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libstdc++: Add missing clear_padding in __atomic_float constructor

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For 80-bit long double we need to clear the padding bits on
construction.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_base.h (__atomic_float::__atomic_float(Fp)):
	Clear padding.
	* testsuite/29_atomics/atomic_float/compare_exchange_padding.cc:
	New test.

Signed-off-by: xndcn <xndchn@gmail.com>

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
This commit is contained in:
xndcn 2024-02-16 11:00:13 +00:00 • committed by Jonathan Wakely
parent 38958ac987
commit 0adc8c5f14
2 changed files with 54 additions and 1 deletions
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2
libstdc++-v3/include/bits/atomic_base.h
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@ -1283,7 +1283,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
constexpr
__atomic_float(_Fp __t) : _M_fp(__t)
{ }
{ __atomic_impl::__clear_padding(_M_fp); }
__atomic_float(const __atomic_float&) = delete;
__atomic_float& operator=(const __atomic_float&) = delete;

53
libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc Normal file
View file

@ -0,0 +1,53 @@
// { dg-do run { target c++20 } }
// { dg-options "-O0" }
// { dg-additional-options "[atomic_link_flags [get_multilibs]] -latomic" }
#include <atomic>
#include <cstring>
#include <limits>
#include <testsuite_hooks.h>
template<typename T>
void __attribute__((noinline,noipa))
fill_padding(T& f)
{
T mask;
std::memset(&mask, 0xff, sizeof(T));
__builtin_clear_padding(&mask);
unsigned char* ptr_f = (unsigned char*)&f;
unsigned char* ptr_mask = (unsigned char*)&mask;
for (unsigned i = 0; i < sizeof(T); i++)
{
if (ptr_mask[i] == 0x00)
{
ptr_f[i] = 0xff;
}
}
}
void
test01()
{
// test for long double with padding (float80)
if constexpr (std::numeric_limits<long double>::digits == 64)
{
long double f = 0.5f; // long double has padding bits on x86
fill_padding(f);
std::atomic<long double> as{ f }; // padding cleared on constructor
long double t = 1.5;
as.fetch_add(t);
long double s = f + t;
t = as.load();
VERIFY(s == t); // padding ignored on comparison
fill_padding(s);
VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
fill_padding(f);
VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
}
}
int main()
{
test01();
}