From 1927a869d4917cb7bf47cc069dde574e01c26de3 Mon Sep 17 00:00:00 2001 From: Peter Reid Date: Fri, 4 Sep 2015 22:10:39 -0400 Subject: [PATCH] Fix for issue #28012: pow overflow inconsistency 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. --- src/libcore/num/mod.rs | 26 +++++++++++--------------- 1 file changed, 11 insertions(+), 15 deletions(-) diff --git a/src/libcore/num/mod.rs b/src/libcore/num/mod.rs index 05c7e8b8de4..127f8d3b5a2 100644 --- a/src/libcore/num/mod.rs +++ b/src/libcore/num/mod.rs @@ -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 }