xcode Clang hasn't followed suit yet, and still
defaults to -fcommon.

Compiling with -fcommon causes uninitialized global variables to
be treated as "common" (which allows multiple object files to have
similar definitions).

Common variables seem to have the issue that their intended alignment
isn't signaled, so the linker assumes that they may need alignment
according to their full size.

With large global tables, this can lead to linker warnings like
this, with Xcode 16.3:

    ld: warning: reducing alignment of section __DATA,__common from 0x8000 to 0x4000 because it exceeds segment maximum alignment

This can be reproduced with a small snippet like this:

    char table[16385];
    int main(int argc, char* argv[]) { return 0; }

Compiling with -fno-common avoids this issue and warning, and
matches the default behaviour of other compilers. (Compiling with
-fno-common also avoids the risk of accidentally accepting
duplicate definitions of global variables, as long as they are
uninitialized.)
---
 configure | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/configure b/configure
index ee270b770c..45c99239a8 100755
--- a/configure
+++ b/configure
@@ -5846,6 +5846,13 @@ case $target_os in
             clang_version=$($cc -dumpversion)
             test ${clang_version%%.*} -eq 11 && add_cflags -fno-stack-check
         fi
+
+        # Xcode Clang doesn't default to -fno-common while upstream llvm.org
+        # Clang (and GCC) do. This avoids linker warnings on Xcode 16.3 about
+        # "reducing alignment of section __DATA,__common from 0x8000 to 0x4000
+        # because it exceeds segment maximum alignment".
+        check_cflags -fno-common
+
         ;;
     msys*)
         die "Native MSYS builds are discouraged, please use the MINGW environment."
-- 
2.39.5 (Apple Git-154)

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Re: [FFmpeg-devel] [PATCH] configure: Enable -fno-common for Darwin targets, avoid linker warnings

[Martin Storsjö]

On Tue, 29 Apr 2025, Martin Storsjö wrote:

> Since GCC 10 and llvm.org Clang 11, -fno-common is the default.
> However Apple's Xcode Clang hasn't followed suit yet, and still
> defaults to -fcommon.
>
> Compiling with -fcommon causes uninitialized global variables to
> be treated as "common" (which allows multiple object files to have
> similar definitions).
>
> Common variables seem to have the issue that their intended alignment
> isn't signaled, so the linker assumes that they may need alignment
> according to their full size.
>
> With large global tables, this can lead to linker warnings like
> this, with Xcode 16.3:
>
>    ld: warning: reducing alignment of section __DATA,__common from 0x8000 to 0x4000 because it exceeds segment maximum alignment
>
> This can be reproduced with a small snippet like this:
>
>    char table[16385];
>    int main(int argc, char* argv[]) { return 0; }
>
> Compiling with -fno-common avoids this issue and warning, and
> matches the default behaviour of other compilers. (Compiling with
> -fno-common also avoids the risk of accidentally accepting
> duplicate definitions of global variables, as long as they are
> uninitialized.)
> ---
> configure | 7 +++++++
> 1 file changed, 7 insertions(+)

Will push soon.

// Martin
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Re: [FFmpeg-devel] [PATCH] configure: Enable -fno-common for Darwin targets, avoid linker warnings

[Martin Storsjö]

On Fri, 2 May 2025, Martin Storsjö wrote:

> On Tue, 29 Apr 2025, Martin Storsjö wrote:
>
>> Since GCC 10 and llvm.org Clang 11, -fno-common is the default.
>> However Apple's Xcode Clang hasn't followed suit yet, and still
>> defaults to -fcommon.
>> 
>> Compiling with -fcommon causes uninitialized global variables to
>> be treated as "common" (which allows multiple object files to have
>> similar definitions).
>> 
>> Common variables seem to have the issue that their intended alignment
>> isn't signaled, so the linker assumes that they may need alignment
>> according to their full size.
>> 
>> With large global tables, this can lead to linker warnings like
>> this, with Xcode 16.3:
>>
>>    ld: warning: reducing alignment of section __DATA,__common from 0x8000 
>> to 0x4000 because it exceeds segment maximum alignment
>> 
>> This can be reproduced with a small snippet like this:
>>
>>    char table[16385];
>>    int main(int argc, char* argv[]) { return 0; }
>> 
>> Compiling with -fno-common avoids this issue and warning, and
>> matches the default behaviour of other compilers. (Compiling with
>> -fno-common also avoids the risk of accidentally accepting
>> duplicate definitions of global variables, as long as they are
>> uninitialized.)
>> ---
>> configure | 7 +++++++
>> 1 file changed, 7 insertions(+)
>
> Will push soon.

Pushed. I'll go ahead and backport this to a couple release branches as 
well.

// Martin
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Re: [FFmpeg-devel] [PATCH] configure: Enable -fno-common for Darwin targets, avoid linker warnings

[Marvin Scholz]

On 5 May 2025, at 10:45, Martin Storsjö wrote:

> On Fri, 2 May 2025, Martin Storsjö wrote:
>
>> On Tue, 29 Apr 2025, Martin Storsjö wrote:
>>
>>> Since GCC 10 and llvm.org Clang 11, -fno-common is the default.
>>> However Apple's Xcode Clang hasn't followed suit yet, and still
>>> defaults to -fcommon.
>>>
>>> Compiling with -fcommon causes uninitialized global variables to
>>> be treated as "common" (which allows multiple object files to have
>>> similar definitions).
>>>
>>> Common variables seem to have the issue that their intended alignment
>>> isn't signaled, so the linker assumes that they may need alignment
>>> according to their full size.
>>>
>>> With large global tables, this can lead to linker warnings like
>>> this, with Xcode 16.3:
>>>
>>>    ld: warning: reducing alignment of section __DATA,__common from 0x8000 to 0x4000 because it exceeds segment maximum alignment
>>>
>>> This can be reproduced with a small snippet like this:
>>>
>>>    char table[16385];
>>>    int main(int argc, char* argv[]) { return 0; }
>>>
>>> Compiling with -fno-common avoids this issue and warning, and
>>> matches the default behaviour of other compilers. (Compiling with
>>> -fno-common also avoids the risk of accidentally accepting
>>> duplicate definitions of global variables, as long as they are
>>> uninitialized.)
>>> ---
>>> configure | 7 +++++++
>>> 1 file changed, 7 insertions(+)
>>
>> Will push soon.
>
> Pushed. I'll go ahead and backport this to a couple release branches as well.

Good idea, thanks for the warning fix.

>
> // Martin
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> ffmpeg-devel mailing list
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