Thanks for giving us the official rules. That makes it even more important
to avoid "a = b = c" constructs, especially for volatile data. I knew about
function argument evaluation ordering (or lack thereof), but I was not sure
about such multiple assignments (in fact, I was wrong about them !).
David
> Hi,
>
> ANSI-C (C99) says about the assignment operators (6.5.16.4):
>
> The order of evaluation of the operands is unspecified.
>
> This means it's compiler dependent and allowed to change the order e. g.
every day or in random order (i would prefer random order).
> This is also true for many other operators.
> It's the same with function arguments; the order in which the arguments
are evaluated is not defined; some compiler do it from left to right, some
from right to left and other do optimize it to a case dependent order.
>
> You can by an exemplar of the standard at www.ansi.org. It's an pdf for
about 15 $ which can be paied online with a visa card and will be mailed in
a few minutes.
>
> Rolf
>
>
> mspgcc-users@lists.sourceforge.net schrieb am 16.08.04 14:39:04:
> >
> > Hi,
> >
> > First off, I would say that writing "a = b = c" is a bad idea, precisely
> > because it is not clear what it means. There are several
interpretations,
> > some of which might be the same - what the author thinks it means, what
the
> > reader thinks it means, what the compiler thinks it means, what ANSI C
says
> > it means, what the reader thinks the author thinks it means, and so on.
> >
> > To my knowledge, ANSI C says "a = b = c" means "b = c; a = b;", which is
> > precisely how mspgcc is interpreting it. Since the registers you are
using
> > are special registers, and therefore treated as volatile, this means
that
> > IE2 must be re-read before assigning the value to a. When you use your
> > defines, the variables are not volatile, so gcc already knows what value
it
> > has put into IE2, and can re-use it without reading IE2 - after all,
since
> > IE2 is now declared not volatile, it must (as far as gcc knows) contain
the
> > value you wrote to it.
> >
> > As far as I can see, mspgcc is therefore giving you exactly the code you
> > asked for. The correct way to write your code would therefore be:
> > IE1 = 0;
> > IE2 = 0;
> >
> >
> > David
> >
> >
> > > Hallo,
> > >
> > > <disclaimer>
> > > I don't claim i've found the bug in mspgcc I use, but it seems to me
> > > that the behaviour I've found in mspgcc is at least not ANSI C
> > > conformant (I don't have access to real ANSI C specification but the
> > > information I've found in ANSI by Kernighan & Ritchi confirm my
> > > suspicions).
> > > </disclaimer>
> > >
> > > The operator '=' assigns the right side operant, which can be lvalue
or
> > > not lvalue expression to left side operand which has to be lvalue.
> > > But what's the value of the expression (a = b)? According to my
> > > knowledge it's not lvalue but value of the b operand assigned.
> > > How mspgcc behaves?
> > >
> > > Consider the little app, compiled for msp430x147
(only -mmcpu=msp430x147
> > > for compiler given):
> > >
> > > int
> > > main() {
> > > IE1 = IE2 = 0;
> > > return 0;
> > > }
> > >
> > > The interesting is the expression IE1 = IE2 = 0;
> > >
> > > When IE1 and IE2 are defined exactly as in headers for msp430f14x as:
> > > sfrb(IE1,0x0000);
> > > sfrb(IE2,0x0001);
> > > The expression is compiled into (indepenant on level of optimisation)
> > > as:
> > >
> > > mov.b #llo(0), &0x0001 ; 9 *movqi3/3 [length = 2]
> > > mov.b &0x0001, r15 ; 10 *movqi3/5 [length = 2]
> > > mov.b r15, &0x0000 ; 11 *movqi3/2 [length = 2]
> > >
> > > It takes 3 words in mem, 11 cycles for execution according to my
> > > knowledge.
> > >
> > > When IE1 and IE2 are defined as:
> > > #define IE1 (*(unsigned char*)0x0000)
> > > #define IE2 (*(unsigned char*)0x0001)
> > > The expression is compiled into (independant on level of optimisation
> > > given by -O option)
> > > as:
> > >
> > > mov.b #llo(0), &1 ; 11 *movqi3/3 [length = 2]
> > > mov.b #llo(0), &0 ; 12 *movqi3/3 [length = 2]
> > >
> > > It takes 2 words in mem, 8 cycles for execution.
> > >
> > > And last but not least: the second output seems to be correct code,
the
> > > first is not !!!
> > > (the second approach has one drawback: It doesn't produce good debug
> > > symbols, but one could make simple workaround for it with std library
> > > and linker).
> > >
> > > Consider the situation with:
> > > - interrupts (the memory cell content can always be changed in
between)
> > > - special registers - some bits are read-only or undefined.
> > >
> > > After execution the expression produces the values:
> > > IE2: 0
> > > IE1: 1
> > > Probably because IE2 has some undefined bits.
> > >
> > > The mspgcc & C-library version I've tried was gcc-3.2.3 with patches
> > > from mspgcc CVS from 30.July.2004.
> > > I hope that mspgcc maintainers would have some ideas how to solve it,
> > >
> > > regards
> > >
> > >
> > >
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> >
> >
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