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A field f in a structure composed of DWARF pieces may be located in multiple pieces, where the first and last of those may contain bits from other fields as well. So when writing to f, the beginning of the first and the end of the last of those pieces may have to be skipped. But the logic in write_pieced_value for handling one of those pieces is flawed when the first and last piece are the same, i.e., f is contained in a single piece: < - - - - - - - - - piece_size - - - - - - - - - -> +-------------------------------------------------+ | skipped_bits | f_bits | / / / / / / / / / / | +-------------------------------------------------+ The current logic determines the size of the sub-piece to operate on by limiting the piece size to the bit size of f and then subtracting the skipped bits: min (piece_size, f_bits) - skipped_bits Instead of: min (piece_size - skipped_bits, f_bits) So the resulting sub-piece size is corrupted, leading to wrong handling of this piece in write_pieced_value. Note that the same bug was already found in read_pieced_value and fixed there (but not in write_pieced_value), see PR 15391. This patch swaps the calculations, bringing them into the same (correct) order as in read_pieced_value. gdb/ChangeLog: * dwarf2loc.c (write_pieced_value): Fix order of calculations for size capping. gdb/testsuite/ChangeLog: * gdb.dwarf2/var-pieces.exp: Add test case for modifying a variable at nonzero offset.
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