Thank you for your careful review!
> But we don't need a new one if it's going to be used in exactly one test and
> if the new option does the same thing for all targets that run the test.
Got it, thanks. Now add option "-latomic" directly, but it still rely
on the trick "[atomic_link_flags [get_multilibs]]"
> No, because the patch is supposed to prevent the infinite loop, and so
> there's no need to stop it looping after 10s. It won't loop at all.
Thanks, deleted.
> We only need to clear padding for long double, not float and double, right?
Yes, actually there is a check "if constexpr
(__atomic_impl::__maybe_has_padding<_Fp>())".
But "__atomic_impl::__clear_padding(_M_fp); " is indeed simply, so fixed here.
> Why can't we run this on all targets?
Got it, now target option deleted.
> There's no reason to use __builtin_memset here, just include <cstring> and
> use std::memcpy.
Thanks, fixed.
> It definitely does have padding, just say "long double has padding bits on
> x86"
Thanks, fixed.
So here comes the latest patch:
---
libstdc++-v3/ChangeLog:
* include/bits/atomic_base.h: add __builtin_clear_padding in
__atomic_float constructor.
* testsuite/29_atomics/atomic_float/compare_exchange_padding.cc: New test.
Signed-off-by: xndcn <xnd...@gmail.com>
---
libstdc++-v3/include/bits/atomic_base.h | 2 +-
.../atomic_float/compare_exchange_padding.cc | 53 +++++++++++++++++++
2 files changed, 54 insertions(+), 1 deletion(-)
create mode 100644
libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
diff --git a/libstdc++-v3/include/bits/atomic_base.h
b/libstdc++-v3/include/bits/atomic_base.h
index f4ce0fa53..cdd6f07da 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -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;
diff --git
a/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
new file mode 100644
index 000000000..eeace251c
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
@@ -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 (int 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 float comparing
+ 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();
+}
--
2.25.1
