libstd: modify wrong shift width #6108
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borrow = *elem << (uint::bits - n_bits); The code above contains a bug that the value of the right operand of the shift operator exceeds the size of the left operand, because sizeof(*elem) == 32, and 0 <= n_bits < 32 in 64bit architecture. If `--opt-level` option is not given to rustc, the code above runs as if the right operand is `(uint::bits - n_bits) % 32`, but if --opt-level is given, `borrow` is always zero. I wonder why this bug is not catched in the libstd's testsuite (I try the `rustc --test --opt-level=2 bigint.rs` before fixing the bug, but the unittest passes normally.)
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`std::bigint` contains the following code. ```rust borrow = *elem << (uint::bits - n_bits); ``` The code above contains a bug that the value of the right operand of the shift operator exceeds the size of the left operand, because sizeof(*elem) == 32, and 0 <= n_bits < 32 in 64bit architecture. If `--opt-level` option is not given to rustc, the code above runs as if the right operand is `(uint::bits - n_bits) % 32`, but if --opt-level is given, `borrow` is always zero. I wonder why this bug is not catched in the libstd's testsuite (I try the `rustc --test --opt-level=2 bigint.rs` before fixing the bug, but the unittest passes normally.) This pull request also removes the implicit vector copies in `bigint.rs`.
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std::bigintcontains the following code.The code above contains a bug that the value of the right operand of the shift operator exceeds the size of the left operand,
because sizeof(*elem) == 32, and 0 <= n_bits < 32 in 64bit architecture.
If
--opt-leveloption is not given to rustc, the code above runs as if the right operand is(uint::bits - n_bits) % 32,but if --opt-level is given,
borrowis always zero.I wonder why this bug is not catched in the libstd's testsuite (I try the
rustc --test --opt-level=2 bigint.rsbefore fixing the bug,but the unittest passes normally.)
This pull request also removes the implicit vector copies in
bigint.rs.