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

Andreas Arnez
2016-11-15T18:58:02Z

[-- Attachment #1: Type: text/plain, Size: 288 bytes --]

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:

[-- Attachment #2: copy_bitwise.c --]
[-- Type: text/plain, Size: 1870 bytes --]

/* 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;
}
}