https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109986
--- Comment #3 from Ivan Sorokin <vanyacpp at gmail dot com> --- I tried to investigate why GCC is able to simplify `(a | b) ^ a` and `(a | ~b) ^ a` from comment 2, but not similarly looking `(~a | b) ^ a` from comment 0. `(a | b) ^ a` matches the following pattern from match.pd: /* (X | Y) ^ X -> Y & ~ X*/ (simplify (bit_xor:c (convert1? (bit_ior:c @@0 @1)) (convert2? @0)) (if (tree_nop_conversion_p (type, TREE_TYPE (@0))) (convert (bit_and @1 (bit_not @0))))) `(a | ~b) ^ a` matches another pattern: /* (~X | C) ^ D -> (X | C) ^ (~D ^ C) if (~D ^ C) can be simplified. */ (simplify (bit_xor:c (bit_ior:cs (bit_not:s @0) @1) @2) (bit_xor (bit_ior @0 @1) (bit_xor! (bit_not! @2) @1))) With substitution `X = b, C = a, D = a` it gives: (b | a) ^ (~a ^ a) (b | a) ^ -1 ~(b | a) `(~a | b) ^ a` is not simplifiable by this pattern because it requires that `~D ^ C` is simplifiable further, but `~a ^ b` is not. In any case, even if it were applicable it would produce `(a | b) ^ (~a ^ b)` which has more operations than the original expression.
