[flang][runtime] Don't let random seed queries change the sequence

[klausler]

Summary

When the current seed of the pseudo-random generator is queried
with CALL RANDOM_SEED(GET=n), that query should not change the
stream of pseudo-random numbers produced by CALL RANDOM_NUMBER().

[klausler]

Clean up after a rebase gone somewhat wrong.

flang/runtime/random.cpp

 
 static Lock lock;
 static Generator generator;
+static std::optional<GeneratedWord> nextValue;
+
+// Call only with lock held
+static GeneratedWord GetNextValue() {
+  GeneratedWord result;
+  if (nextValue.has_value()) {
+    result = *nextValue;
+    nextValue.reset();
+  } else {
+    result = generator();
+  }
+  return result;
+}
 
 template <typename REAL, int PREC>
 inline void Generate(const Descriptor &harvest) {

   {
     CriticalSection critical{lock};
     for (std::size_t j{0}; j < elements; ++j) {
-      Int fraction{generator()};
-      if constexpr (words > 1) {
-        for (std::size_t k{1}; k < words; ++k) {
-          static constexpr auto rangeMask{(GeneratedWord{1} << rangeBits) - 1};
-          GeneratedWord word{(generator() - generator.min()) & rangeMask};
-          fraction = (fraction << rangeBits) | word;
+      while (true) {
+        Int fraction{GetNextValue()};
+        if constexpr (words > 1) {
+          for (std::size_t k{1}; k < words; ++k) {
+            static constexpr auto rangeMask{
+                (GeneratedWord{1} << rangeBits) - 1};
+            GeneratedWord word{(GetNextValue() - generator.min()) & rangeMask};
+            fraction = (fraction << rangeBits) | word;
+          }
+        }
+        fraction >>= words * rangeBits - PREC;
+        REAL next{std::ldexp(static_cast<REAL>(fraction), -(PREC + 1))};
+        if (next >= 0.0 && next < 1.0) {
+          *harvest.Element<REAL>(at) = next;
+          break;
         }
       }
       fraction >>= words * rangeBits - PREC;

   {
     CriticalSection critical{lock};
     generator.seed(seed);
+    nextValue = seed;
   }
 }
 

   GeneratedWord seed;
   {
     CriticalSection critical{lock};
-    seed = generator();
-    generator.seed(seed);
+    seed = GetNextValue();
+    nextValue = seed;
   }
   switch (kind) {
   case 4: