Different ways of suppressing 'uninitialized variable warnings' in C
Compilers are made to recognize certain constructs as indications that the author intended something deliberately, when the compiler would otherwise warn about it. For example, given
if (b = a), GCC and Clang both warn that an assignment is being used as a conditional, but they do not warn about
if ((b = a)) even though it is equivalent in terms of the C standard. This particular construct with extra parentheses has simply been set as a way to tell the compiler the author truly intends this code.
x = x has been set as a way to tell GCC not to warn about
x being uninitialized. There are times where a function may appear to have a code path in which an object is used without being initialized, but the author knows the function is intended not to be used with parameters that would ever cause that particular code path to be executed and, for reasons of efficiency, they want to silence the compiler warning rather than add an initialization that is not actually necessary for program correctness.
Clang was presumably designed not to recognize GCC’s idiom for this and needed a different method.