[5/8] Add narrow_bit_field_mem

[Richard Sandiford]

Eric Botcazou <ebotcazou@adacore.com> writes:
>> This patch splits out a fairly common operation: that of narrowing a MEM
>> to a particular mode and adjusting the bit number accordingly.
>> 
>> I've kept with "bit_field" rather than "bitfield" for consistency with
>> the callers, although we do have "bitfield" in "adjust_bitfield_address".
>
> My bad.  Feel free to rename it.

TBH, I prefer "bitfield", so I'll leave it be :-)

>> gcc/
>> 	* expmed.c (narrow_bit_field_mem): New function.
>> 	(store_bit_field_using_insv, store_bit_field_1, store_fixed_bit_field)
>> 	(extract_bit_field_1): Use it.
>
> This is a good cleanup but...
>
>> Index: gcc/expmed.c
>> ===================================================================
>> --- gcc/expmed.c	2012-10-30 19:25:44.797368678 +0000
>> +++ gcc/expmed.c	2012-10-30 19:25:47.730368671 +0000
>> @@ -387,6 +387,23 @@ mode_for_extraction (enum extraction_pat
>>    return data->operand[opno].mode;
>>  }
>> 
>> +/* Adjust bitfield memory MEM so that it points to the first unit of
>> +   mode MODE that contains the bitfield at bit position BITNUM.
>> +   Set NEW_BITNUM to the bit position of the field within the
>> +   new memory.  */
>> +
>> +static rtx
>> +narrow_bit_field_mem (rtx mem, enum machine_mode mode,
>> +		      unsigned HOST_WIDE_INT bitnum,
>> +		      unsigned HOST_WIDE_INT &new_bitnum)
>> +{
>> +  unsigned int unit = GET_MODE_BITSIZE (mode);
>> +  unsigned HOST_WIDE_INT offset = bitnum / unit * GET_MODE_SIZE (mode);
>> +  mem = adjust_bitfield_address (mem, mode, offset);
>> +  new_bitnum = bitnum % unit;
>> +  return mem;
>> +}
>
> Ugh. :-)  I cannot see any advantages in using references here except for...
>
>> +    /* Get a reference to the first byte of the field.  */
>> +    xop0 = narrow_bit_field_mem (xop0, byte_mode, bitnum, bitnum);
>
>> +    op0 = narrow_bit_field_mem (op0, byte_mode, bitnum, bitnum);
>
> ... mightily confusing the reader here.

Would it be OK with a pointer, but keeping the interface the same?
That's certainly fine by me.

That's one of the things I'm not sure about after the C++ conversion:
I've noticed some references creep in, but when should we use references
and when pointers?  I think Richard B made a comment about using references
for things that can't be null.

Richard