RFR: 8348286: [AIX] clang 17 introduces new warning Wtentative-Definitions which produces Build errors

[Erik Joelsson]

On Wed, 22 Jan 2025 15:34:36 GMT, Joachim Kern <jkern at openjdk.org> wrote:

> We (SAP) try to introduce the ‘IBM Open XL C/C++ for AIX 17.1.2’ (based on clang 17) as a feasible compiler for jdk25, because in combination with the 17.1.3 runtime this would enable the support for ubsan.
> Unfortunately, the new compiler comes along with a new set of compiler warnings turned into errors by -Werror.
> One of the warnings is -Wtentative-definitions. It is fired when a variable definition in a header is found:
> /jdk/src/java.desktop/share/native/libawt/awt/image/imageInitIDs.h:36:20: error: possible missing 'extern' on global variable definition in header [-Werror,-Wtentative-definitions]
>    36 | IMGEXTERN jfieldID g_BImgRasterID;
>       | ^
> From now on headers allow only extern declarations of variables. The definition must take place in a c/cpp file. This means e.g.
> imageInitIDs.h:36:20
>    36 extern jfieldID g_BImgRasterID;
> and the corresponding c-File
>    jfieldID g_BImgRasterID;
> 
> The other possible solution would be to compile everything with
> -Wno-tentative-definitions
> which could be set in flags-cflags.m4, if compiling with clang 17.

Do you know if this is a new warning in clang 17 or an existing warning that started behaving differently, and possibly buggy? If the warning is working as intended, and we have no intention of adhering to it, then just add the disable flag without a version check. If you suspect that this is buggy behavior in the compiler, then we may need to investigate more to understand what compiler versions are affected and do a more targeted disabling.

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PR Comment: https://git.openjdk.org/jdk/pull/23236#issuecomment-2607997961