[libc] Make 'rand()' thread-safe using atomics instead of TLS

libc/src/stdlib/CMakeLists.txt

@@ -304,6 +304,7 @@ add_entrypoint_object(
   DEPENDS
     .rand_util
     libc.include.stdlib
+    libc.src.__support.threads.sleep
 )
 
 add_entrypoint_object(

libc/src/stdlib/rand.cpp

@@ -8,19 +8,25 @@
 
 #include "src/stdlib/rand.h"
 #include "src/__support/common.h"
+#include "src/__support/threads/sleep.h"
 #include "src/stdlib/rand_util.h"
 
 namespace LIBC_NAMESPACE {
 
 // An implementation of the xorshift64star pseudo random number generator. This
 // is a good general purpose generator for most non-cryptographics applications.
 LLVM_LIBC_FUNCTION(int, rand, (void)) {
-  unsigned long x = rand_next;
-  x ^= x >> 12;
-  x ^= x << 25;
-  x ^= x >> 27;
-  rand_next = x;
-  return static_cast<int>((x * 0x2545F4914F6CDD1Dul) >> 32) & RAND_MAX;
+  unsigned long orig = rand_next.load(cpp::MemoryOrder::RELAXED);
+  for (;;) {
+    unsigned long x = orig;
+    x ^= x >> 12;
+    x ^= x << 25;
+    x ^= x >> 27;
+    if (rand_next.compare_exchange_strong(orig, x, cpp::MemoryOrder::ACQUIRE,
+                                          cpp::MemoryOrder::RELAXED))
+      return static_cast<int>((x * 0x2545F4914F6CDD1Dul) >> 32) & RAND_MAX;
+    sleep_briefly();
+  }
 }
 
 } // namespace LIBC_NAMESPACE

libc/src/stdlib/rand_util.cpp

@@ -7,18 +7,13 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/stdlib/rand_util.h"
+#include "src/__support/CPP/atomic.h"
 #include "src/__support/macros/attributes.h"
 
 namespace LIBC_NAMESPACE {
 
-#ifdef LIBC_TARGET_ARCH_IS_GPU
-// FIXME: Local GPU memory cannot be initialized so we cannot currently provide
-// a standard compliant default value.
-ThreadLocal<unsigned long> rand_next;
-#else
-// C standard 7.10p2: If 'rand' is called before 'srand' it is to proceed as if
-// the 'srand' function was called with a value of '1'.
-LIBC_THREAD_LOCAL unsigned long rand_next = 1;
-#endif
+//  C standard 7.10p2: If 'rand' is called before 'srand' it is to
+// proceed as if the 'srand' function was called with a value of '1'.
+cpp::Atomic<unsigned long> rand_next = 1;
 
 } // namespace LIBC_NAMESPACE

libc/src/stdlib/rand_util.h

@@ -9,33 +9,15 @@
 #ifndef LLVM_LIBC_SRC_STDLIB_RAND_UTIL_H
 #define LLVM_LIBC_SRC_STDLIB_RAND_UTIL_H
 
-#include "src/__support/GPU/utils.h"
+#include "src/__support/CPP/atomic.h"
 #include "src/__support/macros/attributes.h"
 
 namespace LIBC_NAMESPACE {
 
-#ifdef LIBC_TARGET_ARCH_IS_GPU
-// Implement thread local storage on the GPU using local memory. Each thread
-// gets its slot in the local memory array and is private to the group.
-// TODO: We need to implement the 'thread_local' keyword on the GPU. This is an
-// inefficient and incomplete stand-in until that is done.
-template <typename T> class ThreadLocal {
-private:
-  static constexpr long MAX_THREADS = 1024;
-  [[clang::loader_uninitialized]] static inline gpu::Local<T>
-      storage[MAX_THREADS];
-
-public:
-  LIBC_INLINE operator T() const { return storage[gpu::get_thread_id()]; }
-  LIBC_INLINE void operator=(const T &value) {
-    storage[gpu::get_thread_id()] = value;
-  }
-};
-
-extern ThreadLocal<unsigned long> rand_next;
-#else
-extern LIBC_THREAD_LOCAL unsigned long rand_next;
-#endif
+// The ISO C standard does not explicitly require thread-safe behavior for the
+// generic `rand()` function. Some implementations expect it however, so we
+// provide it here.
+extern cpp::Atomic<unsigned long> rand_next;
 
 } // namespace LIBC_NAMESPACE
 

libc/src/stdlib/srand.cpp

@@ -12,6 +12,8 @@
 
 namespace LIBC_NAMESPACE {
 
-LLVM_LIBC_FUNCTION(void, srand, (unsigned int seed)) { rand_next = seed; }
+LLVM_LIBC_FUNCTION(void, srand, (unsigned int seed)) {
+  rand_next.store(seed, cpp::MemoryOrder::RELAXED);
+}
 
 } // namespace LIBC_NAMESPACE

libc/test/src/stdlib/rand_test.cpp

@@ -23,15 +23,12 @@ TEST(LlvmLibcRandTest, UnsetSeed) {
     vals[i] = val;
   }
 
-  // FIXME: The GPU implementation cannot initialize the seed correctly.
-#ifndef LIBC_TARGET_ARCH_IS_GPU
   // The C standard specifies that if 'srand' is never called it should behave
   // as if 'srand' was called with a value of 1. If we seed the value with 1 we
   // should get the same sequence as the unseeded version.
   LIBC_NAMESPACE::srand(1);
   for (size_t i = 0; i < 1000; ++i)
     ASSERT_EQ(LIBC_NAMESPACE::rand(), vals[i]);
-#endif
 }
 
 TEST(LlvmLibcRandTest, SetSeed) {