Hi Linus,
On Sun, Jan 5, 2025 at 10:51 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, 3 Jan 2025 at 01:22, Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> > Perhaps this is an issue with sparse?
>
> No, this is just m68k doing disgusting things that happen to work with
> gcc, because gcc considers casts to be lvalues (which in turn is comes
> from some horrid C++ thing, since in C++ casting is a whole magical
> extra complexity).
>
> IOW, this m68k pattern is horrendous, and sparse quite reasonably
> complains about it:
>
> #define umul_ppmm(w1, w0, u, v) \
> __asm__ ("mulu%.l %3,%1:%0" \
> : "=d" ((USItype)(w0)), \
> "=d" ((USItype)(w1)) \
> : "%0" ((USItype)(u)), \
> "dmi" ((USItype)(v)))
>
> notice how it has two register outputs (the "=d"), and the destination
> of said output is not a proper lvalue, but a cast expression.
>
> I think you could just remove the cast. Afaik the w0/w1 arguments come from
>
> #define __umulsidi3(u, v) \
> ({DIunion __w; \
> umul_ppmm (__w.s.high, __w.s.low, u, v); \
> __w.ll; })
>
> and __w.s.high and __w.s.low are from struct DIstruct, which uses
> "SItype". So all the cast does is to change the signedness of the
> variable, but that has no *meaning* when you assign to it and the
> sizes match.
>
> Alternatively, you could just use the right types explicitly and write
> the umul_ppmm() macro something like
>
> #define umul_ppmm(w1, w0, u, v) do { \
> USItype __w0, __w1; \
> __asm__ ("mulu%.l %3,%1:%0" \
> : "=d" (__w0) \
> "=d" (__w1) \
> : "%0" ((USItype)(u)), \
> "dmi" ((USItype)(v))); \
> w0 = __w0; w1 = __w1; } while (0)
>
> NOTE! UNTESTED! Treat the above as a "something like this, perhaps".
Thank you, using intermediate variables instead of casts for the output
operands gets rid of the lvalue-related sparse warnings.
That leaves us with the "not addressable" sparse error. Apparently
that goes away by dropping the cast on the last input operand,
so I will introduce intermediate variables for all input operands, too.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
