Re: Use nonzero bits to refine range in split_constant_offset (PR 81635)

[Richard Biener <richard dot guenther at gmail dot com>]

On Fri, Feb 2, 2018 at 3:12 PM, Richard Sandiford
<richard.sandiford@linaro.org> wrote:
> This patch is part 2 of the fix for PR 81635.  It means that
> split_constant_offset can handle loops like:
>
>   for (unsigned int i = 0; i < n; i += 4)
>     {
>       a[i] = ...;
>       a[i + 1] = ...;
>     }
>
> CCP records that "i" must have its low 2 bits clear, but we don't
> include this information in the range of "i", which remains [0, +INF].
> I tried making set_nonzero_bits update the range info in the same
> way that set_range_info updates the nonzero bits, but it regressed
> cases like vrp117.c and made some other tests worse.
>
> vrp117.c has a multiplication by 10, so CCP can infer that the low bit
> of the result is clear.  If we included that in the range, the range
> would go from [-INF, +INF] to [-INF, not-quite-+INF].  However,
> the multiplication is also known to overflow in all cases, so VRP
> saturates the result to [INT_MAX, INT_MAX].  This obviously creates a
> contradiction with the nonzero bits, and intersecting the new saturated
> range with an existing not-quite-+INF range would make us drop to
> VR_UNDEFINED.  We're prepared to fold a comparison with an [INT_MAX,
> INT_MAX] value but not with a VR_UNDEFINED value.
>
> The other problems were created when intersecting [-INF, not-quite-+INF]
> with a useful VR_ANTI_RANGE like ~[-1, 1].  The intersection would
> keep the former range rather than the latter.
>
> The patch therefore keeps the adjustment local to split_constant_offset
> for now, but adds a helper routine so that it's easy to move this later.
>
> Tested on aarch64-linux-gnu, x86_64-linux-gnu and powerpc64le-linux-gnu.
> OK to install?
>
> Richard
>
>
> 2018-02-02  Richard Sandiford  <richard.sandiford@linaro.org>
>
> gcc/
>         PR tree-optimization/81635
>         * wide-int.h (wi::round_down_for_mask, wi::round_up_for_mask): Declare.
>         * wide-int.cc (wi::round_down_for_mask, wi::round_up_for_mask)
>         (test_round_for_mask): New functions.
>         (wide_int_cc_tests): Call test_round_for_mask.
>         * tree-vrp.h (intersect_range_with_nonzero_bits): Declare.
>         * tree-vrp.c (intersect_range_with_nonzero_bits): New function.
>         * tree-data-ref.c (split_constant_offset_1): Use it to refine the
>         range returned by get_range_info.
>
> gcc/testsuite/
>         PR tree-optimization/81635
>         * gcc.dg/vect/bb-slp-pr81635-3.c: New test.
>         * gcc.dg/vect/bb-slp-pr81635-4.c: Likewise.
>
> Index: gcc/wide-int.h
> ===================================================================
> --- gcc/wide-int.h      2018-02-02 14:03:53.964530009 +0000
> +++ gcc/wide-int.h      2018-02-02 14:03:54.185521788 +0000
> @@ -3308,6 +3308,8 @@ gt_pch_nx (trailing_wide_ints <N> *, voi
>    wide_int set_bit_in_zero (unsigned int, unsigned int);
>    wide_int insert (const wide_int &x, const wide_int &y, unsigned int,
>                    unsigned int);
> +  wide_int round_down_for_mask (const wide_int &, const wide_int &);
> +  wide_int round_up_for_mask (const wide_int &, const wide_int &);
>
>    template <typename T>
>    T mask (unsigned int, bool);
> Index: gcc/wide-int.cc
> ===================================================================
> --- gcc/wide-int.cc     2018-02-02 14:03:53.964530009 +0000
> +++ gcc/wide-int.cc     2018-02-02 14:03:54.185521788 +0000
> @@ -2132,6 +2132,70 @@ wi::only_sign_bit_p (const wide_int_ref
>    return only_sign_bit_p (x, x.precision);
>  }
>
> +/* Return VAL if VAL has no bits set outside MASK.  Otherwise round VAL
> +   down to the previous value that has no bits set outside MASK.
> +   This rounding wraps for signed values if VAL is negative and
> +   the top bit of MASK is clear.
> +
> +   For example, round_down_for_mask (6, 0xf1) would give 1 and
> +   round_down_for_mask (24, 0xf1) would give 17.  */
> +
> +wide_int
> +wi::round_down_for_mask (const wide_int &val, const wide_int &mask)
> +{
> +  /* Get the bits in VAL that are outside the mask.  */
> +  wide_int extra_bits = wi::bit_and_not (val, mask);
> +  if (extra_bits == 0)
> +    return val;
> +
> +  /* Get a mask that includes the top bit in EXTRA_BITS and is all 1s
> +     below that bit.  */
> +  unsigned int precision = val.get_precision ();
> +  wide_int lower_mask = wi::mask (precision - wi::clz (extra_bits),
> +                                 false, precision);
> +
> +  /* Clear the bits that aren't in MASK, but ensure that all bits
> +     in MASK below the top cleared bit are set.  */
> +  return (val & mask) | (mask & lower_mask);
> +}
> +
> +/* Return VAL if VAL has no bits set outside MASK.  Otherwise round VAL
> +   up to the next value that has no bits set outside MASK.  The rounding
> +   wraps if there are no suitable values greater than VAL.
> +
> +   For example, round_up_for_mask (6, 0xf1) would give 16 and
> +   round_up_for_mask (24, 0xf1) would give 32.  */
> +
> +wide_int
> +wi::round_up_for_mask (const wide_int &val, const wide_int &mask)
> +{
> +  /* Get the bits in VAL that are outside the mask.  */
> +  wide_int extra_bits = wi::bit_and_not (val, mask);
> +  if (extra_bits == 0)
> +    return val;
> +
> +  /* Get a mask that is all 1s above the top bit in EXTRA_BITS.  */
> +  unsigned int precision = val.get_precision ();
> +  wide_int upper_mask = wi::mask (precision - wi::clz (extra_bits),
> +                                 true, precision);
> +
> +  /* Get the bits of the mask that are above the top bit in EXTRA_BITS.  */
> +  upper_mask &= mask;
> +
> +  /* Conceptually we need to:
> +
> +     - clear bits of VAL outside UPPER_MASK
> +     - add the lowest bit in UPPER_MASK to VAL (or add 0 if UPPER_MASK is 0)
> +     - propagate the carry through the bits of VAL in UPPER_MASK
> +
> +     If (~VAL & UPPER_MASK) is nonzero, the carry eventually
> +     reaches that bit and the process leaves all lower bits clear.
> +     If (~VAL & UPPER_MASK) is zero then the result is also zero.  */
> +  wide_int tmp = wi::bit_and_not (upper_mask, val);
> +
> +  return (val | tmp) & -tmp;
> +}
> +
>  /*
>   * Private utilities.
>   */
> @@ -2384,6 +2448,53 @@ test_overflow ()
>      }
>  }
>
> +/* Test the round_{down,up}_for_mask functions.  */
> +
> +static void
> +test_round_for_mask ()
> +{
> +  unsigned int prec = 18;
> +  ASSERT_EQ (17, wi::round_down_for_mask (wi::shwi (17, prec),
> +                                         wi::shwi (0xf1, prec)));
> +  ASSERT_EQ (17, wi::round_up_for_mask (wi::shwi (17, prec),
> +                                       wi::shwi (0xf1, prec)));
> +
> +  ASSERT_EQ (1, wi::round_down_for_mask (wi::shwi (6, prec),
> +                                        wi::shwi (0xf1, prec)));
> +  ASSERT_EQ (16, wi::round_up_for_mask (wi::shwi (6, prec),
> +                                       wi::shwi (0xf1, prec)));
> +
> +  ASSERT_EQ (17, wi::round_down_for_mask (wi::shwi (24, prec),
> +                                         wi::shwi (0xf1, prec)));
> +  ASSERT_EQ (32, wi::round_up_for_mask (wi::shwi (24, prec),
> +                                       wi::shwi (0xf1, prec)));
> +
> +  ASSERT_EQ (0x011, wi::round_down_for_mask (wi::shwi (0x22, prec),
> +                                            wi::shwi (0x111, prec)));
> +  ASSERT_EQ (0x100, wi::round_up_for_mask (wi::shwi (0x22, prec),
> +                                          wi::shwi (0x111, prec)));
> +
> +  ASSERT_EQ (100, wi::round_down_for_mask (wi::shwi (101, prec),
> +                                          wi::shwi (0xfc, prec)));
> +  ASSERT_EQ (104, wi::round_up_for_mask (wi::shwi (101, prec),
> +                                        wi::shwi (0xfc, prec)));
> +
> +  ASSERT_EQ (0x2bc, wi::round_down_for_mask (wi::shwi (0x2c2, prec),
> +                                            wi::shwi (0xabc, prec)));
> +  ASSERT_EQ (0x800, wi::round_up_for_mask (wi::shwi (0x2c2, prec),
> +                                          wi::shwi (0xabc, prec)));
> +
> +  ASSERT_EQ (0xabc, wi::round_down_for_mask (wi::shwi (0xabd, prec),
> +                                            wi::shwi (0xabc, prec)));
> +  ASSERT_EQ (0, wi::round_up_for_mask (wi::shwi (0xabd, prec),
> +                                      wi::shwi (0xabc, prec)));
> +
> +  ASSERT_EQ (0xabc, wi::round_down_for_mask (wi::shwi (0x1000, prec),
> +                                            wi::shwi (0xabc, prec)));
> +  ASSERT_EQ (0, wi::round_up_for_mask (wi::shwi (0x1000, prec),
> +                                      wi::shwi (0xabc, prec)));
> +}
> +
>  /* Run all of the selftests within this file, for all value types.  */
>
>  void
> @@ -2393,6 +2504,7 @@ wide_int_cc_tests ()
>    run_all_wide_int_tests <offset_int> ();
>    run_all_wide_int_tests <widest_int> ();
>    test_overflow ();
> +  test_round_for_mask ();
>  }
>
>  } // namespace selftest
> Index: gcc/tree-vrp.h
> ===================================================================
> --- gcc/tree-vrp.h      2018-02-02 14:03:53.964530009 +0000
> +++ gcc/tree-vrp.h      2018-02-02 14:03:54.184521826 +0000
> @@ -61,6 +61,8 @@ extern void extract_range_from_unary_exp
>                                            tree op0_type);
>
>  extern bool vrp_operand_equal_p (const_tree, const_tree);
> +extern enum value_range_type intersect_range_with_nonzero_bits
> +  (enum value_range_type, wide_int *, wide_int *, const wide_int &, signop);
>
>  struct assert_info
>  {
> Index: gcc/tree-vrp.c
> ===================================================================
> --- gcc/tree-vrp.c      2018-02-02 14:03:53.964530009 +0000
> +++ gcc/tree-vrp.c      2018-02-02 14:03:54.184521826 +0000
> @@ -171,6 +171,53 @@ vrp_val_is_min (const_tree val)
>               && operand_equal_p (val, type_min, 0)));
>  }
>
> +/* VR_TYPE describes a range with mininum value *MIN and maximum
> +   value *MAX.  Restrict the range to the set of values that have
> +   no bits set outside NONZERO_BITS.  Update *MIN and *MAX and
> +   return the new range type.
> +
> +   SGN gives the sign of the values described by the range.  */
> +
> +enum value_range_type
> +intersect_range_with_nonzero_bits (enum value_range_type vr_type,
> +                                  wide_int *min, wide_int *max,
> +                                  const wide_int &nonzero_bits,
> +                                  signop sgn)
> +{
> +  if (vr_type == VR_RANGE)
> +    {
> +      *max = wi::round_down_for_mask (*max, nonzero_bits);
> +
> +      /* Check that the range contains at least one valid value.  */
> +      if (wi::gt_p (*min, *max, sgn))
> +       return VR_UNDEFINED;
> +
> +      *min = wi::round_up_for_mask (*min, nonzero_bits);
> +      gcc_checking_assert (wi::le_p (*min, *max, sgn));
> +    }
> +  if (vr_type == VR_ANTI_RANGE)
> +    {
> +      *max = wi::round_up_for_mask (*max, nonzero_bits);
> +
> +      /* If the calculation wrapped, we now have a VR_RANGE whose
> +        lower bound is *MAX and whose upper bound is *MIN.  */
> +      if (wi::gt_p (*min, *max, sgn))
> +       {
> +         std::swap (*min, *max);
> +         *max = wi::round_down_for_mask (*max, nonzero_bits);
> +         gcc_checking_assert (wi::le_p (*min, *max, sgn));
> +         return VR_RANGE;
> +       }
> +
> +      *min = wi::round_down_for_mask (*min, nonzero_bits);
> +      gcc_checking_assert (wi::le_p (*min, *max, sgn));
> +
> +      /* Check whether we now have an empty set of values.  */
> +      if (*min - 1 == *max)
> +       return VR_UNDEFINED;
> +    }
> +  return vr_type;
> +}
>
>  /* Set value range VR to VR_UNDEFINED.  */
>
> Index: gcc/tree-data-ref.c
> ===================================================================
> --- gcc/tree-data-ref.c 2018-02-02 14:03:53.964530009 +0000
> +++ gcc/tree-data-ref.c 2018-02-02 14:03:54.184521826 +0000
> @@ -721,7 +721,13 @@ split_constant_offset_1 (tree type, tree
>                 if (TREE_CODE (tmp_var) != SSA_NAME)
>                   return false;
>                 wide_int var_min, var_max;
> -               if (get_range_info (tmp_var, &var_min, &var_max) != VR_RANGE)
> +               value_range_type vr_type = get_range_info (tmp_var, &var_min,
> +                                                          &var_max);
> +               wide_int var_nonzero = get_nonzero_bits (tmp_var);
> +               signop sgn = TYPE_SIGN (itype);
> +               if (intersect_range_with_nonzero_bits (vr_type, &var_min,
> +                                                      &var_max, var_nonzero,
> +                                                      sgn) != VR_RANGE)

Above it looks like we could go from VR_RANGE to VR_UNDEFINED.
I'm not sure if the original range-info might be useful in this case -
if it may be
can we simply use only the range info if it was VR_RANGE?

Ok otherwise.
Thanks,
Richard.

>                   return false;
>
>                 /* See whether the range of OP0 (i.e. TMP_VAR + TMP_OFF)
> @@ -729,7 +735,6 @@ split_constant_offset_1 (tree type, tree
>                    operations done in ITYPE.  The addition must overflow
>                    at both ends of the range or at neither.  */
>                 bool overflow[2];
> -               signop sgn = TYPE_SIGN (itype);
>                 unsigned int prec = TYPE_PRECISION (itype);
>                 wide_int woff = wi::to_wide (tmp_off, prec);
>                 wide_int op0_min = wi::add (var_min, woff, sgn, &overflow[0]);
> Index: gcc/testsuite/gcc.dg/vect/bb-slp-pr81635-3.c
> ===================================================================
> --- /dev/null   2018-02-02 09:03:36.168354735 +0000
> +++ gcc/testsuite/gcc.dg/vect/bb-slp-pr81635-3.c        2018-02-02 14:03:54.183521863 +0000
> @@ -0,0 +1,62 @@
> +/* { dg-do compile } */
> +/* { dg-additional-options "-fno-tree-loop-vectorize" } */
> +/* { dg-require-effective-target vect_double } */
> +/* { dg-require-effective-target lp64 } */
> +
> +void
> +f1 (double *p, double *q, unsigned int n)
> +{
> +  p = (double *) __builtin_assume_aligned (p, sizeof (double) * 2);
> +  q = (double *) __builtin_assume_aligned (q, sizeof (double) * 2);
> +  for (unsigned int i = 0; i < n; i += 4)
> +    {
> +      double a = q[i] + p[i];
> +      double b = q[i + 1] + p[i + 1];
> +      q[i] = a;
> +      q[i + 1] = b;
> +    }
> +}
> +
> +void
> +f2 (double *p, double *q, unsigned int n)
> +{
> +  p = (double *) __builtin_assume_aligned (p, sizeof (double) * 2);
> +  q = (double *) __builtin_assume_aligned (q, sizeof (double) * 2);
> +  for (unsigned int i = 0; i < n; i += 2)
> +    {
> +      double a = q[i] + p[i];
> +      double b = q[i + 1] + p[i + 1];
> +      q[i] = a;
> +      q[i + 1] = b;
> +    }
> +}
> +
> +void
> +f3 (double *p, double *q, unsigned int n)
> +{
> +  p = (double *) __builtin_assume_aligned (p, sizeof (double) * 2);
> +  q = (double *) __builtin_assume_aligned (q, sizeof (double) * 2);
> +  for (unsigned int i = 0; i < n; i += 6)
> +    {
> +      double a = q[i] + p[i];
> +      double b = q[i + 1] + p[i + 1];
> +      q[i] = a;
> +      q[i + 1] = b;
> +    }
> +}
> +
> +void
> +f4 (double *p, double *q, unsigned int start, unsigned int n)
> +{
> +  p = (double *) __builtin_assume_aligned (p, sizeof (double) * 2);
> +  q = (double *) __builtin_assume_aligned (q, sizeof (double) * 2);
> +  for (unsigned int i = start & -2; i < n; i += 2)
> +    {
> +      double a = q[i] + p[i];
> +      double b = q[i + 1] + p[i + 1];
> +      q[i] = a;
> +      q[i + 1] = b;
> +    }
> +}
> +
> +/* { dg-final { scan-tree-dump-times "basic block vectorized" 4 "slp1" } } */
> Index: gcc/testsuite/gcc.dg/vect/bb-slp-pr81635-4.c
> ===================================================================
> --- /dev/null   2018-02-02 09:03:36.168354735 +0000
> +++ gcc/testsuite/gcc.dg/vect/bb-slp-pr81635-4.c        2018-02-02 14:03:54.183521863 +0000
> @@ -0,0 +1,47 @@
> +/* { dg-do compile } */
> +/* { dg-additional-options "-fno-tree-loop-vectorize" } */
> +/* { dg-require-effective-target lp64 } */
> +
> +void
> +f1 (double *p, double *q, unsigned int n)
> +{
> +  p = (double *) __builtin_assume_aligned (p, sizeof (double) * 2);
> +  q = (double *) __builtin_assume_aligned (q, sizeof (double) * 2);
> +  for (unsigned int i = 0; i < n; i += 1)
> +    {
> +      double a = q[i] + p[i];
> +      double b = q[i + 1] + p[i + 1];
> +      q[i] = a;
> +      q[i + 1] = b;
> +    }
> +}
> +
> +void
> +f2 (double *p, double *q, unsigned int n)
> +{
> +  p = (double *) __builtin_assume_aligned (p, sizeof (double) * 2);
> +  q = (double *) __builtin_assume_aligned (q, sizeof (double) * 2);
> +  for (unsigned int i = 0; i < n; i += 3)
> +    {
> +      double a = q[i] + p[i];
> +      double b = q[i + 1] + p[i + 1];
> +      q[i] = a;
> +      q[i + 1] = b;
> +    }
> +}
> +
> +void
> +f3 (double *p, double *q, unsigned int start, unsigned int n)
> +{
> +  p = (double *) __builtin_assume_aligned (p, sizeof (double) * 2);
> +  q = (double *) __builtin_assume_aligned (q, sizeof (double) * 2);
> +  for (unsigned int i = start; i < n; i += 2)
> +    {
> +      double a = q[i] + p[i];
> +      double b = q[i + 1] + p[i + 1];
> +      q[i] = a;
> +      q[i + 1] = b;
> +    }
> +}
> +
> +/* { dg-final { scan-tree-dump-not "basic block vectorized" "slp1" } } */