[coreboot-gerrit] Change in ...coreboot[master]: lib.h: Add convenient memcpy() variants

[Jacob Garber (Code Review)]

Jacob Garber has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/32740 )

Change subject: lib.h: Add convenient memcpy() variants
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Patch Set 1:

We can use _Generic from C11 and the __typeof__ extension from GCC to detect if something is a pointer or an array. First, we can define the macro

#define IS_COMPATIBLE(x, y) _Generic((x), __typeof__(y) : true, default : false)

This will switch on the type of x and check if it is compatible with y. Next, detecting if something is an array can be done using

#define IS_ARRAY(x) !IS_COMPATIBLE(&*x, x)

For pointers, & and * are inverses, so &*x and x will have the same type. For an array x of type T[N] however, &*x will have the type *T, which is incompatible with T[N]. For example,

int x[2];
    int *y;
    static_assert(IS_ARRAY(x));  // will succeed!
    static_assert(IS_ARRAY(y));  // will fail!

void test(int x[2]) {
        // will fail, since x is not an array even though it looks like one
        static_assert(IS_ARRAY(x));
    }

I think having a macro (or macros) specifically for copying arrays would be useful - it is certainly helpful in the patches Nico has based on this commit.

More _Generic macro magic can be read about here, which is the web page that inspired this post.
http://www.robertgamble.net/2012/01/c11-generic-selections.html