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From: Jonathan Wakely <redi@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r12-6905] libstdc++: Avoid overflow in ranges::advance(i, n, bound) Date: Thu, 27 Jan 2022 22:29:14 +0000 (GMT) [thread overview] Message-ID: <20220127222914.20E9D3858430@sourceware.org> (raw) https://gcc.gnu.org/g:f21f22d1baf7e90f3edbfc48040c76fb14103803 commit r12-6905-gf21f22d1baf7e90f3edbfc48040c76fb14103803 Author: Jonathan Wakely <jwakely@redhat.com> Date: Wed Jan 26 16:08:51 2022 +0000 libstdc++: Avoid overflow in ranges::advance(i, n, bound) When (bound - i) or n is the most negative value of its type, the negative of the value will overflow. Instead of abs(n) >= abs(bound - i) use n >= (bound - i) when positive and n <= (bound - i) when negative. The function has a precondition that they must have the same sign, so this works correctly. The precondition check can be moved into the else branch, and simplified. The standard requires calling ranges::advance(i, bound) even if i==bound is already true, which is technically observable, but that's pointless. We can just return n in that case. Similarly, for i!=bound but n==0 we are supposed to call ranges::advance(i, n), but that's pointless. An LWG issue to allow omitting the pointless calls is expected to be filed. libstdc++-v3/ChangeLog: * include/bits/ranges_base.h (ranges::advance): Avoid signed overflow. Do nothing if already equal to desired result. * testsuite/24_iterators/range_operations/advance_overflow.cc: New test. Diff: --- libstdc++-v3/include/bits/ranges_base.h | 15 +++++---- .../range_operations/advance_overflow.cc | 37 ++++++++++++++++++++++ 2 files changed, 46 insertions(+), 6 deletions(-) diff --git a/libstdc++-v3/include/bits/ranges_base.h b/libstdc++-v3/include/bits/ranges_base.h index 08e99ebc5d3..3c5f4b1790a 100644 --- a/libstdc++-v3/include/bits/ranges_base.h +++ b/libstdc++-v3/include/bits/ranges_base.h @@ -756,20 +756,23 @@ namespace ranges { const auto __diff = __bound - __it; - // n and bound must not lead in opposite directions: - __glibcxx_assert(__n == 0 || __diff == 0 || (__n < 0 == __diff < 0)); - const auto __absdiff = __diff < 0 ? -__diff : __diff; - const auto __absn = __n < 0 ? -__n : __n;; - if (__absn >= __absdiff) + if (__diff == 0) + return __n; + else if (__diff > 0 ? __n >= __diff : __n <= __diff) { (*this)(__it, __bound); return __n - __diff; } - else + else if (__n != 0) [[likely]] { + // n and bound must not lead in opposite directions: + __glibcxx_assert(__n < 0 == __diff < 0); + (*this)(__it, __n); return 0; } + else + return 0; } else if (__it == __bound || __n == 0) return __n; diff --git a/libstdc++-v3/testsuite/24_iterators/range_operations/advance_overflow.cc b/libstdc++-v3/testsuite/24_iterators/range_operations/advance_overflow.cc new file mode 100644 index 00000000000..0fadcd6e99a --- /dev/null +++ b/libstdc++-v3/testsuite/24_iterators/range_operations/advance_overflow.cc @@ -0,0 +1,37 @@ +// { dg-options "-std=gnu++20" } +// { dg-do compile { target c++20 } } + +// Public domain testcase from Casey Carter, send to LWG list on 2021-07-24. +// +// Here's a compile-only test case for which n is INT_MIN, which will overflow +// if simply negated to get |n|: https://godbolt.org/z/M7Wz1nW58. + +#include <cassert> +#include <iterator> +#include <limits> + +struct I { + using difference_type = int; + using value_type = int; + + int x; + + constexpr int operator*() const { return x; } + constexpr I& operator++() { ++x; return *this; } + constexpr I operator++(int) { ++x; return {x - 1}; } + constexpr bool operator==(const I&) const = default; + + constexpr int operator-(const I& that) const { return x - that.x; } + + constexpr I& operator--() { --x; return *this; } + constexpr I operator--(int) { --x; return {x - 1}; } +}; +static_assert(std::bidirectional_iterator<I>); +static_assert(std::sized_sentinel_for<I, I>); + +constexpr bool test() { + using L = std::numeric_limits<int>; + I i{-2}; + return std::ranges::advance(i, L::min(), I{-4}) == L::min() + 2; +} +static_assert(test());
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