Hi!
On Wed, Jul 08, 2020 at 10:00:01AM -0400, Mathieu Desnoyers wrote:
> >> So perhaps you have code like
> >>
> >> int *p;
> >> int x;
> >> ...
> >> asm ("lwz %0,%1" : "=r"(x) : "m"(*p));
> >
> > We indeed have explicit "lwz" and "stw" instructions in there.
> >
> >>
> >> where that last line should actually read
> >>
> >> asm ("lwz%X1 %0,%1" : "=r"(x) : "m"(*p));
> >
> > Indeed, turning those into "lwzx" and "stwx" seems to fix the issue.
> >
> > There has been some level of extra CPP macro coating around those
> > instructions
> > to
> > support both ppc32 and ppc64 with the same assembly. So adding %X[arg] is
> > not
> > trivial.
> > Let me see what can be done here.
>
> I did the following changes which appear to generate valid asm.
> See attached corresponding .S output.
>
> I grepped for uses of "m" asm operand in Linux powerpc code and noticed it's
> pretty much
> always used with e.g. "lwz%U1%X1". I could find one blog post discussing that
> %U is about
> update flag, and nothing about %X. Are those documented ?
Historically, no machine-specific output modifiers were documented.
For GCC 10 i added a few (in
https://gcc.gnu.org/onlinedocs/gcc-10.1.0/gcc/Machine-Constraints.html#Machine-Constraints
), but not all (that user code should use!) yet.
> Although it appears to generate valid asm, I have the feeling I'm relying on
> undocumented
> features here. :-/
It is supported for 30 years or so now. GCC itself uses this a *lot*
internally as well. It works, and it will work forever.
> -#define STORE_WORD "std "
> -#define LOAD_WORD "ld "
> -#define LOADX_WORD "ldx "
> +#define STORE_WORD(arg) "std%U[" __rseq_str(arg) "]%X["
> __rseq_str(arg) "] " /* To memory ("m" constraint) */
> +#define LOAD_WORD(arg) "lwd%U[" __rseq_str(arg) "]%X[" __rseq_str(arg) "] "
> /* From memory ("m" constraint) */
That cannot work (you typoed "ld" here).
Some more advice about this code, pretty generic stuff:
The way this all uses r17 will likely not work reliably.
The way multiple asm statements are used seems to have missing
dependencies between the statements.
Don't try to work *against* the compiler. You will not win.
Alternatively, write assembler code, if that is what you actually want
to do? Not C code.
And done macro-mess this, you want to be able to debug it, and you need
other people to be able to read it!
Segher
