SMT2 back-end: do not flatten flattened arrays #7144
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This PR now includes two (in some ways: unrelated) bugfixes. In order for the one test that is flipped from broken-smt-backend to works-with-Z3 both of these bugfixes are required. The second commit (which is about byte operator lowering) touches code that is most heavily exercised when using the SMT back-end, where all byte operators are rewritten via this code path. |
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## develop #7144 +/- ##
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convert_expr may flatten an array anyway, in which case flatten_array would generate select statements over bitvectors instead of arrays (as flatten_array itself invokes convert_expr to supposedly construct the array expression). Also, make sure the same flattening logic applies to 1-element structs.
When updating an array/vector at a non-constant byte offset the update value may affect multiple array/vector elements. The last such element needs to be updated with however many bytes have not yet been used from the update value. With a non-constant byte offset the size of remaining bytes may be zero. Therefore, make the tail update use symbolic offsets rather than constants. When fixing this, it also became apparent that we must not use byte extracts of non-constant extraction size to build update elements. Instead, the update offset should be non-constant, and will actually need to be negative on such occasions.
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convert_expr may flatten an array anyway, in which case flatten_array would generate select statements over bitvectors instead of arrays (as flatten_array itself invokes convert_expr to supposedly construct the array expression).