I don't think so, and even where it would be, GCC's IR can evolve. But GP was talking about the limitations of C, not GCC. C's memory model puts some limits on

I don't think so, and even where it would be, GCC's IR can evolve. But GP was talking about the limitations of C, not GCC. C's memory model puts some limits on what a compiler can do that other languages might not. Fortran is usually given as an example of a language that can be faster than C, because the compiler is allowed to do more. Rust might get there as well, at least in very specific circumstances. Given that GCC also has a good Fortran frontend, I'd assume that their IR is already more powerful than what's needed to compile C alone.