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From: Patrick Palka <ppalka@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r14-205] libstdc++: Fix __max_diff_type::operator>>= for negative values Date: Mon, 24 Apr 2023 17:41:44 +0000 (GMT) [thread overview] Message-ID: <20230424174144.8D5213858C39@sourceware.org> (raw) https://gcc.gnu.org/g:83470a5cd4c3d233e1d55b5e5553e1b9c553bf28 commit r14-205-g83470a5cd4c3d233e1d55b5e5553e1b9c553bf28 Author: Patrick Palka <ppalka@redhat.com> Date: Mon Apr 24 13:39:54 2023 -0400 libstdc++: Fix __max_diff_type::operator>>= for negative values This patch fixes sign bit propagation when right-shifting a negative __max_diff_type value by more than one, a bug that our existing test coverage didn't expose until r14-159-g03cebd304955a6 fixed the front end's 'signed typedef-name' handling that the test relies on (which is a non-standard extension to the language grammar). 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 the non-standard 'signed typedef-name'. Add some compile-time tests for right-shifting a negative __max_diff_type value by more than one. Diff: --- libstdc++-v3/include/bits/max_size_type.h | 3 ++- libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc | 12 ++++++++++-- 2 files changed, 12 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..54c26ba2b4b 100644 --- a/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc +++ b/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc @@ -26,8 +26,14 @@ 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 = __int128; +#else +using signed_rep_t = long long; +#endif static_assert(sizeof(max_size_t) == sizeof(max_diff_t)); +static_assert(sizeof(rep_t) == sizeof(signed_rep_t)); static_assert(std::regular<max_size_t>); static_assert(std::totally_ordered<max_size_t>); @@ -54,6 +60,8 @@ 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 +196,7 @@ 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 +254,7 @@ 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__;
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