1 files changed, 70 insertions, 0 deletions
diff --git a/libc/fuzzing/__support/uint_fuzz.cpp b/libc/fuzzing/__support/uint_fuzz.cpp
new file mode 100644
index 000000000000..f48f00d3b4ba
--- /dev/null
+++ b/libc/fuzzing/__support/uint_fuzz.cpp
@@ -0,0 +1,70 @@
+#include "src/__support/CPP/bit.h"
+#include "src/__support/UInt.h"
+#include "src/string/memory_utils/inline_memcpy.h"
+
+using namespace LIBC_NAMESPACE;
+
+// Helper function when using gdb / lldb to set a breakpoint and inspect values.
+template <typename T> void debug_and_trap(const char *msg, T a, T b) {
+  __builtin_trap();
+}
+
+#define DEBUG_AND_TRAP()
+
+#define TEST_BINOP(OP)                                                         \
+  if ((a OP b) != (static_cast<T>(BigInt(a) OP BigInt(b))))                    \
+    debug_and_trap(#OP, a, b);
+
+#define TEST_SHIFTOP(OP)                                                       \
+  if ((a OP b) != (static_cast<T>(BigInt(a) OP b)))                            \
+    debug_and_trap(#OP, a, b);
+
+#define TEST_FUNCTION(FUN)                                                     \
+  if (FUN(a) != FUN(BigInt(a)))                                                \
+    debug_and_trap(#FUN, a, b);
+
+// Test that basic arithmetic operations of BigInt behave like their scalar
+// counterparts.
+template <typename T, typename BigInt> void run_tests(T a, T b) {
+  TEST_BINOP(+)
+  TEST_BINOP(-)
+  TEST_BINOP(*)
+  if (b != 0)
+    TEST_BINOP(/)
+  if (b >= 0 && b < cpp::numeric_limits<T>::digits) {
+    TEST_SHIFTOP(<<)
+    TEST_SHIFTOP(>>)
+  }
+  if constexpr (!BigInt::SIGNED) {
+    TEST_FUNCTION(cpp::has_single_bit)
+    TEST_FUNCTION(cpp::countr_zero)
+    TEST_FUNCTION(cpp::countl_zero)
+    TEST_FUNCTION(cpp::countl_one)
+    TEST_FUNCTION(cpp::countr_one)
+  }
+}
+
+// Reads a T from libfuzzer data.
+template <typename T> T read(const uint8_t *data, size_t &remainder) {
+  T out = 0;
+  constexpr size_t T_SIZE = sizeof(T);
+  const size_t copy_size = remainder < T_SIZE ? remainder : T_SIZE;
+  inline_memcpy(&out, data, copy_size);
+  remainder -= copy_size;
+  return out;
+}
+
+template <typename T, typename BigInt>
+void run_tests(const uint8_t *data, size_t size) {
+  const auto a = read<T>(data, size);
+  const auto b = read<T>(data, size);
+  run_tests<T, BigInt>(a, b);
+}
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  // unsigned
+  run_tests<uint64_t, BigInt<64, false, uint16_t>>(data, size);
+  // signed
+  run_tests<int64_t, BigInt<64, true, uint16_t>>(data, size);
+  return 0;
+}