It's decided by function use_register_for_decl in gcc: https://github.com/gcc-mirror/gcc/blob/releases/gcc-12/gcc/function.cc#L2151 https://github.com/gcc-mirro

It's decided by function use_register_for_decl in gcc: https://github.com/gcc-mirror/gcc/blob/releases/gcc-12/gcc/function.cc#L2151 https://github.com/gcc-mirror/gcc/blob/releases/gcc-12/gcc/f... With -g -O0 register is only used in special cases like using the register keyword. The memory accesses are also easily visible by disassembling the compiled binary. Performance of resulting binary at -O0 is also rougly similar to performance of binary produced by Tiny C Compiler, which doesn't implement register allocation at all.