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
