GCC Bugzilla Bug 24961

There is no way to specify a constraint for an operand of an asm statement that
may only be a destination operand (ie. the first operand of the st insn).  If
you try to use the "m" constraint together with the %P operand modifier you can
get invalid asm code when %P expands to a register instead of a number.  For
example, the Linux kernel uses this asm statement:

asm volatile ("\n[1:]\tst"#n" %1=%r2%P1\t// %0 gets overwritten by exception
handler\n"
              "\t.xdata4 \"__ex_table\", 1b-., 1f-.\n"
              "[1:]"
              : "=r"(__pu_r8) : "m"(__m(addr)), "rO"(val), "0"(__pu_r8));

Occassionally you'll get an assembler error because the POST_MODIFY is expanded
into a register.

Confirmed.

I believe 25008 will have to be fixed before we can add a working constraint
letter for this.  It should be possible to generate a testcase for this by
taking the example from 25008, changing the "S" to an "m", and then changing
foo to a valid store instruction syntax so as to get the desired assembler
error.  I'll worry about that later when I need a testcase.