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Re: Fix inchash handling of wide_ints (PR91242)


On Mon, Jul 29, 2019 at 11:11 AM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> inchash::hash::add_wide_int operated directly on the raw encoding
> of the wide_int, including any redundant upper bits.  The problem
> with that is that the upper bits are only defined for some wide-int
> storage types (including wide_int itself).  wi::to_wide(tree) instead
> returns a value that is extended according to the signedness of the
> type (so that wi::to_widest can use the same encoding) while rtxes
> have the awkward special case of BI, which can be zero-extended
> rather than sign-extended.
>
> In the PR, we computed a hash for a "normal" sign-extended wide_int
> while the existing entries hashed wi::to_wide(tree).  This gives
> different results for unsigned types that have the top bit set.
>
> The patch fixes that by hashing the canonical sign-extended form even
> if the raw encoding happens to be different.
>
> Tested on aarch64-linux-gnu (with and without SVE), armeb-eabi
> and x86_64-linux-gnu.  OK to install?

But only the most significant elt needs this treatment, no?  So
we can save some compile-time in the hasing by not doing
it for all elts.

> Richard
>
>
> 2019-07-29  Richard Sandiford  <richard.sandiford@arm.com>
>
> gcc/
>         * wide-int.h (generic_wide_int::sext_elt): New function.
>         * inchash.h (hash::add_wide_int): Use it instead of elt.
>
> Index: gcc/wide-int.h
> ===================================================================
> --- gcc/wide-int.h      2019-07-10 19:41:26.391898059 +0100
> +++ gcc/wide-int.h      2019-07-29 10:08:12.048610030 +0100
> @@ -730,6 +730,7 @@ class GTY(()) generic_wide_int : public
>    /* Public accessors for the interior of a wide int.  */
>    HOST_WIDE_INT sign_mask () const;
>    HOST_WIDE_INT elt (unsigned int) const;
> +  HOST_WIDE_INT sext_elt (unsigned int) const;
>    unsigned HOST_WIDE_INT ulow () const;
>    unsigned HOST_WIDE_INT uhigh () const;
>    HOST_WIDE_INT slow () const;
> @@ -909,6 +910,23 @@ generic_wide_int <storage>::elt (unsigne
>      return this->get_val ()[i];
>  }
>
> +/* Like elt, but sign-extend beyond the upper bit, instead of returning
> +   the raw encoding.  */
> +template <typename storage>
> +inline HOST_WIDE_INT
> +generic_wide_int <storage>::sext_elt (unsigned int i) const
> +{
> +  HOST_WIDE_INT elt_i = elt (i);
> +  if (!is_sign_extended)
> +    {
> +      unsigned int precision = this->get_precision ();
> +      unsigned int lsb = i * HOST_BITS_PER_WIDE_INT;
> +      if (precision - lsb < HOST_BITS_PER_WIDE_INT)
> +       elt_i = sext_hwi (elt_i, precision - lsb);
> +    }
> +  return elt_i;
> +}
> +
>  template <typename storage>
>  template <typename T>
>  inline generic_wide_int <storage> &
> Index: gcc/inchash.h
> ===================================================================
> --- gcc/inchash.h       2019-03-08 18:15:36.704740334 +0000
> +++ gcc/inchash.h       2019-07-29 10:08:12.048610030 +0100
> @@ -85,7 +85,7 @@ hashval_t iterative_hash_hashval_t (hash
>    {
>      add_int (x.get_len ());
>      for (unsigned i = 0; i < x.get_len (); i++)
> -      add_hwi (x.elt (i));
> +      add_hwi (x.sext_elt (i));
>    }
>
>    /* Hash in pointer PTR.  */


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