[flang-commits] [PATCH] D127023: [flang][runtime] Don't let random seed queries change the sequence

[Peter Klausler via Phabricator via flang-commits]

This revision was automatically updated to reflect the committed changes.
Closed by commit rGea1a69d66dc7: [flang][runtime] Don't let random seed queries change the sequence (authored by klausler).

Changed prior to commit:
  https://reviews.llvm.org/D127023?vs=434220&id=434266#toc

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D127023/new/

https://reviews.llvm.org/D127023

Files:
  flang/runtime/random.cpp


Index: flang/runtime/random.cpp
===================================================================
--- flang/runtime/random.cpp
+++ flang/runtime/random.cpp
@@ -40,6 +40,19 @@
 
 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) {
@@ -55,12 +68,12 @@
     CriticalSection critical{lock};
     for (std::size_t j{0}; j < elements; ++j) {
       while (true) {
-        Int fraction{generator()};
+        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{(generator() - generator.min()) & rangeMask};
+            GeneratedWord word{(GetNextValue() - generator.min()) & rangeMask};
             fraction = (fraction << rangeBits) | word;
           }
         }
@@ -161,6 +174,7 @@
   {
     CriticalSection critical{lock};
     generator.seed(seed);
+    nextValue = seed;
   }
 }
 
@@ -183,8 +197,8 @@
   GeneratedWord seed;
   {
     CriticalSection critical{lock};
-    seed = generator();
-    generator.seed(seed);
+    seed = GetNextValue();
+    nextValue = seed;
   }
   switch (kind) {
   case 4:


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