[libc] Change rand implementation so all tests pass in both 32- and 6…

…4-bit systems (#98692)

This patch makes rand select different algorithms depending on the arch.
This is needed to avoid a test failure in 32-bit systems where the LSB
of rand was not uniform enough when the 64-bit constants are used in
32-bit systems.

libc/src/stdlib/rand.cpp

@@ -14,20 +14,39 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
-// 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 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();
+
+  // An implementation of the xorshift64star pseudo random number generator.
+  // This is a good general purpose generator for most non-cryptographics
+  // applications.
+  if constexpr (sizeof(void *) == sizeof(uint64_t)) {
+    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();
+    }
+  } else {
+    // This is the xorshift32 pseudo random number generator, slightly different
+    // from the 64-bit star version above, as the previous version fails to
+    // generate uniform enough LSB in 32-bit systems.
+    for (;;) {
+      unsigned long x = orig;
+      x ^= x >> 13;
+      x ^= x << 27;
+      x ^= x >> 5;
+      if (rand_next.compare_exchange_strong(orig, x, cpp::MemoryOrder::ACQUIRE,
+                                            cpp::MemoryOrder::RELAXED))
+        return static_cast<int>(x * 1597334677ul) & RAND_MAX;
+      sleep_briefly();
+    }
   }
+  __builtin_unreachable();
 }
 
 } // namespace LIBC_NAMESPACE_DECL