Fix inchash handling of wide_ints (PR91242)

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.

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.

From-SVN: r273881

gcc/ChangeLog

+2019-07-29  Richard Sandiford  <richard.sandiford@arm.com>
+
+	* wide-int.h (generic_wide_int::sext_elt): New function.
+	* inchash.h (hash::add_wide_int): Use it instead of elt.
+
 2019-07-29  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
 
 	* config/arm/arm-builtins.c (acle_builtin_data): Expand VAR1 to

gcc/inchash.h

@@ -85,7 +85,7 @@ class 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.  */

gcc/wide-int.h

@@ -730,6 +730,7 @@ 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 (unsigned int i) const
     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> &