Re: Add an array_mode_supported_p target hook

[Richard Sandiford <richard.sandiford@linaro.org>]

Richard Earnshaw <rearnsha@arm.com> writes:
>> +/* Implements target hook array_mode_supported_p.  */
>> +
>> +static bool
>> +arm_array_mode_supported_p (enum machine_mode mode,
>> +			    unsigned HOST_WIDE_INT nelems)
>> +{
>> +  if (TARGET_NEON
>> +      && (VALID_NEON_DREG_MODE (mode) || VALID_NEON_QREG_MODE (mode))
>> +      && (nelems >= 2 && nelems <= 4))
>> +    return true;
>> +
>> +  return false;
>> +}
>
> I'm not sure I understand why this is limited to 4 or fewer elements. A
> Q reg of chars would surely be 16 elements.

The mode here is the mode of the array element, which for the cases
we're interested in would be something like V4HI (D) or V4SI (Q).
nelems says how many of those (in our case, vector) elements there
are in the array.

The element range we want is 1-4 because that matches the number
of vectors that can be loaded by the vld1-vld4 instructions.
We don't include 1 because arrays of one element are already
treated as having the same mode as their element.

Richard