[Bug target/27006] New: Invalid altivec constant loading code

[uweigand at gcc dot gnu dot org]

When compiling the following code with -O0 -maltivec:

typedef union
{
  int i[4];
  __attribute__((altivec(vector__))) int v;
} vec_int4;

int main (void)
{
   vec_int4 i1;

   i1.v = (__attribute__((altivec(vector__))) int){31, 31, 31, 31};
   printf ("%d\n", i1.i[0]);

   return 0;
}

the output printed is 30, not 31.

The load of the vector constant is done by the following pair
of instructions:

        vspltisw 0,15
        vadduwm 0,0,0

which are generated by this splitter in altivec.md:

(define_split
  [(set (match_operand:VI 0 "altivec_register_operand" "")
        (match_operand:VI 1 "easy_vector_constant_add_self" ""))]
  "TARGET_ALTIVEC && reload_completed"
  [(set (match_dup 0) (match_dup 3))
   (set (match_dup 0) (plus:VI (match_dup 0)
                               (match_dup 0)))]
{
  rtx dup = gen_easy_altivec_constant (operands[1]);
  rtx const_vec;

  /* Divide the operand of the resulting VEC_DUPLICATE, and use
     simplify_rtx to make a CONST_VECTOR.  */
  XEXP (dup, 0) = simplify_const_binary_operation (ASHIFTRT, QImode,
                                                   XEXP (dup, 0), const1_rtx);
  const_vec = simplify_rtx (dup);

  if (GET_MODE (const_vec) == <MODE>mode)
    operands[3] = const_vec;
  else
    operands[3] = gen_lowpart (<MODE>mode, const_vec);
})

Now, easy_vector_constand_add_self accepts all constants between
16 and 31, where I think it should really only be accepting *even*
constants.

The test is really implemented in rs6000.h:

#define EASY_VECTOR_15_ADD_SELF(n) (!EASY_VECTOR_15((n))        \
                                    && EASY_VECTOR_15((n) >> 1))

and adding a condition ((n) & 1) == 0 here fixes the problem.

Is this the proper solution?


-- 
           Summary: Invalid altivec constant loading code
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: uweigand at gcc dot gnu dot org
GCC target triplet: powerpc-*-linux*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27006