On Mi, 2011-02-02 at 19:05 +0100, Drasko DRASKOVIC wrote: > On Wed, Feb 2, 2011 at 6:55 PM, Stefan Schulze Frielinghaus > <stefan@xxxxxxxxxxxx> wrote: > >> But have you tried comparing the outputs of what -O0 outputs to say -O2 ? > > > > The problem with using anything else then -O0 is that it enables other > > optimization techniques, e.g. constant propagation (the example of the > > first mail would be scaled down to a simple "return 0;") which I do not > > want. Therefore, I would like to compile my code without any > > optimizations except register allocation. > > Hi Stefan, > keep in mind that "register" keyword is only a **hint** given to > compiler to do register optimization. Compiler is not obliged to > listen to your hints, and it probably does not do so without > optimization turned on (I am not sure if it can be forced). Yes that's true. I only used it in my examples to demonstrate what result I actually wanted. > BTW, you should also keep in mind that ANSI C does not allow for > taking the address of a register object; this restriction does not > apply to C++. However, if the address-of operator (&) is used on an > object, the compiler must put the object in a location for which an > address can be represented. In practice, this means in memory instead > of in a register. Because of this restriction GCC will ignore the > register keyword on variables whos address is taken at any point in > the program. So, ensure that somewhere in your code you are not using > the addresses operator on these variables. This wouldn't be a problem for me because in ANSI C, as you already mentioned, the address-of operator is not allowed for variables which reside in a register, ergo the variable cannot stay in a register and needs to be spilled on the stack. That is perfectly fine for me. I'm more interested in all other variables. Kind regards, Stefan
