[libc++] avoid a GCC -Wsigned-compare warning where time_t is unsigned

[yamt]

Summary

Note: For some reasons, GCC -Wall enables the warning for C++
while it doesn't for C.

[Mordante]

Thanks for your patch! One suggestion.

libcxx/src/condition_variable.cpp

66

This is consistent with the next line.

[yamt]

libcxx/src/condition_variable.cpp

66

my understanding is that s.count() here is long long.
ts_sec is time_t, which can be narrower. (eg. int)

[Mordante]

libcxx/src/condition_variable.cpp

66

Good point, ts_sec, might indeed be a smaller type. (There is no guarantee count is long long, but it most likely is.)

I still wonder whether this change fixes anything, in glibc time_t is signed
https://www.gnu.org/software/libc/manual/html_node/Time-Types.html

Maybe it's safer to cast the result to the common type of these two types, WDYT?

[yamt]

libcxx/src/condition_variable.cpp

66

you are right this is not necessary for glibc.

however, for other platforms, time_t can be uint32_t, uint64_t, etc.
cf. https://github.com/apache/nuttx/pull/8201

[Mordante]

We want to finish the review queue for libc++ due to moving to GitHub PRs. Do you want to continue this patch or should I finish if for you?

libcxx/src/condition_variable.cpp

66

Maybe we should use cmp_less which has solved this issue. We require C++20 for the dylib.

[yamt]

libcxx/src/condition_variable.cpp

66

Maybe we should use cmp_less which has solved this issue.

maybe. i wasn't aware of the function. thank you.

We require C++20 for the dylib.

can you explain a bit? (i'm not a C++ expert.)

[Mordante]

libcxx/src/condition_variable.cpp

66

C++20 is the version of C++ we use so the version from 2020. This means we can use cmp_less.

libcxx/src/condition_variable.cpp

if (!lk.owns_lock())
                             "condition_variable::timed wait: mutex not locked");
     nanoseconds d = tp.time_since_epoch();
     if (d > nanoseconds(0x59682F000000E941))
         d = nanoseconds(0x59682F000000E941);
     __libcpp_timespec_t ts;
     seconds s = duration_cast<seconds>(d);
     typedef decltype(ts.tv_sec) ts_sec;
     _LIBCPP_CONSTEXPR ts_sec ts_sec_max = numeric_limits<ts_sec>::max();
-    if (s.count() < ts_sec_max)
-    {
-        ts.tv_sec = static_cast<ts_sec>(s.count());
-        ts.tv_nsec = static_cast<decltype(ts.tv_nsec)>((d - s).count());
-    }
-    else
-    {
-        ts.tv_sec = ts_sec_max;
-        ts.tv_nsec = giga::num - 1;
+    if ((unsigned long long)s.count() < ts_sec_max) {
+      ts.tv_sec  = static_cast<ts_sec>(s.count());
+    } else {
+      ts.tv_sec  = ts_sec_max;
     }
     int ec = __libcpp_condvar_timedwait(&__cv_, lk.mutex()->native_handle(), &ts);
     if (ec != 0 && ec != ETIMEDOUT)
         __throw_system_error(ec, "condition_variable timed_wait failed");
 }
 
 void
 notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk)