On Thu, Oct 22, 2020 at 4:47 PM Qing Zhao <qing.z...@oracle.com> wrote:
>
> Hi, Uros,
>
> > On Oct 21, 2020, at 9:45 AM, Qing Zhao via Gcc-patches
> > <gcc-patches@gcc.gnu.org> wrote:
> >>>>
> >>
> >> Something like this:
> >>
> >> --cut here--
> >> long double
> >> __attribute__ ((noinline))
> >> test (long double a, long double b)
> >> {
> >> long double r = a + b;
> >>
> >> asm volatile ("fldz; \
> >> fldz; \
> >> fldz; \
> >> fldz; \
> >> fldz; \
> >> fldz; \
> >> fldz; \
> >> fstp %%st(0); \
> >> fstp %%st(0); \
> >> fstp %%st(0); \
> >> fstp %%st(0); \
> >> fstp %%st(0); \
> >> fstp %%st(0); \
> >> fstp %%st(0)" : : "X"(r));
> >> return r;
> >> }
> >>
> >> int
> >> main ()
> >> {
> >> long double a = 1.1, b = 1.2;
> >>
> >> long double c = test (a, b);
> >>
> >> printf ("%Lf\n", c);
> >>
> >> return 0;
> >> }
> >> --cut here—
> >
> >
> > Okay, so,
> >
> > 1. First compute how many st registers need to be zeroed, num_of_zeroed_st
> > 2. Then issue (8 - num_of_zeroed_st) fldz to push 0 to the stack to clear
> > all the dead stack slots;
> > 3. Then issue (8 - num_of_zeroed_st) fstp %st(0) to pop the stack and empty
> > the stack.
> >
>
> How to generate such asm volatile insn at i386 backend? Is there any code in
> i386 backend I can refer for this ?
fldz is plain move of zero to XF register, fstp is generated from an
XF move of FIRST_STACK_REG to itself with REG_DEAD note added:
#(insn 366 128 129 9 (set (reg:XF 8 st)
# (reg:XF 8 st)) "test.c":711:14 110 {*movxf_internal}
# (expr_list:REG_DEAD (reg:XF 8 st)
# (nil)))
Uros.
