[Mspgcc-users] Code for bitfield manipulation

[David Brown]

Hi,

I'm using bitfields more and more these days - they are much more
debugger-friendly than the traditional #define's and flag bytes, and let the
compiler do more useful error-checking.

However, one thing that would make them much nicer would be more efficient
code for assignment.  For example,

#define true 1
#define false 0
typedef unsigned char byte;
typedef union {
    struct {
        byte a : 1; byte b : 1; byte c : 1; padding : 5;
    };
    byte raw;
} tFlags;

tFlags x, y;

void test1(void)
{
    x.b = y.c;
}

void test2(void)
{
    if (y.c) x.b = true; else x.b = false;
}


The two functions test1() and test2() have the same efffect, except that
test1() generates a range of shifts and masking operations, whereas test2()
uses a smaller and faster arrangement of bit tests and jumps.  In some
cases, the direct bit manipulation is best, such as when the same bit is
accessed in two structures.  But would it be possible to automatically
generate the jump form when that is more efficient?

mvh.

David