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

[Luis Machado <lgustavo@codesourcery.com>]

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.