On Thu, Oct 07, 2021 at 08:04:23PM -0500, Paul A. Clarke wrote:
> On Thu, Oct 07, 2021 at 06:39:06PM -0500, Segher Boessenkool wrote:
> > > + __asm__ __volatile__ ("mffsce %0" : "=f" (__fpscr_save.__fr));
> >
> > The __volatile__ does likely not do what you want. As far as I can see
> > you do not want one here anyway?
> >
> > "volatile" does not order asm wrt fp insns, which you likely *do* want.
>
> Reading the GCC docs, it looks like the "volatile" qualifier for "asm"
> has no effect at all (6.47.1):
>
> | The optional volatile qualifier has no effect. All basic asm blocks are
> | implicitly volatile.
>
> So, it could be removed without concern.
This is not a basic asm (it contains a ":"; that is not just an easy way
to see it, it is the *definition* of basic vs. extended asm).
The manual explains:
"""
Note that the compiler can move even 'volatile asm' instructions
relative to other code, including across jump instructions. For
example, on many targets there is a system register that controls the
rounding mode of floating-point operations. Setting it with a 'volatile
asm' statement, as in the following PowerPC example, does not work
reliably.
asm volatile("mtfsf 255, %0" : : "f" (fpenv));
sum = x + y;
The compiler may move the addition back before the 'volatile asm'
statement. To make it work as expected, add an artificial dependency to
the 'asm' by referencing a variable in the subsequent code, for example:
asm volatile ("mtfsf 255,%1" : "=X" (sum) : "f" (fpenv));
sum = x + y;
"""
> > You do not need any of that __ either.
>
> I'm surprised that I don't. A .h file needs to be concerned about the
> namespace it inherits, no?
These are local variables in a function though. You get such
complexities in macros, but never in functions, where everything is
scoped. Local variables are a great thing. And macros are a bad thing!
Segher
