From: Patrick Palka <ppalka@redhat.com>
To: gcc-patches@gcc.gnu.org
Cc: libstdc++@gcc.gnu.org, Patrick Palka <ppalka@redhat.com>
Subject: [PATCH] libstdc++: Fix __max_diff_type::operator>>= for negative values
Date: Mon, 24 Apr 2023 12:23:37 -0400 [thread overview]
Message-ID: <20230424162337.104065-1-ppalka@redhat.com> (raw)
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
next reply other threads:[~2023-04-24 16:23 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-24 16:23 Patrick Palka [this message]
2023-04-24 16:28 ` Patrick Palka
2023-04-24 16:39 ` Jonathan Wakely
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