[flang] [llvm] [flang-rt] Runtime implementation of extended intrinsic function SECNDS() (PR #152021)

[Peter Klausler via llvm-commits]

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@@ -303,6 +304,75 @@ void FORTRAN_PROCEDURE_NAME(qsort)(int *array, int *len, int *isize,
 // PERROR(STRING)
 void RTNAME(Perror)(const char *str) { perror(str); }
 
+// GNU extension function SECNDS(refTime)
+float FORTRAN_PROCEDURE_NAME(secnds)(float *refTime) {
+  constexpr float FAIL_SECNDS{-1.0f}; // Failure code for this function
+  // Failure code for time functions that return std::time_t
+  constexpr std::time_t FAIL_TIME{std::time_t{-1}};
+  constexpr std::time_t TIME_UNINITIALIZED{std::time_t{0}};
+  if (!refTime) {
+    return FAIL_SECNDS;
+  }
+  std::time_t now{std::time(nullptr)};
+  if (now == FAIL_TIME) {
+    return FAIL_SECNDS;
+  }
+  // In float result, we can only precisely store 2^24 seconds, which
+  // comes out to about 194 days. Thus, need to pick a starting point,
+  // which will allow us to keep the time diffs as precise as possible.
+  // Given the description of this function, midnight of the current
+  // day is the best starting point.
+  static std::atomic<std::time_t> startingPoint{TIME_UNINITIALIZED};
+  // "Acquire" will give us writes from other threads.
+  std::time_t localStartingPoint{startingPoint.load(std::memory_order_acquire)};
+  // Initialize startingPoint if we haven't initialized it yet or
+  // if we were passed 0.0f, which indicates to compute seconds from
+  // current day's midnight.
+  if (localStartingPoint == TIME_UNINITIALIZED || *refTime < 0.5f) {
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klausler wrote:

You don't need the `f` suffix, and the comparison will be true for both `0.` and `-0.` inputs.  If that's what was documented, it's best to stick to it.

https://github.com/llvm/llvm-project/pull/152021