[Bug target/39250] unsigned char times 64U produces long slow loop

["aesok at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #1 from aesok at gcc dot gnu dot org  2009-02-23 19:10 -------
Hi.

The GCC always use a shift for optimizing multiply by power of 2 constant.

expr.c:expand_expr_real_1:8680
....
      /* Check for a multiplication with matching signedness.  */
      else if (TREE_CODE (TREE_OPERAND (exp, 0)) == NOP_EXPR
          && TREE_CODE (type) == INTEGER_TYPE
          && (TYPE_PRECISION (TREE_TYPE (TREE_OPERAND (TREE_OPERAND (exp, 0),
0)))
              < TYPE_PRECISION (TREE_TYPE (TREE_OPERAND (exp, 0))))
          && ((TREE_CODE (TREE_OPERAND (exp, 1)) == INTEGER_CST
               && int_fits_type_p (TREE_OPERAND (exp, 1),
                                   TREE_TYPE (TREE_OPERAND (TREE_OPERAND (exp,
0), 0)))
               /* Don't use a widening multiply if a shift will do.  */
               && ((GET_MODE_BITSIZE (TYPE_MODE (TREE_TYPE (TREE_OPERAND (exp,
1))))
                    > HOST_BITS_PER_WIDE_INT)
                   || exact_log2 (TREE_INT_CST_LOW (TREE_OPERAND (exp, 1))) <
0))

expmed.c:expand_mult
...
      if (coeff != 0)
        {
          /* Special case powers of two.  */
          if (EXACT_POWER_OF_2_OR_ZERO_P (coeff))
            return expand_shift (LSHIFT_EXPR, mode, op0,
                                 build_int_cst (NULL_TREE, floor_log2 (coeff)),
                                 target, unsignedp);


For the AVR target for multiply by 2 with using  a shift give better code,
but for multiply by 4,8, ... using a shift is bad and for code size and for
speed. 

I think this optimization should not be hard coded, but should be chosen
based on the insn cost data. Perhaps there are other targets, which is better
to use multiplication rather than a shift.

Anatoly.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39250