https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108799
Bug ID: 108799
Summary: Improper deprecation diagnostic for rsp clobber
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: andrew.cooper3 at citrix dot com
Target Milestone: ---
Originally from LKML.
https://lore.kernel.org/lkml/y9lfmq%2fr1%2fpep...@biznet-home.integral.gnuweeb.org/
Slightly modified example: https://godbolt.org/z/xx76nEvKM
Given:
static void clobber_redzone_buggy(void)
{
register unsigned long rsp asm("rsp");
unsigned long fl;
asm volatile ("pushf; popq %[fl]"
: [fl] "=r" (fl)
, "+r" (rsp)
:
: //"rsp"
);
}
static void set_red_zone(long *mem, long val)
{
__asm__ volatile ("movq %[val], %[mem]"
: [mem] "=m" (*mem)
: [val] "r" (val));
}
static long get_red_zone(long *mem)
{
long ret;
__asm__ volatile ("movq %[in], %[out]"
: [out] "=r" (ret)
: [in] "m" (*mem));
return ret;
}
long a_leaf_func_with_red_zone(void)
{
long x;
set_red_zone(&x, 1);
clobber_redzone_buggy();
/* The correct retval is 1 */
return get_red_zone(&x);
}
gcc generates:
a_leaf_func_with_red_zone:
movl $1, %eax
movq %rax, -8(%rsp)
pushf
popq %rax
movq -8(%rsp), %rax
ret
which is buggy because the asm clobbers the same redzone slot as `x` occupies.
Swapping the "+r"(rsp) constraint for an explicit "rsp" clobber generates:
a_leaf_func_with_red_zone:
pushq %rbp
movl $1, %eax
movq %rsp, %rbp
subq $16, %rsp
movq %rax, -8(%rbp)
pushf
popq %rax
movq -8(%rbp), %rax
leave
ret
which seems to do the right thing. It sets up a full stack frame and avoids
using the redzone.
However, doing so yields:
warning: listing the stack pointer register 'rsp' in a clobber list is
deprecated [-Wdeprecated]
note: the value of the stack pointer after an 'asm' statement must be the
same as it was before the statement
The note is incorrect. For ABIs with a redzone, the requirement is stricter
than simply preserving the value of the stack pointer.
The warning suggests that there ought to be a different way to express "this
clobbers the redzone", but there doesn't appear to be any other way. If this
is the case, why is it deprecated?
