Re: typeof and operands in named address spaces

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Nov 5, 2020 at 1:16 PM Alexander Monakov via Gcc
<gcc@gcc.gnu.org> wrote:
>
>
>
> On Thu, 5 Nov 2020, Uros Bizjak wrote:
>
> > On Thu, Nov 5, 2020 at 12:38 PM Alexander Monakov <amonakov@ispras.ru> wrote:
> > >
> > > On Thu, 5 Nov 2020, Uros Bizjak via Gcc wrote:
> > >
> > > > > What is the usecase for stripping the address space for asm operands?
> > > >
> > > > Please see the end of [2], where the offset to <mem> is passed in %rsi
> > > > to the call to this_cpu_cmpxchg16b_emu. this_cpu_cmpxchg16b_emu
> > > > implements access with PER_CPU_VAR((%rsi)), which expands to
> > > > %gs:(%rsi), so it is the same as %gs:<mem> in cmpxchg16b alternative.
> > > > The offset is loaded by lea <mem>, %rsi to %rsi reg.
> > >
> > > I see, thanks. But then with the typeof-stripping-address-space solution
> > > you'd be making a very evil cast (producing address of an object that
> > > does not actually exist in the generic address space). I can write such
> > > a solution, but it is clearly Undefined Behavior:
> > >
> > > #define strip_as(mem) (*(__typeof(0?(mem):(mem))*)(intptr_t)&(mem))
> > >
> > > void foo(__seg_fs int *x)
> > > {
> > >   asm("# %0" :: "m"(x[1]));
> > >   asm("# %0" :: "m"(strip_as(x[1])));
> > > }
> > >
> > > yields
> > >
> > > foo:
> > >         # %fs:4(%rdi)
> > >         # 4(%rdi)
> > >         ret
> > >
> > >
> > > I think a clean future solution is adding a operand modifier that would
> > > print the memory operand without the segment prefix.
> >
> > I was also thinking of introducing of operand modifier, but Richi
> > advises the following:
> >
> > --cut here--
> > typedef __UINTPTR_TYPE__ uintptr_t;
> >
> > __seg_fs int x;
> >
> > uintptr_t test (void)
> > {
> >  uintptr_t *p = (uintptr_t *)(uintptr_t) &x;
> >  uintptr_t addr;
> >
> >  asm volatile ("lea %1, %0" : "=r"(addr) : "m"(*p));
> >
> >  return addr;
> > }
>
> This is even worse undefined behavior compared to my solution above:
> this code references memory in uintptr_t type, while mine preserves the
> original type via __typeof. So this can visibly break with TBAA (though
> the kernel uses -fno-strict-aliasing, so this particular concern wouldn't
> apply there).
>
> If you don't care about preserving sizeof and type you can use a cast to char:
>
> #define strip_as(mem) (*(char *)(intptr_t)&(mem))

But in the end, on x86 the (uintptr_t)&x cast yields you exactly
the offset from the segment register, no?  The casting back
to (uintrptr_t *) and the "dereference" is just because the
inline asm is not able to build the lea otherwise?  that said,
sth like

 asm volatile ("lea fs:%1, %0" : "=r"(addr) : "r" ((uintptr_t)&x));

with the proper asm template should likely be used.

Of course in case the kernel wants transparent handling of
non-fs and fs-based cases that will be off.

Richard.

>
> Alexander