David and others,
to my expreience arithemtics with signed bit fields produce much much larger
code, because the sign extension is very costly.
Try:
struct {
int a:3;
} stest;
void test(void)
{
if (a < 3)
a++;
}
Same with unsigned produces much smaller code.
Moreover, fields of 1 bit width should lead to exactly the same code, no
matter if signed or unsigned. But they don't. Why?
Regards,
Ralf
"David Dyck" <[email protected]> schrieb im Newsbeitrag
news:[email protected]...
> On Tue, 26 Aug 2003 at 18:05 -0700, Steve Underwood
<[email protected]...:
>
> > +<sect1 id="tips-and-tricks">
>
> > +
> > +<listitem><para>When defining bit fields, try to use signed integers.
This produces more compact code
> > + that bit fields of unsigned integers.</para></listitem>
>
> I'd like to understand this suggestion a bit more, as if I want to store
0, 1, 2, or 3 in a bitfield,
> it seems like you are suggesting that I should use 3 bits instead of the
expected 2 bits that
> would be required in an unsigned bit field. What can be done to make
msp430-gcc to generate better
> code for the unsigned integer bit field also?
>
>
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