On Thu, Nov 5, 2020 at 1:32 PM Uros Bizjak <ubiz...@gmail.com> wrote:
>
> On Thu, Nov 5, 2020 at 1:24 PM Richard Biener
> <richard.guent...@gmail.com> wrote:
>
> > > 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));
>
> No, this is not how LEA operates. It needs a memory input operand. The
> above will report "operand type mismatch for 'lea'" error.
The following will work:
asm volatile ("lea (%1), %0" : "=r"(addr) : "r"((uintptr_t)&x));
Uros.
