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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/97979] [11 Regression]: Segmentation fault
 with "-O3 -fno-toplevel-reorder -fno-tree-ccp" since
 r11-5271-g4866b2f5db117f9e
Date: Wed, 25 Nov 2020 08:36:58 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97979

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
/* Fold (X {&,^,|} C2) << C1 into (X << C1) {&,^,|} (C2 << C1)
   (X {&,^,|} C2) >> C1 into (X >> C1) & (C2 >> C1).  */
(for shift (lshift rshift)
 (for bit_op (bit_and bit_xor bit_ior)
  (simplify
   (shift (convert?:s (bit_op:s @0 INTEGER_CST@2)) INTEGER_CST@1)
   (if (tree_nop_conversion_p (type, TREE_TYPE (@0)))
    (with { tree mask =3D int_const_binop (shift, fold_convert (type, @2), =
@1); }
     (bit_op (shift (convert @0) @1) { mask; }))))))

and mask ends up NULL.

 tree mask =3D int_const_binop (shift, fold_convert (type, captures[3]),
captures[4]);
(gdb) p debug_tree (captures[3])
 <integer_cst 0x7ffff6953138 type <integer_type 0x7ffff68255e8 int> constant
65535>
(gdb) p debug_tree (captures[4])
 <integer_cst 0x7ffff682a3a8 type <integer_type 0x7ffff6825690 unsigned int>
constant 4294967294>
(gdb) p debug_tree (type)
 <integer_type 0x7ffff68255e8 int public SI

so we push in mismatched signs and folding interprets the unsigned shift
amout as signed?=