The main loop of simplify_comparison starts with:

      if (GET_MODE_CLASS (mode) != MODE_INT
          && ! (mode == VOIDmode
                && (GET_CODE (op0) == COMPARE || COMPARISON_P (op0))))
        break;

So VOIDmode is only acceptable when comparing a COMPARE,
EQ, NE, etc. operand against a constant.  After this, the loop
calls simplify_compare_const to:

  (a) bring the constant op1 closer to 0 where possible and
  (b) use nonzero_bits and num_sign_bit_copies to get a simpler
      constant.

(a) works for both integer and VOID modes, (b) is specific
to integer modes.

The loop then has a big switch statement that handles further
simplifications.  This switch statement checks for COMPARISON_P
codes but not for COMPARE.

This patch uses scalar_int_mode to make the split between
(a) and (b) more explicit.  It also takes the COMPARISON_P
handling out of the switch statement and does it first,
so that the rest of the loop can treat the mode as a
scalar_int_mode.

2017-08-30  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* combine.c (simplify_compare_const): Check that the mode is a
	scalar_int_mode (rather than VOIDmode) before testing its
	precision.
	(simplify_comparison): Move COMPARISON_P handling out of the
	loop and restrict the latter part of the loop to scalar_int_modes.
	Check is_a <scalar_int_mode> before calling HWI_COMPUTABLE_MODE_P
	and when considering SUBREG_REGs.  Use is_int_mode instead of
	checking GET_MODE_CLASS against MODE_INT.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r251495