Are the Optimization Keywords in C and C++ Reasonable?

Question

So we've all heard the don't-use-register line, the reasoning being that trying to out-optimize a compiler is a fool's errand.

register, from what I know, doesn't actually state anything about CPU registers, just that a given variable can't be referenced indirectly. I'll hazard a guess that it's often referred to as obsolete because compilers can detect a lack of addressing automatically thus making such optimizations transparent.

But if we're firm on that argument, can't it be levelled at every optimization-driven keyword in C? Why do we use inline and C99's restrict for example?

I suppose that some things like aliasing make deducing some optimizations hard or even impossible, so where is the line drawn before we start venturing into Sufficiently Smart Compiler territory?

Where should the line should be drawn in C and C++ between spoon-feeding a compiler optimization information and assuming it knows what it's doing?

EDIT: Jens Gustedt pointed out that my conflating of C and C++ isn't right since two of the keywords have semantic differences and one doesn't exist in standard C++. I had a good link about register in C++ which I'll add if I find it...

Answer 1

I would agree that register and inline are somewhat similar in this respect. If the compiler can see the body of the callee while compiling a call site, it should be able to make a good decision on inlining. The use of the inline keyword in both C and C++ has more to do with the mechanics of making the body of the function visible than with anything else.

restrict, however, is different. When compiling a function, the compiler has no idea of what the call sites are going to be. Being able to assume no aliasing can enable optimizations that would otherwise be impossible.