[reland][libc] Refactor BigInt (#87613)

This is a reland of #86137 with a fix for platforms / compiler that do
not support trivially constructible int128 types.

libc/fuzzing/CMakeLists.txt

@@ -1,6 +1,7 @@
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=fuzzer")
 add_custom_target(libc-fuzzer)
 
+add_subdirectory(__support)
 # TODO(#85680): Re-enable math fuzzing after headers are sorted out
 # add_subdirectory(math)
 add_subdirectory(stdlib)

libc/fuzzing/__support/CMakeLists.txt

@@ -0,0 +1,7 @@
+add_libc_fuzzer(
+  uint_fuzz
+  SRCS
+    uint_fuzz.cpp
+  DEPENDS
+    libc.src.__support.uint
+)

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;
+}

libc/src/__support/FPUtil/dyadic_float.h

@@ -58,9 +58,9 @@ template <size_t Bits> struct DyadicFloat {
   // significant bit.
   LIBC_INLINE constexpr DyadicFloat &normalize() {
     if (!mantissa.is_zero()) {
-      int shift_length = static_cast<int>(mantissa.clz());
+      int shift_length = cpp::countl_zero(mantissa);
       exponent -= shift_length;
-      mantissa.shift_left(static_cast<size_t>(shift_length));
+      mantissa <<= static_cast<size_t>(shift_length);
     }
     return *this;
   }
@@ -233,7 +233,7 @@ LIBC_INLINE constexpr DyadicFloat<Bits> quick_add(DyadicFloat<Bits> a,
     result.sign = a.sign;
     result.exponent = a.exponent;
     result.mantissa = a.mantissa;
-    if (result.mantissa.add(b.mantissa)) {
+    if (result.mantissa.add_overflow(b.mantissa)) {
       // Mantissa addition overflow.
       result.shift_right(1);
       result.mantissa.val[DyadicFloat<Bits>::MantissaType::WORD_COUNT - 1] |=