knowing how a compiler works in the most unusual situations is always
interesting...and this discussion is no different.
however, a compiler is just a machine with a lot of rules, so you
cannot expect it to be "clever" anytime, even if correct code is a
necessity and efficiency is only a plus;
but in this case, reading the C source code, I honestly can't
understand for sure what it meant....if I were to translate manually
that code into asm, I would have asked a clarification to the
programmer: did you want to sample twice the port and then add these
two samples ? (say for making an average) or did you want two sample
the port once and double that reading ?
IMHO the answer wasn't clear, even if I considered all the volatile qualifiers;
so I wouldn't try to add another complicated rule to gcc, but I would
rather write a better C code...an unambiguous one that the "simple"
rules of any compiler can understand.
for sampling twice: x = P6IN; x += P6IN;
for doubling the sampling: x = P6IN * 2;
the compiler won't mistake.
By the way, I think it's also better to change your initial example:
unsigned two;
void foo(void)
{
two = P6IN + P6IN;
}
into:
unsigned two;
void foo(void)
unsigned int tmp;
{
tmp = P6IN + P6IN; // <----- tmp = 2 * P6IN; // <----- tmp =
P6IN; tmp+= P6IN;
two = tmp;
}
or even better a function. it's more efficient (and easier to compile)
if you make the calculations through local variables.
Roberto
2006/7/1, Grant Edwards <gra...@visi.com>:
On 2006-06-30, Chris Liechti <cliec...@gmx.net> wrote:
>>>>Just for fun, I tried compiling the code with "two" changed to
>>>>an unsigned char. My mps430 compiler (3.2.3) then gives
>>>>
>>>> mov.b &P6IN, r15
>>>> rla.b r15
>>>> mov.b r15, &two
>>>> ret
>>>>
>>>>In other words, it makes the same mistake you did and
>>>>disregards the "volatile" qualifier. This is far more serious
>>>>than the original question - it is incorrect code, rather than
>>>>just inefficient code.
>> Rather than just complaining, I should try to fix it. Is this
>> something that could be fixed by somebody like me who's only
>> done minor hacking on GCC?
>
> if you already have an idea how the backend of gcc works i
> could think of "yes"
I've read descriptions of the backend, but never worked on it.
I've only tweaked a few things in the front end.
> gcc likes to output code for volatile acceess as
> load/modify/store, even when with the MSP430 a single mov,
> bis, bic/and would be fine too. there are extra optinizations
> by mspgcc to generate efficent code in such a case. that might
> also explain the different code at the two optimization
> levels.
It does generate correct code for -O0:
foo:
mov.b &0x0034, r15
add.b &0x0034, r15
mov.b r15, &two
ret
At -O1 and -O2, it generates the incorrect code as previously
noted.
> maybe someone could check with -mno-volatile-workaround
No difference. It still generates incorrect code for the
-O[12] case when the destination matches the width of the
source (both 8 bits wide). For the original case where the
destination i s16 bits wide, it stell generates an extra mov.b
and an extraneous and.b #-1,r15.
>> I see that there are several more recent versions of mspgcc in
>> CVS. Are any of the newer ones ready for production use? If
>> not, is 3.2.3 still being maintained?
>
> 3.2.3 is our "stable" version
Is it getting bug fixes?
> the latest 4.x should be the next version, but it's not yet
> ready
--
Grant Edwards grante Yow! YOU'D cry too if it
at happened to YOU!!
visi.com
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