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Fix for issue #28012: pow overflow inconsistency

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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.
This commit is contained in:
Peter Reid 2015-09-04 22:10:39 -04:00
parent 1110f1e873
commit 1927a869d4
1 changed files with 11 additions and 15 deletions
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26
src/libcore/num/mod.rs
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@ -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;
}
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
}