[Bug target/87376] [avr] Miscompilation with __memx and long long addition

[gjl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87376

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2018-10-11
     Ever confirmed|0                           |1
      Known to fail|                            |8.0.1

--- Comment #1 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
Hi Senthil, confirmed with v8.0.1 and -O2 -mmcu=atmega128 and test case

extern const __memx long long a asm ("a2");
const long long b = 0x1122334455667788;
const __memx long long a2 = 0x123456789abcdef1;

__attribute((noinline,noclone))
long long add (void) { return a + b; }

int main()
{
    if (add() != a2 + b)
        __builtin_abort();
    return 0;
}

TER propagates parameter register DI:18 into QImode subregs of DI, and we get
the following insn in expand:

(insn ... (parallel [
     (set (reg:QI 21 r21 [+3 ])
          (mem/u/c:QI (reg/f:PSI 51) [1 a+3 S1 A8 AS7]))
     (clobber (reg:QI 22 r22))
     (clobber (reg:QI 21 r21))
     (clobber (reg:HI 30 r30))]))

and a similar insn for R22.  R21 and R22 correspond to a+3 and a+4,
respectively.  The insn is xloadqi_A and emit by movqi expander.

An (untested) fix is to avoid overlaps of the destination register and a
clobbered register, e.g. by means of loading the destination into a new pseudo: 

if (!avr_xload_libgcc_p (<MODE>mode))
  ...
else
  {
    if (REG_P (dest)
        && (reg_operlaps_mentioned_p (dest, all_regs_rtx[21])
            || reg_operlaps_mentioned_p (dest, gen_rtx_REG (<MODE>mode, 22))
            || reg_operlaps_mentioned_p (dest, reg_z)))
      {
        // PR87376: Avoid overlaps of hard reg destination and clobber(s).
        rtx reg = gen_reg_rtx (<MODE>mode);
        emit_insn (gen_xload<mode>_A (reg, src));
        emit_move_insn (dest, reg);
      }
    else
      emit_insn (gen_xload<mode>_A (dest, src));
  }