David Brown wrote:
>
> 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?
>
How about
void test3(void)
{
x.b = (y.c == true);
}
Garst
