On Wed, 13 Sep 2006, Ricardo Wiggers wrote:
> Steve Hosgood escreveu:
> > Steve Underwood wrote:
> >
> > <snip>
> >> I'm not
> >> sure if MIPs ever changed. Now, the question is, what do people really
> >> expect on a thing like the MSP430.
> >>
> >> If you try something like
> >>
> >> struct
> >> {
> >> int x;
> >> char y;
> >> } xxx[10];
> >>
> >> and loop through the array accessing x, all is well. If you pack the
> >> structure it fails with mspgcc. The only real difference in the
> >> generated code is the loop steps 3 bytes at a time instead of 4. Now, if
> >> you try this with the IAR compiler for the MSP430, things still work
> >> with the packed structure. However, to achieve that complex code is
> >> produced when the structure is packed, and simple code is produced when
> >> it is not. Is this something people would like to see in mspgcc?
> >>
> >>
> >
> > It's an interesting dilemma. My opinion is that it should do what the
> > IAR compiler does, but generate a warning. Alternatively, fail to
> > compile with an error message explaining the issue.
> >
> > The big thing about compiling for compact embedded environments where
> > the MSP430 is often found is that there just isn't the code space to
> > allow code bloat due to possible user mistakes like packing
> > inappropriate structs. A warning would be the least I'd expect.
> >
Hi Steve,
I think there are situations where it is appropriate to use packed
structs. When implementing a network protocol with a fixed frame or
packet structure packed structs come in quite handy:
struct
{
long addr;
char flags;
char data[17];
} my_packet __attribute__((packed));
I have also seen people using unions here in order to apply a different
internal structure to a packet fixed in size.
In this case application requirements clash with MSP430 alignments. I
see the mspgcc compiler to be the one resolving this somehow rather than
failing with an error.
What do you think?
Marten
