Re: [PATCH v5 1/7] bits: split the definition of the asm and non-asm GENMASK()

David Laight
2025-03-09T01:59:00Z

On Fri, 7 Mar 2025 18:58:08 +0900
Vincent Mailhol <mailhol.vincent@wanadoo.fr> wrote:

> On 07/03/2025 at 04:23, David Laight wrote:
> > On Thu, 06 Mar 2025 20:29:52 +0900
> > Vincent Mailhol via B4 Relay <devnull+mailhol.vincent.wanadoo.fr@kernel.org> wrote:
> >
> >> From: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
> >>
> >> In an upcoming change, GENMASK() and its friends will indirectly
> >> depend on sizeof() which is not available in asm.
> >>
> >> Instead of adding further complexity to __GENMASK() to make it work
> >> for both asm and non asm, just split the definition of the two
> >> variants.
> > ...
> >> +#else /* defined(__ASSEMBLY__) */
> >> +
> >> +#define GENMASK(h, l) __GENMASK(h, l)
> >> +#define GENMASK_ULL(h, l) __GENMASK_ULL(h, l)
> >
> > What do those actually expand to now?
> > As I've said a few times both UL(0) and ULL(0) are just (0) for __ASSEMBLY__
> > so the expansions of __GENMASK() and __GENMASK_ULL() contained the
> > same numeric constants.
>
> Indeed, in asm, the UL(0) and ULL(0) expands to the same thing: 0.
>
> But the two macros still expand to something different on 32 bits
> architectures:
>
> * __GENMASK:
>
> (((~(0)) << (l)) & (~(0) >> (32 - 1 - (h))))
>
> * __GENMASK_ULL:
>
> (((~(0)) << (l)) & (~(0) >> (64 - 1 - (h))))
>
> On 64 bits architecture these are the same.

I've just fed those into godbolt (https://www.godbolt.org/z/Ter6WE9qE) as:
int fi(void)
{
int v;
asm("mov $(((~(0)) << (8)) & (~(0) >> (32 - 1 - (15)))),%0": "=r" (v));
return v -(((~(0u)) << (8)) & (~(0u) >> (32 - 1 - (15))));
}

gas warns:
:4: Warning: 0xffffffffff00 shortened to 0xffffff00

unsigned long long fll(void)
{
unsigned long long v;
asm("mov $(((~(0)) << (8)) & (~(0) >> (64 - 1 - (15)))),%0": "=r" (v));
return v -(((~(0ull)) << (8)) & (~(0ull) >> (64 - 1 - (15))));
}

(for other architectures you'll need to change the opcode)

For x86 and x86-32 the assembler seems to be doing 64bit maths with unsigned
right shifts - so the second function (with the 64 in it) generates 0xff00.
I doubt a 32bit only assembler does 64bit maths, but the '>> 48' above
might get masked to a '>> 16' by the cpu and generate the correct result.

So __GENMASK() is likely to be broken for any assembler that supports 64bits
when generating 32bit code.
__GENMASK_ULL() works (assuming all have unsigned >>) on 64bit assemblers
(even when generating 32bit code). It may work on some 32bit assemblers.

Since most uses in the header files will be GENMASK() I doubt (hope) no
asm code actually uses the values!
The headers assemble - but that is about all that can be said.

Bags of worms :-)

David