Re: [PATCH] libdw: pre-compute leb128 loop limits

[Mark Wielaard <mjw at redhat dot com>]

On Mon, 2014-12-15 at 12:18 -0800, Josh Stone wrote:
> On Fedora 21, this appears to be slightly faster, although pretty close
> to noise levels.  Mark, can you see if this helps the performance slip
> on your el7 system?

It is slightly faster ~0.5 secs on ~55 secs.

>    /* Unrolling 0 like uleb128 didn't prove to benefit optimization.  */
> -  for (unsigned int i = 0; i < len_leb128 (acc) && *addrp < end; ++i)
> +  const size_t max = __libdw_max_len_leb128 (*addrp, end);
> +  for (size_t i = 0; i < max; ++i)
>      get_sleb128_step (acc, *addrp, i);
>    /* Other implementations set VALUE to INT_MAX in this
>       case.  So we better do this as well.  */

Unrolling this does seem to give an addition ~0.2 seconds win.
So both are mostly in the noise, but they do seem to help a little.

But not as much as upgrading GCC :)

Cheers,

Mark