[PATCH][v4] GIMPLE store merging pass

Kyrill Tkachov kyrylo.tkachov@foss.arm.com
Mon Oct 3 13:02:00 GMT 2016


Hi Richard,
another question as I'm working through your comments...

On 29/09/16 11:45, Richard Biener wrote:
>
>> +      /* The region from the byte array that we're inserting into.  */
>> +      tree ptr_wide_int
>> +	= native_interpret_expr (dest_int_type, ptr + first_byte,
>> +				 total_bytes);
>> +
>> +      gcc_assert (ptr_wide_int);
>> +      wide_int dest_wide_int
>> +	= wi::to_wide (ptr_wide_int, TYPE_PRECISION (dest_int_type));
>> +      wide_int expr_wide_int
>> +	= wi::to_wide (tmp_int, byte_size * BITS_PER_UNIT);
>> +      if (BYTES_BIG_ENDIAN)
>> +	{
>> +	  unsigned int insert_pos
>> +	    = byte_size * BITS_PER_UNIT - bitlen - (bitpos % BITS_PER_UNIT);
>> +	  dest_wide_int
>> +	    = wi::insert (dest_wide_int, expr_wide_int, insert_pos, bitlen);
>> +	}
>> +      else
>> +	dest_wide_int = wi::insert (dest_wide_int, expr_wide_int,
>> +				    bitpos % BITS_PER_UNIT, bitlen);
>> +
>> +      tree res = wide_int_to_tree (dest_int_type, dest_wide_int);
>> +      native_encode_expr (res, ptr + first_byte, total_bytes, 0);
>> +
> OTOH this whole dance looks as complicated and way more expensive than
> using native_encode_expr into a temporary buffern and then a
> manually implemented "bit-merging" of it at ptr + first_byte + bitpos.
> AFAICS that operation is even endianess agnostic.

If the quantity we're inserting at a non-byte boundary
is more than a byte wide we still have to shift the value
to position properly across the bytes it straddles, so I don't
see how we can avoid creating a wide_int here.
Consider inserting a 10-bit value at bitposition 3 (I hope the mailer
doesn't screw up the indentation):
value:  xxxxxxxxxx
before: |--------||--------|
         | byte 1 || byte 2 |
after:  |---xxxxx||xxxxx---|

We'll native_encode_expr the value into a two-byte buffer but then we can't
just shift each byte by 3 to insert it into the destination buffer, we need
to form the whole 10-bit value and shift is as a whole to not lose any bits.

And if a value crosses bytes then we need to care about BYTES_BIG_ENDIAN when
writing the bytes back into the buffer, no?

Thanks,
Kyrill



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