Fix nb_iterations_estimate calculation in tree-vect-loop.c

vect_transform_loop has to reduce three iteration counts by
the vectorisation factor: nb_iterations_upper_bound,
nb_iterations_likely_upper_bound and nb_iterations_estimate.
All three are latch execution counts rather than loop body
execution counts.  The calculations were taking that into
account for the first two, but not for nb_iterations_estimate.

This patch updates the way the calculations are done to fix
this and to add a bit more commentary about what is going on.

Tested on aarch64-linux-gnu and x86_64-linux-gnu.  OK to install?

Thanks,
Richard


[ This patch is part of the SVE series posted here:
  https://gcc.gnu.org/ml/gcc/2016-11/msg00030.html ]

gcc/
2016-11-15  Richard Sandiford  <richard.sandiford@arm.com>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

	* tree-vect-loop.c (vect_transform_loop): Protect the updates of
	all three iteration counts with an any_* test.  Use a single update
	for each count.  Fix the calculation of nb_iterations_estimate.

On Tue, Nov 15, 2016 at 1:57 PM, Richard Sandiford
<richard.sandiford@arm.com> wrote:
> vect_transform_loop has to reduce three iteration counts by
> the vectorisation factor: nb_iterations_upper_bound,
> nb_iterations_likely_upper_bound and nb_iterations_estimate.
> All three are latch execution counts rather than loop body
> execution counts.  The calculations were taking that into
> account for the first two, but not for nb_iterations_estimate.
>
> This patch updates the way the calculations are done to fix
> this and to add a bit more commentary about what is going on.
>
> Tested on aarch64-linux-gnu and x86_64-linux-gnu.  OK to install?

Ok.

Richard.

> Thanks,
> Richard
>
>
> [ This patch is part of the SVE series posted here:
>   https://gcc.gnu.org/ml/gcc/2016-11/msg00030.html ]
>
> gcc/
> 2016-11-15  Richard Sandiford  <richard.sandiford@arm.com>
>             Alan Hayward  <alan.hayward@arm.com>
>             David Sherwood  <david.sherwood@arm.com>
>
>         * tree-vect-loop.c (vect_transform_loop): Protect the updates of
>         all three iteration counts with an any_* test.  Use a single update
>         for each count.  Fix the calculation of nb_iterations_estimate.
>
> diff --git a/gcc/tree-vect-loop.c b/gcc/tree-vect-loop.c
> index 1cd9c72..53570f3 100644
> --- a/gcc/tree-vect-loop.c
> +++ b/gcc/tree-vect-loop.c
> @@ -7043,27 +7043,25 @@ vect_transform_loop (loop_vec_info loop_vinfo)
>    /* Reduce loop iterations by the vectorization factor.  */
>    scale_loop_profile (loop, GCOV_COMPUTE_SCALE (1, vf),
>                       expected_iterations / vf);
> -  if (LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo))
> -    {
> -      if (loop->nb_iterations_upper_bound != 0)
> -        loop->nb_iterations_upper_bound = loop->nb_iterations_upper_bound - 1;
> -      if (loop->nb_iterations_likely_upper_bound != 0)
> -        loop->nb_iterations_likely_upper_bound
> -          = loop->nb_iterations_likely_upper_bound - 1;
> -    }
> -  loop->nb_iterations_upper_bound
> -    = wi::udiv_floor (loop->nb_iterations_upper_bound + 1, vf) - 1;
> -  loop->nb_iterations_likely_upper_bound
> -    = wi::udiv_floor (loop->nb_iterations_likely_upper_bound + 1, vf) - 1;
> -
> +  /* The minimum number of iterations performed by the epilogue.  This
> +     is 1 when peeling for gaps because we always need a final scalar
> +     iteration.  */
> +  int min_epilogue_iters = LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo) ? 1 : 0;
> +  /* +1 to convert latch counts to loop iteration counts,
> +     -min_epilogue_iters to remove iterations that cannot be performed
> +       by the vector code.  */
> +  int bias = 1 - min_epilogue_iters;
> +  /* In these calculations the "- 1" converts loop iteration counts
> +     back to latch counts.  */
> +  if (loop->any_upper_bound)
> +    loop->nb_iterations_upper_bound
> +      = wi::udiv_floor (loop->nb_iterations_upper_bound + bias, vf) - 1;
> +  if (loop->any_likely_upper_bound)
> +    loop->nb_iterations_likely_upper_bound
> +      = wi::udiv_floor (loop->nb_iterations_likely_upper_bound + bias, vf) - 1;
>    if (loop->any_estimate)
> -    {
> -      loop->nb_iterations_estimate
> -       = wi::udiv_floor (loop->nb_iterations_estimate, vf);
> -       if (LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo)
> -          && loop->nb_iterations_estimate != 0)
> -        loop->nb_iterations_estimate = loop->nb_iterations_estimate - 1;
> -    }
> +    loop->nb_iterations_estimate
> +      = wi::udiv_floor (loop->nb_iterations_estimate + bias, vf) - 1;
>
>    if (dump_enabled_p ())
>      {
>