Re: [PATCH] Analyze niter for until-wrap condition [PR101145]

["Bin.Cheng" <amker.cheng@gmail.com>]

On Thu, Jul 1, 2021 at 10:06 AM Jiufu Guo via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> For code like:
> unsigned foo(unsigned val, unsigned start)
> {
>   unsigned cnt = 0;
>   for (unsigned i = start; i > val; ++i)
>     cnt++;
>   return cnt;
> }
>
> The number of iterations should be about UINT_MAX - start.
>
> There is function adjust_cond_for_loop_until_wrap which
> handles similar work for const bases.
> Like adjust_cond_for_loop_until_wrap, this patch enhance
> function number_of_iterations_cond/number_of_iterations_lt
> to analyze number of iterations for this kind of loop.
>
> Bootstrap and regtest pass on powerpc64le, is this ok for trunk?
>
> gcc/ChangeLog:
>
>         PR tree-optimization/101145
>         * tree-ssa-loop-niter.c
>         (number_of_iterations_until_wrap): New function.
>         (number_of_iterations_lt): Invoke above function.
>         (adjust_cond_for_loop_until_wrap):
>         Merge to number_of_iterations_until_wrap.
>         (number_of_iterations_cond): Update invokes for
>         adjust_cond_for_loop_until_wrap and number_of_iterations_lt.
>
> gcc/testsuite/ChangeLog:
>
>         PR tree-optimization/101145
>         * gcc.dg/vect/pr101145.c: New test.
>         * gcc.dg/vect/pr101145.inc: New test.
>         * gcc.dg/vect/pr101145_1.c: New test.
>         * gcc.dg/vect/pr101145_2.c: New test.
>         * gcc.dg/vect/pr101145_3.c: New test.
> ---
>  gcc/testsuite/gcc.dg/vect/pr101145.c   | 187 +++++++++++++++++++++++++
>  gcc/testsuite/gcc.dg/vect/pr101145.inc |  63 +++++++++
>  gcc/testsuite/gcc.dg/vect/pr101145_1.c |  15 ++
>  gcc/testsuite/gcc.dg/vect/pr101145_2.c |  15 ++
>  gcc/testsuite/gcc.dg/vect/pr101145_3.c |  15 ++
>  gcc/tree-ssa-loop-niter.c              | 150 +++++++++++---------
>  6 files changed, 380 insertions(+), 65 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.dg/vect/pr101145.c
>  create mode 100644 gcc/testsuite/gcc.dg/vect/pr101145.inc
>  create mode 100644 gcc/testsuite/gcc.dg/vect/pr101145_1.c
>  create mode 100644 gcc/testsuite/gcc.dg/vect/pr101145_2.c
>  create mode 100644 gcc/testsuite/gcc.dg/vect/pr101145_3.c
>

> diff --git a/gcc/tree-ssa-loop-niter.c b/gcc/tree-ssa-loop-niter.c
> index b5add827018..06db6a36ef8 100644
> --- a/gcc/tree-ssa-loop-niter.c
> +++ b/gcc/tree-ssa-loop-niter.c
> @@ -1473,6 +1473,86 @@ assert_loop_rolls_lt (tree type, affine_iv *iv0, affine_iv *iv1,
>      }
>  }
>
> +/* Determines number of iterations of loop whose ending condition
> +   is IV0 < IV1 which likes:  {base, -C} < n,  or n < {base, C}.
> +   The number of iterations is stored to NITER.  */
> +
> +static bool
> +number_of_iterations_until_wrap (class loop *, tree type, affine_iv *iv0,
> +                                affine_iv *iv1, class tree_niter_desc *niter)
> +{
> +  tree niter_type = unsigned_type_for (type);
> +  tree max, min;
> +
> +  if (POINTER_TYPE_P (type))
> +    {
> +      max = fold_convert (type, TYPE_MAX_VALUE (niter_type));
> +      min = fold_convert (type, TYPE_MIN_VALUE (niter_type));
> +    }
> +  else
> +    {
> +      max = TYPE_MAX_VALUE (type);
> +      min = TYPE_MIN_VALUE (type);
> +    }
> +
> +  tree high = max, low = min, one = build_int_cst (niter_type, 1);
> +  tree step;
> +
> +  /* n < {base, C}. */
> +  if (integer_zerop (iv0->step) && TREE_CODE (iv1->step) == INTEGER_CST
> +      && !tree_int_cst_sign_bit (iv1->step))
> +    {
> +      step = iv1->step;
> +      niter->niter = fold_build2 (MINUS_EXPR, niter_type, max, iv1->base);
max/iv1->base could be of pointer type, not sure if this is canonical though.

> +      if (TREE_CODE (iv1->base) == INTEGER_CST)
> +       low = fold_build2 (MINUS_EXPR, type, iv1->base, one);
> +      else if (TREE_CODE (iv0->base) == INTEGER_CST)
> +       low = iv0->base;
> +    }
> +  /* {base, -C} < n. */
> +  else if (TREE_CODE (iv0->step) == INTEGER_CST
> +          && tree_int_cst_sign_bit (iv0->step) && integer_zerop (iv1->step))
> +    {
> +      step = fold_build1 (NEGATE_EXPR, TREE_TYPE (iv0->step), iv0->step);
> +      niter->niter = fold_build2 (MINUS_EXPR, niter_type, iv0->base, min);
> +      if (TREE_CODE (iv0->base) == INTEGER_CST)
> +       high = fold_build2 (PLUS_EXPR, type, iv0->base, one);
> +      else if (TREE_CODE (iv1->base) == INTEGER_CST)
> +       high = iv1->base;
> +    }
> +  else
> +    return false;
> +
> +  /* (delta + step - 1) / step */
> +  step = fold_convert (niter_type, step);
> +  niter->niter = fold_convert (niter_type, niter->niter);
> +  niter->niter = fold_build2 (PLUS_EXPR, niter_type, niter->niter, step);
> +  niter->niter = fold_build2 (FLOOR_DIV_EXPR, niter_type, niter->niter, step);
> +
> +  tree m = fold_build2 (MINUS_EXPR, niter_type, high, low);
> +  m = fold_convert (niter_type, m);
> +  mpz_t mstep, tmp, mmax;
> +  mpz_init (mstep);
> +  mpz_init (tmp);
> +  mpz_init (mmax);
> +  wi::to_mpz (wi::to_wide (step), mstep, UNSIGNED);
> +  wi::to_mpz (wi::to_wide (m), mmax, UNSIGNED);
> +  mpz_add (tmp, mmax, mstep);
> +  mpz_sub_ui (tmp, tmp, 1);
> +  mpz_fdiv_q (tmp, tmp, mstep);
> +  niter->max = widest_int::from (wi::from_mpz (niter_type, tmp, false),
> +                                TYPE_SIGN (niter_type));
This computation is similar to function number_of_iterations_lt, could
we factor it out into an independent function?

> +  mpz_clear (mstep);
> +  mpz_clear (tmp);
> +
> +  niter->may_be_zero
> +    = fold_build2 (LE_EXPR, boolean_type_node, iv1->base, iv0->base);
If iv0->base and iv1->base are constant and iv1->base <= iv0->base,
the number of iteration is actually zero, but here we rely on
may_be_zero (== true), which loses information.  Could we specially
handle this case and do a fast return?

Could you test this on some more targets(x86, aarch64) please?  Otherwise LGTM.

Thanks,
bin
> +
> +  niter->control.no_overflow = false;
> +
> +  return true;
> +}
> +
>  /* Determines number of iterations of loop whose ending condition
>     is IV0 < IV1.  TYPE is the type of the iv.  The number of
>     iterations is stored to NITER.  BNDS bounds the difference
> @@ -1501,6 +1581,11 @@ number_of_iterations_lt (class loop *loop, tree type, affine_iv *iv0,
>        niter->bound = iv0->base;
>      }
>
> +  /* {base, -C} < n,  or n < {base, C} */
> +  if (tree_int_cst_sign_bit (iv0->step)
> +      || (!integer_zerop (iv1->step) && !tree_int_cst_sign_bit (iv1->step)))
> +    return number_of_iterations_until_wrap (loop, type, iv0, iv1, niter);
> +
>    delta = fold_build2 (MINUS_EXPR, niter_type,
>                        fold_convert (niter_type, iv1->base),
>                        fold_convert (niter_type, iv0->base));
> @@ -1665,62 +1750,6 @@ dump_affine_iv (FILE *file, affine_iv *iv)
>      }
>  }
>
> -/* Given exit condition IV0 CODE IV1 in TYPE, this function adjusts
> -   the condition for loop-until-wrap cases.  For example:
> -     (unsigned){8, -1}_loop < 10        => {0, 1} != 9
> -     10 < (unsigned){0, max - 7}_loop   => {0, 1} != 8
> -   Return true if condition is successfully adjusted.  */
> -
> -static bool
> -adjust_cond_for_loop_until_wrap (tree type, affine_iv *iv0, tree_code *code,
> -                                affine_iv *iv1)
> -{
> -  /* Only support simple cases for the moment.  */
> -  if (TREE_CODE (iv0->base) != INTEGER_CST
> -      || TREE_CODE (iv1->base) != INTEGER_CST)
> -    return false;
> -
> -  tree niter_type = unsigned_type_for (type), high, low;
> -  /* Case: i-- < 10.  */
> -  if (integer_zerop (iv1->step))
> -    {
> -      /* TODO: Should handle case in which abs(step) != 1.  */
> -      if (!integer_minus_onep (iv0->step))
> -       return false;
> -      /* Give up on infinite loop.  */
> -      if (*code == LE_EXPR
> -         && tree_int_cst_equal (iv1->base, TYPE_MAX_VALUE (type)))
> -       return false;
> -      high = fold_build2 (PLUS_EXPR, niter_type,
> -                         fold_convert (niter_type, iv0->base),
> -                         build_int_cst (niter_type, 1));
> -      low = fold_convert (niter_type, TYPE_MIN_VALUE (type));
> -    }
> -  else if (integer_zerop (iv0->step))
> -    {
> -      /* TODO: Should handle case in which abs(step) != 1.  */
> -      if (!integer_onep (iv1->step))
> -       return false;
> -      /* Give up on infinite loop.  */
> -      if (*code == LE_EXPR
> -         && tree_int_cst_equal (iv0->base, TYPE_MIN_VALUE (type)))
> -       return false;
> -      high = fold_convert (niter_type, TYPE_MAX_VALUE (type));
> -      low = fold_build2 (MINUS_EXPR, niter_type,
> -                        fold_convert (niter_type, iv1->base),
> -                        build_int_cst (niter_type, 1));
> -    }
> -  else
> -    gcc_unreachable ();
> -
> -  iv0->base = low;
> -  iv0->step = fold_convert (niter_type, integer_one_node);
> -  iv1->base = high;
> -  iv1->step = build_int_cst (niter_type, 0);
> -  *code = NE_EXPR;
> -  return true;
> -}
> -
>  /* Determine the number of iterations according to condition (for staying
>     inside loop) which compares two induction variables using comparison
>     operator CODE.  The induction variable on left side of the comparison
> @@ -1855,15 +1884,6 @@ number_of_iterations_cond (class loop *loop,
>        return true;
>      }
>
> -  /* Handle special case loops: while (i-- < 10) and while (10 < i++) by
> -     adjusting iv0, iv1 and code.  */
> -  if (code != NE_EXPR
> -      && (tree_int_cst_sign_bit (iv0->step)
> -         || (!integer_zerop (iv1->step)
> -             && !tree_int_cst_sign_bit (iv1->step)))
> -      && !adjust_cond_for_loop_until_wrap (type, iv0, &code, iv1))
> -    return false;
> -
>    /* OK, now we know we have a senseful loop.  Handle several cases, depending
>       on what comparison operator is used.  */
>    bound_difference (loop, iv1->base, iv0->base, &bnds);
> --
> 2.17.1
>