[Bug fortran/118862] UBSAN: shift exponent too large since r15-7345-gc2a0ee58865c5a

[jakub at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118862

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
One would need to rewrite gfc_conv_powi to work on wide_ints.  Another option
is just punt at trying to optimize it, if (lhs_prec > HOST_BITS_PER_WIDE_INT)
return 0;
Though, maybe in such case the n & mask could be just n, because the mask in
infinite precision would have all the low 64-bits set and perhaps some more.

So perhaps
--- gcc/trans-expr.cc.jj        2025-02-08 08:49:56.573946067 +0100
+++ gcc/trans-expr.cc   2025-02-13 22:57:05.373836717 +0100
@@ -3604,7 +3604,7 @@ gfc_conv_cst_int_power (gfc_se * se, tre
        if (bit_size(rhs) < bit_size(lhs))  ! Checked here.
         return lhs ** rhs;

-       mask = (1 < bit_size(a) - 1) / 2;
+       mask = 1 << (bit_size(a) - 1) / 2;
        return lhs ** (n & rhs);
      }
    if (rhs > bit_size(lhs))  ! Checked here.
@@ -3624,13 +3624,13 @@ gfc_conv_cst_uint_power (gfc_se * se, tr
   tree vartmp_odd[POWI_TABLE_SIZE], vartmp_even[POWI_TABLE_SIZE];

   /* Anything ** 0 is one.  */
-  if (tree_int_cst_sgn (rhs) == 0)
+  if (integer_zerop (rhs) == 0)
     {
       se->expr = build_int_cst (type, 1);
       return 1;
     }

-  if (!wi::fits_shwi_p (wrhs))
+  if (!wi::fits_uhwi_p (wrhs))
     return 0;

   n = wrhs.to_uhwi ();
@@ -3642,19 +3642,19 @@ gfc_conv_cst_uint_power (gfc_se * se, tr
                            tmp, build_int_cst (type, 1));

   lhs_prec = TYPE_PRECISION (type);
-  rhs_prec = TYPE_PRECISION (TREE_TYPE(rhs));
+  rhs_prec = TYPE_PRECISION (TREE_TYPE (rhs));

-  if (rhs_prec >= lhs_prec)
+  if (rhs_prec >= lhs_prec && lhs_prec < HOST_BITS_PER_WIDE_INT)
     {
-      unsigned HOST_WIDE_INT mask;
-      mask = (((unsigned HOST_WIDE_INT) 1) << (lhs_prec - 1)) - 1;
+      unsigned HOST_WIDE_INT mask
+       = (HOST_WIDE_INT_1U << (lhs_prec - 1)) - 1;
       n_odd = n & mask;
     }
   else
     n_odd = n;

   memset (vartmp_odd, 0, sizeof (vartmp_odd));
-  vartmp_odd[0] = build_int_cst(type, 1);
+  vartmp_odd[0] = build_int_cst (type, 1);
   vartmp_odd[1] = lhs;
   odd_branch = gfc_conv_powi (se, n_odd, vartmp_odd);
   even_branch = NULL_TREE;
@@ -3666,7 +3666,7 @@ gfc_conv_cst_uint_power (gfc_se * se, tr
       if (n_odd != n)
        {
          memset (vartmp_even, 0, sizeof (vartmp_even));
-         vartmp_even[0] = build_int_cst(type, 1);
+         vartmp_even[0] = build_int_cst (type, 1);
          vartmp_even[1] = lhs;
          even_branch = gfc_conv_powi (se, n, vartmp_even);
        }

Completely untested.