Re: [PATCH 3/3] Optimize byte-aligned copies in copy_bitwise()

Luis Machado
2016-11-14T15:38:08Z

On 11/14/2016 09:02 AM, Andreas Arnez wrote:
> The function copy_bitwise used for copying DWARF pieces can potentially
> be invoked for large chunks of data. For instance, consider a large
> struct one of whose members is currently located in a register. In this
> case copy_bitwise would still copy the data bitwise in a loop, which is
> much slower than necessary.
>
> This change uses memcpy for the large part instead, if possible.
>
> gdb/ChangeLog:
>
> * dwarf2loc.c (copy_bitwise): Use memcpy for the middle part, if
> it is byte-aligned.
> ---
> gdb/dwarf2loc.c | 27 +++++++++++++++++++++++----
> 1 file changed, 23 insertions(+), 4 deletions(-)
>
> diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
> index 3a241a8..26f6bd8 100644
> --- a/gdb/dwarf2loc.c
> +++ b/gdb/dwarf2loc.c
> @@ -1547,11 +1547,30 @@ copy_bitwise (gdb_byte *dest, ULONGEST dest_offset,
> {
> size_t len = nbits / 8;
>
> - while (len--)
> + /* Use a faster method for byte-aligned copies. */
> + if (avail == 0)
> {
> - buf |= *(bits_big_endian ? source-- : source++) << avail;
> - *(bits_big_endian ? dest-- : dest++) = buf;
> - buf >>= 8;
> + if (bits_big_endian)
> + {
> + dest -= len;
> + source -= len;
> + memcpy (dest + 1, source + 1, len);
> + }
> + else
> + {
> + memcpy (dest, source, len);
> + dest += len;
> + source += len;
> + }
> + }
> + else
> + {
> + while (len--)
> + {
> + buf |= *(bits_big_endian ? source-- : source++) << avail;

Same as patch 2/3 about the construct.

> + *(bits_big_endian ? dest-- : dest++) = buf;
> + buf >>= 8;
> + }
> }
> nbits %= 8;
> }
>

Otherwise looks sane to me.