disable all obvious elimination when compiling with gcc (without changing my source code!)

I want to keep all dead code (or anything that is even obviously can be optimized) when compiling with gcc, but even with -O0, some dead code are still optimized. How can I keep all code without changing my source code? The sample code is as follows, and when compiling with g++ -S -O0 main.cc, the if-statement will be optimized in assembly code (there will be no cmpl and jmp code).

int main() {
  constexpr int a = 123; // or const int a = 0; I do not want to remove `const` or `constexpr` qualifier.
  if (a) // or just if (123)
    return 1;
  return 0;
}

A related question is here: Disable "if(0)" elimination in gcc. But the answers there need you to change your source code (remove const/constexpr qualifier) which I do not want to do.

Could it be that I do not change my source code but only use some compiler flags to achieve this?

This is not possible with GCC to keep the conditional in this case since it is removed during a very early stage of the compilation.

First of all, here is the compilation steps of GCC:

1. Code parsing (syntax & semantics) producing an AST in GENERIC representation (HL-IR)
2. High-level GIMPLE generation (ML-IR)
3. Low-level GIMPLE generation (ML-IR)
4. Tree SSA optimization (ML-IR)
5. RTL generation (LL-IR)
6. Code optimization
7. Assembly generation

The conditional is already removed after the generation of the (theoretically unoptimized) high-level GIMPLE representation. Thus, before any optimization step. One can check this by using the GCC flag -fdump-tree-all and look at the first generated GIMPLE code. Here is the result:

;; Function int main() (null)
;; enabled by -tree-original
{
  const int a = 123;

  <<cleanup_point   const int a = 123;>>;
  return <retval> = 1;
  return <retval> = 0;
}
return <retval> = 0;

One can note that the resulting code is the same with both constexpr and const. Actually, constexpr is treated as a simple const variable in the HL GIMPLE code.

It is hard to know when the conditional is exactly removed in Step 1 as GENERIC is an implementation-dependent internal representation of GCC. It is not very flexible/customizable. AFAIK, it is not even yet possible to generate the AST/GENERIC representation. You can extract it yourself with some GCC plugins, but this is a quite tricky task.