Comment by Joker_vD

I vaguely remember that Clang and GCC used to disagree what the contents of the upper parts of x64 registers when returning some integer types should be (zeroes or garabge), because the PDF that defined Sys V ABI on x64 left such irrelevant details out, so linking together objects produced by those compilers, both of which claimed to follow the same ABI, would produce malfunctioning executable.

> Forcing them writing some specific bit-pattern may lead to suboptimal code generation.

So? Forcing them to compile "return 42;" as "mov eax, 42; ret" also leads to suboptimal code generation: a plain "ret", returning whatever is in rax already, is optimal. It doesn't generate the specific bit pattern for 42 but that's a small price for the improved efficiency, isn't it?