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

[Peter Klausler via llvm-commits]

================
@@ -303,6 +303,47 @@ 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 time_t FAIL_TIME{std::time_t{-1}};
+  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.
+  // Given the description of this function, midnight of the current
+  // day is the best starting point.
+  static time_t startingPoint{0};
----------------
klausler wrote:

Be sure to not create a dependence on the C++ runtime library binaries from libflang_rt.runtime.

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