restrict-qualifier

Consider the following code: void doesnt_modify(const int *); int foo(int *n) { *n = 42; doesnt_modify(n); return *n; } where the definition of doesnt_modify isn’t visible for the compiler. Thus, it must assume, that doesnt_modify changes the object n points to and must read *n before the return (the last line cannot be replaced by return 42; ). Assume, doesnt_modify doesn’t modify *n . I thought about the following to allow the optimization: int foo_r(int *n) { *n = 42; { /* New scope is important, I think. */ const int *restrict n_restr = n; doesnt_modify(n_restr); return *n_restr; } } This

Is there any practical difference between the following prototypes? void f(const int *p); void f(const int *restrict p); void f(const int *volatile p); The section C11 6.7.6.3/15 (final sentence) says that top-level qualifiers are not considered for the purposes of determining type compatibility, i.e. it is permitted for the function definition to have different top-level qualifiers on its parameters than the prototype declaration had. However (unlike C++) it does not say that they are ignored completely. In the case of const this is clearly moot; however in the case of volatile and restrict

I was browsing through some documentation and questions/answers and saw it mentioned. I read a brief description, stating that it would be basically a promise from the programmer that the pointer won't be used to point somewhere else. Can anyone offer some realistic cases where its worth actually using this? Michael restrict says that the pointer is the only thing that accesses the underlying object. It eliminates the potential for pointer aliasing, enabling better optimization by the compiler. For instance, suppose I have a machine with specialized instructions that can multiply vectors of

Has anyone ever seen any numbers/analysis on whether or not use of the C/C++ restrict keyword in gcc/g++ actual provides any significant performance boost in reality (and not just in theory)? I've read various articles recommending / disparaging its use, but I haven't ran across any real numbers practically demonstrating either sides arguments. EDIT I know that restrict is not officially part of C++, but it is supported by some compilers and I've read a paper by Christer Ericson which strongly recommends its usage. Nils Pipenbrinck The restrict keyword does a difference. I've seen improvements