This patch treats the mode associated with an integer constant as a
scalar_mode.  We can't use the more natural-sounding scalar_int_mode
because we also use (const_int 0) for bounds-checking modes.  (It might
be worth adding a bounds-specific code instead, but that's for another
day.)

This exposes a latent bug in simplify_immed_subreg, which for
vectors of CONST_WIDE_INTs would pass the vector mode rather than
the element mode to rtx_mode_t.

I think the:

		  /* We can get a 0 for an error mark.  */
		  || GET_MODE_CLASS (mode) == MODE_VECTOR_INT
		  || GET_MODE_CLASS (mode) == MODE_VECTOR_FLOAT

in immed_double_const is dead.  trunc_int_mode (via gen_int_mode)
would go on to ICE if the mode fitted in a HWI, and surely plenty
of other code would be confused to see a const_int be interpreted
as a vector.  We should instead be using CONST0_RTX (mode) if we
need a safe constant for a particular mode.

We didn't try to make these functions take scalar_mode arguments
because in many cases that would be too invasive at this stage.
Maybe it would become feasible in future.  Also, the long-term
direction should probably be to add modes to constant integers
rather than have then as VOIDmode odd-ones-out.  That would remove
the need for rtx_mode_t and thus remove the question whether they
should use scalar_int_mode, scalar_mode or machine_mode.

The patch also uses scalar_mode for the CONST_DOUBLE handling
in loc_descriptor.  In that case the mode can legitimately be
either floating-point or integral.

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

gcc/
	* emit-rtl.c (immed_double_const): Use is_a <scalar_mode> instead
	of separate mode class checks.  Do not allow vector modes here.
	(immed_wide_int_const): Use as_a <scalar_mode>.
	* explow.c (trunc_int_for_mode): Likewise.
	* rtl.h (wi::int_traits<rtx_mode_t>::get_precision): Likewise.
	(wi::shwi): Likewise.
	(wi::min_value): Likewise.
	(wi::max_value): Likewise.
	* dwarf2out.c (loc_descriptor): Likewise.
	* simplify-rtx.c (simplify_immed_subreg): Fix rtx_mode_t argument
	for CONST_WIDE_INT.

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

From-SVN: r251517