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libstd: modify wrong shift width.

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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.)
This commit is contained in:
gifnksm 2013-04-29 11:53:01 +09:00
parent e4ca2da420
commit ffa31d235b
1 changed files with 2 additions and 1 deletions
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3
src/libstd/num/bigint.rs
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@ -623,7 +623,7 @@ impl BigUint {
let mut shifted = ~[];
for self.data.each_reverse |elem| {
shifted = ~[(*elem >> n_bits) | borrow] + shifted;
borrow = *elem << (uint::bits - n_bits);
borrow = *elem << (BigDigit::bits - n_bits);
}
return BigUint::new(shifted);
}
@ -1213,6 +1213,7 @@ mod biguint_tests {
check(~[1 << 2], 2, ~[1]);
check(~[1, 2], 3, ~[1 << (BigDigit::bits - 2)]);
check(~[1, 1, 2], 3 + BigDigit::bits, ~[1 << (BigDigit::bits - 2)]);
check(~[0, 1], 1, ~[0x80000000]);
test_shr_bits();
#[cfg(target_arch = "x86_64")]