Re: typeof and operands in named address spaces

[Alexander Monakov <amonakov@ispras.ru>]

On Thu, 5 Nov 2020, Uros Bizjak via Gcc wrote:

> > 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));

This is the same as a plain move though, and the cast to uintptr_t doesn't
do anything, you can simply pass "r"(&x) to the same effect.

The main advantage of passing a "fake" memory location for use with lea is
avoiding base+offset computation outside the asm. If you're okay with one
extra register tied up by the asm, just pass the address to the asm directly:

void foo(__seg_fs int *x)
{
  asm("# %0 (%1)" :: "m"(x[1]), "r"(&x[1]));
  asm("# %0 (%1)" :: "m"(x[0]), "r"(&x[0]));
}

foo:
        leaq    4(%rdi), %rax
        # %fs:4(%rdi) (%rax)
        # %fs:(%rdi) (%rdi)
        ret

Alexander