C/C++ __restrict type

Question

hello.

Is there a way to define using typedef integral/float type which implies no aliasng?

something equivalent to (but primitive construct):

template < typename T >
struct restrict { T* __restrict data; };

as related question, is it possible to ask gcc what it determines alias/no alias of pointer is?

Answer 1

As noted in the comments, many newer C++ compilers do support the C99 implementation of the restrict type qualifier. Since restrict is not a reserved keyword in C++, the compilers generally use __restrict or __restrict__. Both GCC and Visual C++ document this nicely, with explicit references to C99.

The C++ 1998 standard states that "The typedef specifier shall not ... be combined in a decl-specifier-seq with any kind of specifier except a type-specifier." Essentially, it must be a list of type-specifiers, which includes the two cv-qualifiers, const and volatile.

C99 defines typedef similarly, except that its list of qualifiers includes restrict.

It would seem reasonable to anticipate similar support in typedefs for the nonstandard __restrict... but you never know!

A clever and easy way to test this is as follows:

extern void link_fail();

typedef int *__restrict restricted_int_p;

void test(restricted_int_p a, restricted_int_p b) {
    *a = 1;
    *b = 2;

    if (*a == 2) link_fail();
}

This simply exploits the fact that if the unresolved link_fail symbol is found in the object file, the linker will throw an error. If the compiler is properly restricting the two arguments, then it should know the value of a, even after b is changed. Thus, it should strip the entire if block from the generated object file since it will never be run.

Note that although GCC supported the restrict syntax since at least version 3.0, it really didn't perform the proper optimizations until version 4.4 or so.