This patch fixes sign bit propagation when right-shifting a negative __max_diff_type value by more than one, a bug which our test coverage failed to uncover until r14-159-g03cebd304955a6 fixed the front end's 'signed typedef-name' handling (which is a non-standard extension to the language grammar).
Tested on x86_64-pc-linux-gnu, does this look OK for trunk/13.2/12/11? libstdc++-v3/ChangeLog: * include/bits/max_size_type.h (max_diff_type::operator>>=): Fix propagation of sign bit. * testsuite/std/ranges/iota/max_size_type.cc: Avoid using 'signed typedef-name'. Add compile-time tests for right-shifting a negative __max_diff_type value by more than one. --- libstdc++-v3/include/bits/max_size_type.h | 3 ++- .../testsuite/std/ranges/iota/max_size_type.cc | 11 +++++++++-- 2 files changed, 11 insertions(+), 3 deletions(-) diff --git a/libstdc++-v3/include/bits/max_size_type.h b/libstdc++-v3/include/bits/max_size_type.h index 92b8168d02f..4796135d073 100644 --- a/libstdc++-v3/include/bits/max_size_type.h +++ b/libstdc++-v3/include/bits/max_size_type.h @@ -560,7 +560,8 @@ namespace ranges // Arithmetic right shift. const auto __msb = _M_rep._M_msb; _M_rep >>= __r._M_rep; - _M_rep._M_msb |= __msb; + if (__msb) + _M_rep |= ~(__max_size_type(-1) >> __r._M_rep); return *this; } diff --git a/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc b/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc index 06114c22cae..985acd5a803 100644 --- a/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc +++ b/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc @@ -26,6 +26,11 @@ using max_size_t = std::ranges::__detail::__max_size_type; using max_diff_t = std::ranges::__detail::__max_diff_type; using rep_t = max_size_t::__rep; +#if __SIZEOF_INT128__ +using signed_rep_t = signed __int128; +#else +using signed_rep_t = long long; +#endif static_assert(sizeof(max_size_t) == sizeof(max_diff_t)); @@ -54,6 +59,8 @@ test01() static_assert(max_diff_t(3) % -2 == 1); static_assert(max_diff_t(-3) << 1 == -6); static_assert(max_diff_t(-3) >> 1 == -2); + static_assert(max_diff_t(-3) >> 2 == -1); + static_assert(max_diff_t(-3) >> 10 == -1); static_assert(max_diff_t(3) >> 1 == 1); static_assert(max_diff_t(3) >> 2 == 0); @@ -188,7 +195,7 @@ template<bool signed_p, bool shorten_p> void test02() { - using hw_type = std::conditional_t<signed_p, signed rep_t, rep_t>; + using hw_type = std::conditional_t<signed_p, signed_rep_t, rep_t>; using max_type = std::conditional_t<signed_p, max_diff_t, max_size_t>; using shorten_type = std::conditional_t<shorten_p, hw_type, max_type>; const int hw_type_bit_size = sizeof(hw_type) * __CHAR_BIT__; @@ -246,7 +253,7 @@ template<bool signed_p, bool toggle_base_p> void test03() { - using hw_type = std::conditional_t<signed_p, signed rep_t, rep_t>; + using hw_type = std::conditional_t<signed_p, signed_rep_t, rep_t>; using max_type = std::conditional_t<signed_p, max_diff_t, max_size_t>; using base_type = std::conditional_t<toggle_base_p, hw_type, max_type>; constexpr int hw_type_bit_size = sizeof(hw_type) * __CHAR_BIT__; -- 2.40.0.374.g7580f92ffa