Re: [PATCH] ranger: Optimise irange_union

[Richard Biener <richard.guenther@gmail.com>]

On Sun, Dec 5, 2021 at 10:55 PM Richard Sandiford via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> When compiling an optabs.ii at -O2 with a release-checking build,
> the hottest function in the profile was irange_union.  This patch
> tries to optimise it a bit.  The specific changes are:
>
> - Use quick_push rather than safe_push, since the final number
>   of entries is known in advance.
>
> - Avoid assigning wi::to_wide & co. to a temporary wide_int,
>   such as in:
>
>     wide_int val_j = wi::to_wide (res[j]);
>
>   wi::to_wide returns a wide_int "view" of the in-place INTEGER_CST
>   storage.  Assigning the result to wide_int forces an unnecessary
>   copy to temporary storage.
>
>   This is one area where "auto" helps a lot.  In the end though,
>   it seemed more readable to inline the wi::to_*s rather than
>   use auto.
>
> - Use to_widest_int rather than to_wide_int.  Both are functionally
>   correct, but to_widest_int is more efficient, for three reasons:
>
>   - to_wide returns a wide-int representation in which the most
>     significant element might not be canonically sign-extended.
>     This is because we want to allow the storage of an INTEGER_CST
>     like 0x1U << 31 to be accessed directly with both a wide_int view
>     (where only 32 bits matter) and a widest_int view (where many more
>     bits matter, and where the 32 bits are zero-extended to match the
>     unsigned type).  However, operating on uncanonicalised wide_int
>     forms is less efficient than operating on canonicalised forms.
>
>   - to_widest_int has a constant rather than variable precision and
>     there are never any redundant upper bits to worry about.
>
>   - Using widest_int avoids the need for an overflow check, since
>     there is enough precision to add 1 to any IL constant without
>     wrap-around.
>
> This gives a ~2% compile-time speed up with the test above.
>
> I also tried adding a path for two single-pair ranges, but it
> wasn't a win.
>
> Tested on aarch64-linux-gnu and x86_64-linux-gnu.  OK to install?

OK.

Thanks,
Richard.

> Richard
>
>
> gcc/
>         * value-range.cc (irange::irange_union): Use quick_push rather
>         than safe_push.  Use widest_int rather than wide_int.  Avoid
>         assigning wi::to_* results to wide*_int temporaries.
> ---
>  gcc/value-range.cc | 46 +++++++++++++---------------------------------
>  1 file changed, 13 insertions(+), 33 deletions(-)
>
> diff --git a/gcc/value-range.cc b/gcc/value-range.cc
> index 82509fa55a7..d38d0786072 100644
> --- a/gcc/value-range.cc
> +++ b/gcc/value-range.cc
> @@ -1550,70 +1550,50 @@ irange::irange_union (const irange &r)
>    // the merge is performed.
>    //
>    // [Xi,Yi]..[Xn,Yn]  U  [Xj,Yj]..[Xm,Ym]   -->  [Xk,Yk]..[Xp,Yp]
> -  tree ttype = r.type ();
> -  signop sign = TYPE_SIGN (ttype);
> -
> -  auto_vec<tree, 20> res;
> -  wide_int u1 ;
> -  wi::overflow_type ovf;
> +  auto_vec<tree, 20> res (m_num_ranges * 2 + r.m_num_ranges * 2);
>    unsigned i = 0, j = 0, k = 0;
>
>    while (i < m_num_ranges * 2 && j < r.m_num_ranges * 2)
>      {
>        // lower of Xi and Xj is the lowest point.
> -      if (wi::le_p (wi::to_wide (m_base[i]), wi::to_wide (r.m_base[j]), sign))
> +      if (wi::to_widest (m_base[i]) <= wi::to_widest (r.m_base[j]))
>         {
> -         res.safe_push (m_base[i]);
> -         res.safe_push (m_base[i + 1]);
> +         res.quick_push (m_base[i]);
> +         res.quick_push (m_base[i + 1]);
>           k += 2;
>           i += 2;
>         }
>        else
>         {
> -         res.safe_push (r.m_base[j]);
> -         res.safe_push (r.m_base[j + 1]);
> +         res.quick_push (r.m_base[j]);
> +         res.quick_push (r.m_base[j + 1]);
>           k += 2;
>           j += 2;
>         }
>      }
>    for ( ; i < m_num_ranges * 2; i += 2)
>      {
> -      res.safe_push (m_base[i]);
> -      res.safe_push (m_base[i + 1]);
> +      res.quick_push (m_base[i]);
> +      res.quick_push (m_base[i + 1]);
>        k += 2;
>      }
>    for ( ; j < r.m_num_ranges * 2; j += 2)
>      {
> -      res.safe_push (r.m_base[j]);
> -      res.safe_push (r.m_base[j + 1]);
> +      res.quick_push (r.m_base[j]);
> +      res.quick_push (r.m_base[j + 1]);
>        k += 2;
>      }
>
>    // Now normalize the vector removing any overlaps.
>    i = 2;
> -  int prec = TYPE_PRECISION (ttype);
> -  wide_int max_val = wi::max_value (prec, sign);
>    for (j = 2; j < k ; j += 2)
>      {
> -      wide_int val_im1 = wi::to_wide (res[i - 1]);
> -      if (val_im1 == max_val)
> -       break;
> -      u1 = wi::add (val_im1, 1, sign, &ovf);
> -
> -      // Overflow indicates we are at MAX already.
> -      // A wide int bug requires the previous max_val check
> -      // trigger: gcc.c-torture/compile/pr80443.c  with -O3
> -      if (ovf == wi::OVF_OVERFLOW)
> -       break;
> -
> -      wide_int val_j = wi::to_wide (res[j]);
> -      wide_int val_jp1 = wi::to_wide (res[j+1]);
>        // Current upper+1 is >= lower bound next pair, then we merge ranges.
> -      if (wi::ge_p (u1, val_j, sign))
> +      if (wi::to_widest (res[i - 1]) + 1 >= wi::to_widest (res[j]))
>         {
>           // New upper bounds is greater of current or the next one.
> -         if (wi::gt_p (val_jp1, val_im1, sign))
> -           res [i - 1] = res[j + 1];
> +         if (wi::to_widest (res[j + 1]) > wi::to_widest (res[i - 1]))
> +           res[i - 1] = res[j + 1];
>         }
>        else
>         {
> --
> 2.31.1
>