Is zero initialization of structures guaranteed to wipe padded areas?

Suppose I have the following structure:

typedef struct
{
    unsigned field1 :1;
    unsigned field2 :1;
    unsigned field3 :1;
} mytype;

The first 3 bits will be usable but sizeof(mytype) will return 4 which means 29 bits of padding. My question is, are these padding bits guaranteed by the standard to be zero initialized by the statement:

mytype testfields = {0};

or:

mytype myfields = {1, 1, 1};

Such that it's safe to perform the following memcmp() on the assumption that bits 4..29 will be zero and therefore won't affect the comparison:

if ( memcmp(&myfields, &testfields, sizeof(myfields)) == 0 )
    printf("Fields have no bits set\n");
else
    printf("Fields have bits set\n");