Why do compilers insist on using a callee-saved register here?

Question

Consider this C code:

void foo(void);

long bar(long x) {
    foo();
    return x;
}

When I compile it on GCC 9.3 with either -O3

Answer 1

Why do compilers insist on using a callee-saved register here?

Because most compilers would generate nearly the same code for a given function, and are following global calling conventions defined by the ABI targeted by your compiler.

Notice that with a recent GCC (e.g. GCC 10 in start of 2021) you could compile and link with gcc -O3 -flto -fwhole-program and in some cases get some inline expansion. You can also build GCC from its source code as a cross-compiler, and since GCC is free software, you can improve it to follow your private new calling conventions. Be sure to document your calling conventions first.

If performance matters to you a lot, you can consider writing your own GCC plugin doing even more optimizations. Your compiler plugin could even implement other calling conventions (e.g. using asmjit).

Consider also improving TinyCC or Clang or NWCC to fit your needs.

My opinion is that in many cases it is not worth spending months of your efforts to improve performance by just a few nanoseconds. But your employer/manager/client could disagree. Consider also compiling (or refactoring) significant parts of your software to silicon, e.g. thru VHDL, or using specialized hardware e.g. GPGPU with OpenCL or CUDA.