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Auto merge of #28248 - PeterReid:master, r=alexcrichton

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Overflows in integer pow() computations would be missed if they
preceded a 0 bit of the exponent being processed. This made
calls such as 2i32.pow(1024) not trigger an overflow.

Fixes #28012
This commit is contained in:
bors 2015-09-14 17:10:09 +00:00
commit 9da7706dd6
2 changed files with 27 additions and 15 deletions
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24
src/libcore/num/mod.rs
View file

@ -537,25 +537,21 @@ macro_rules! int_impl {
let mut base = self;
let mut acc = Self::one();
let mut prev_base = self;
let mut base_oflo = false;
while exp > 0 {
while exp > 1 {
if (exp & 1) == 1 {
if base_oflo {
// ensure overflow occurs in the same manner it
// would have otherwise (i.e. signal any exception
// it would have otherwise).
acc = acc * (prev_base * prev_base);
} else {
acc = acc * base;
}
}
prev_base = base;
let (new_base, new_base_oflo) = base.overflowing_mul(base);
base = new_base;
base_oflo = new_base_oflo;
exp /= 2;
base = base * base;
}
// Deal with the final bit of the exponent separately, since
// squaring the base afterwards is not necessary and may cause a
// needless overflow.
if exp == 1 {
acc = acc * base;
}
acc
}

16
src/test/run-fail/overflowing-pow.rs Normal file
View file

@ -0,0 +1,16 @@
// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
// error-pattern:thread '<main>' panicked at 'arithmetic operation overflowed'
// compile-flags: -C debug-assertions
fn main() {
let _x = 2i32.pow(1024);
}