[Bug rtl-optimization/70504] New: FLD, FLD, FXCH emitted instead of FLD, FLD in the needed order

["b7.10110111 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70504

            Bug ID: 70504
           Summary: FLD, FLD, FXCH emitted instead of FLD, FLD in the
                    needed order
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: b7.10110111 at gmail dot com
  Target Milestone: ---

The following code demonstrates the bug:


long double inl_scalbn(long double mant, long double exp)
{
    long double result;
    asm("fscale"
        : "=&t"(result)
        : "%0"(mant),
          "u"(exp)
       );
    return result;
}


With `-O3` option GCC generates the following assembly:


inl_scalbn:
        fld     TBYTE PTR [esp+4]
        fld     TBYTE PTR [esp+16]
        fxch    st(1)
        fscale
        fstp    st(1)
        ret


What's even stranger, I thought it was somehow related to order of function
arguments, but if I switch `mant` and `exp`, the code just switches `fld`
instructions instead of removing `fxch`.

It's clear that in both cases the code could have just loaded the parameters in
the correct order in the first place.