Re: [PATCH 2/3] Fix copy_bitwise()

[Andreas Arnez <arnez at linux dot vnet dot ibm dot com>]

On Mon, Nov 14 2016, Andreas Arnez wrote:

[...]

> diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
> index 93aec1f..3a241a8 100644
> --- a/gdb/dwarf2loc.c
> +++ b/gdb/dwarf2loc.c

[...]

For reference, since the patch itself might be a bit clumsy to read,
here's the resulting function:

/* Copy NBITS bits from SOURCE to DEST starting at the given bit
   offsets.  Use the bit order as specified by BITS_BIG_ENDIAN.
   Source and destination buffers must not overlap.  */

static void
copy_bitwise (gdb_byte *dest, ULONGEST dest_offset,
	      const gdb_byte *source, ULONGEST source_offset,
	      ULONGEST nbits, int bits_big_endian)
{
  unsigned int buf, avail;

  if (nbits == 0)
    return;

  if (bits_big_endian)
    {
      /* Start from the end, then work backwards.  */
      dest_offset += nbits - 1;
      dest += dest_offset / 8;
      dest_offset = 7 - dest_offset % 8;
      source_offset += nbits - 1;
      source += source_offset / 8;
      source_offset = 7 - source_offset % 8;
    }
  else
    {
      dest += dest_offset / 8;
      dest_offset %= 8;
      source += source_offset / 8;
      source_offset %= 8;
    }

  /* Fill BUF with DEST_OFFSET bits from the destination and 8 -
     SOURCE_OFFSET bits from the source.  */
  buf = *(bits_big_endian ? source-- : source++) >> source_offset;
  buf <<= dest_offset;
  buf |= *dest & ((1 << dest_offset) - 1);

  /* NBITS: bits yet to be written; AVAIL: BUF's fill level.  */
  nbits += dest_offset;
  avail = dest_offset + 8 - source_offset;

  /* Flush 8 bits from BUF, if appropriate.  */
  if (nbits >= 8 && avail >= 8)
    {
      *(bits_big_endian ? dest-- : dest++) = buf;
      buf >>= 8;
      avail -= 8;
      nbits -= 8;
    }

  /* Copy the middle part.  */
  if (nbits >= 8)
    {
      size_t len = nbits / 8;

      while (len--)
	{
	  buf |= *(bits_big_endian ? source-- : source++) << avail;
	  *(bits_big_endian ? dest-- : dest++) = buf;
	  buf >>= 8;
	}
      nbits %= 8;
    }

  /* Write the last byte.  */
  if (nbits)
    {
      if (avail < nbits)
	buf |= *source << avail;

      buf &= (1 << nbits) - 1;
      *dest = (*dest & (~0 << nbits)) | buf;
    }
}