[libc] Add fixed point support to printf (#82707)

This patch adds the r, R, k, and K conversion specifiers to printf, with
accompanying tests. They are guarded behind the
LIBC_COPT_PRINTF_DISABLE_FIXED_POINT flag as well as automatic fixed
point support detection.

libc/fuzzing/stdio/CMakeLists.txt

@@ -15,3 +15,17 @@ add_libc_fuzzer(
     libc.src.stdio.snprintf
     libc.src.__support.FPUtil.fp_bits
 )
+
+if(LIBC_COMPILER_HAS_FIXED_POINT)
+  add_libc_fuzzer(
+    printf_fixed_conv_fuzz
+    NEED_MPFR
+    SRCS
+      printf_fixed_conv_fuzz.cpp
+    DEPENDS
+      libc.src.stdio.snprintf
+      libc.src.__support.fixed_point.fx_bits
+    COMPILE_OPTIONS
+      -ffixed-point # TODO: add -ffixed-point to fuzz tests automatically
+  )
+endif()

libc/fuzzing/stdio/printf_fixed_conv_fuzz.cpp

@@ -0,0 +1,133 @@
+//===-- printf_fixed_conv_fuzz.cpp ----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// Fuzzing test for llvm-libc printf %f/e/g/a implementations.
+///
+//===----------------------------------------------------------------------===//
+#include "src/stdio/snprintf.h"
+
+#include "include/llvm-libc-macros/stdfix-macros.h"
+#include "src/__support/fixed_point/fx_bits.h"
+#include "src/__support/fixed_point/fx_rep.h"
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "utils/MPFRWrapper/mpfr_inc.h"
+
+constexpr int MAX_SIZE = 10000;
+
+inline bool simple_streq(char *first, char *second, int length) {
+  for (int i = 0; i < length; ++i)
+    if (first[i] != second[i])
+      return false;
+
+  return true;
+}
+
+inline int clamp(int num, int max) {
+  if (num > max)
+    return max;
+  if (num < -max)
+    return -max;
+  return num;
+}
+
+enum class TestResult {
+  Success,
+  BufferSizeFailed,
+  LengthsDiffer,
+  StringsNotEqual,
+};
+
+template <typename F>
+inline TestResult test_vals(const char *fmt, uint64_t num, int prec,
+                            int width) {
+  typename LIBC_NAMESPACE::fixed_point::FXRep<F>::StorageType raw_num = num;
+
+  auto raw_num_bits = LIBC_NAMESPACE::fixed_point::FXBits<F>(raw_num);
+
+  // This needs to be a float with enough bits of precision to hold the fixed
+  // point number.
+  static_assert(sizeof(long double) > sizeof(long accum));
+
+  // build a long double that is equivalent to the fixed point number.
+  long double ld_num =
+      static_cast<long double>(raw_num_bits.get_integral()) +
+      (static_cast<long double>(raw_num_bits.get_fraction()) /
+       static_cast<long double>(1ll << raw_num_bits.get_exponent()));
+
+  if (raw_num_bits.get_sign())
+    ld_num = -ld_num;
+
+  // Call snprintf on a nullptr to get the buffer size.
+  int buffer_size = LIBC_NAMESPACE::snprintf(nullptr, 0, fmt, width, prec, num);
+
+  if (buffer_size < 0)
+    return TestResult::BufferSizeFailed;
+
+  char *test_buff = new char[buffer_size + 1];
+  char *reference_buff = new char[buffer_size + 1];
+
+  int test_result = 0;
+  int reference_result = 0;
+
+  test_result = LIBC_NAMESPACE::snprintf(test_buff, buffer_size + 1, fmt, width,
+                                         prec, num);
+
+  // The fixed point format is defined to be %f equivalent.
+  reference_result = mpfr_snprintf(reference_buff, buffer_size + 1, "%*.*Lf",
+                                   width, prec, ld_num);
+
+  // All of these calls should return that they wrote the same amount.
+  if (test_result != reference_result || test_result != buffer_size)
+    return TestResult::LengthsDiffer;
+
+  if (!simple_streq(test_buff, reference_buff, buffer_size))
+    return TestResult::StringsNotEqual;
+
+  delete[] test_buff;
+  delete[] reference_buff;
+  return TestResult::Success;
+}
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  // const uint8_t raw_data[] = {0x8d,0x43,0x40,0x0,0x0,0x0,};
+  // data = raw_data;
+  // size = sizeof(raw_data);
+  int prec = 0;
+  int width = 0;
+
+  LIBC_NAMESPACE::fixed_point::FXRep<long accum>::StorageType raw_num = 0;
+
+  // Copy as many bytes of data as will fit into num, prec, and with. Any extras
+  // are ignored.
+  for (size_t cur = 0; cur < size; ++cur) {
+    if (cur < sizeof(raw_num)) {
+      raw_num = (raw_num << 8) + data[cur];
+    } else if (cur < sizeof(raw_num) + sizeof(prec)) {
+      prec = (prec << 8) + data[cur];
+    } else if (cur < sizeof(raw_num) + sizeof(prec) + sizeof(width)) {
+      width = (width << 8) + data[cur];
+    }
+  }
+
+  width = clamp(width, MAX_SIZE);
+  prec = clamp(prec, MAX_SIZE);
+
+  TestResult result;
+  result = test_vals<long accum>("%*.*lk", raw_num, prec, width);
+  if (result != TestResult::Success)
+    __builtin_trap();
+
+  result = test_vals<unsigned long accum>("%*.*lK", raw_num, prec, width);
+  if (result != TestResult::Success)
+    __builtin_trap();
+
+  return 0;
+}