This makes -fno-common the default for all targets because this has performance and code-size benefits and is more language conforming for C code. Additionally, GCC10 also defaults to -fno-common and so we get consistent behaviour with GCC.
With this change, C code that uses tentative definitions as definitions of a variable in multiple translation units will trigger multiple-definition linker errors. Generally, this occurs when the use of the extern keyword is neglected in the declaration of a variable in a header file. In some cases, no specific translation unit provides a definition of the variable. The previous behavior can be restored by specifying -fcommon.
As GCC has switched already, we benefit from applications already being ported and existing documentation how to do this. For example: