https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89502
--- Comment #5 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Uroš Bizjak from comment #3)
> (In reply to H.J. Lu from comment #0)
> > x.s: Assembler messages:
> > x.s:11: Error: can't encode segment `%fs' with 32-bit address
> > x.s:18: Error: can't encode segment `%fs' with 32-bit address
>
> Is this some new warning, since
Yes. it will be in binutils 2.33.
> GNU assembler version 2.31.1-17.fc29
>
> happily assembles
>
> movl %fs:(%edx), %ecx
>
> to:
>
> 8: 64 67 8b 0a mov %fs:(%edx),%ecx
>
> Anyway, the following patch should fix this issue:
>
> --cut here--
> diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
> index e77653d66a4..67872e1c15d 100644
> --- a/gcc/config/i386/i386.c
> +++ b/gcc/config/i386/i386.c
> @@ -18542,6 +18542,10 @@ ix86_print_operand_address_as (FILE *file, rtx addr,
>
> if (!ADDR_SPACE_GENERIC_P (as))
> {
> + /* We can't have 0x67 prefix with segment override. */
> + if (TARGET_64BIT)
> + code = 'q';
> +
> if (ASSEMBLER_DIALECT == ASM_ATT)
> putc ('%', file);
>
> --cut here--
I think we should avoid %fs:(%edx) address. Is that possible to generate
%fs:24 even at -O0? Glibc can do it at -O0:
[hjl@gnu-cfl-1 gcc]$ cat x.i
int
foo (void)
{
int i;
asm volatile ("movl %%fs:%P1, %0" : "=r" (i) : "i" (24));
return i;
}
[hjl@gnu-cfl-1 gcc]$ gcc -S -O0 x.i
[hjl@gnu-cfl-1 gcc]$ cat x.s
.file "x.i"
.text
.globl foo
.type foo, @function
foo:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
#APP
# 5 "x.i" 1
movl %fs:24, %eax
# 0 "" 2
#NO_APP
movl %eax, -4(%rbp)
movl -4(%rbp), %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size foo, .-foo
.ident "GCC: (GNU) 8.3.1 20190223 (Red Hat 8.3.1-2)"
.section .note.GNU-stack,"",@progbits
[hjl@gnu-cfl-1 gcc]$
