[libc] Add implementations of nextafter[f|l] functions.

A differential fuzzer for these functions has also been added.
Along the way, a small correction has been done to the normal/subnormal
limits of x86 long double values.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D94109

3 files changed, 39 insertions, 0 deletions

diff --git a/libc/fuzzing/math/CMakeLists.txt b/libc/fuzzing/math/CMakeLists.txt
index 7c710fcecfdb..af9a5d30765e 100644
--- a/libc/fuzzing/math/CMakeLists.txt
+++ b/libc/fuzzing/math/CMakeLists.txt
@@ -48,3 +48,15 @@ add_libc_fuzzer(
     libc.utils.FPUtil.fputil
     libc.utils.CPP.standalone_cpp
 )
+
+add_libc_fuzzer(
+  nextafter_differential_fuzz
+  SRCS
+    nextafter_differential_fuzz.cpp
+  HDRS
+    TwoInputSingleOutputDiff.h
+  DEPENDS
+    libc.src.math.nextafter
+    libc.src.math.nextafterf
+    libc.src.math.nextafterl
+)
diff --git a/libc/fuzzing/math/Compare.h b/libc/fuzzing/math/Compare.h
index 62ab6c791070..11d54650f66f 100644
--- a/libc/fuzzing/math/Compare.h
+++ b/libc/fuzzing/math/Compare.h
@@ -10,6 +10,7 @@
 #define LLVM_LIBC_FUZZING_MATH_COMPARE_H
 
 #include "utils/CPP/TypeTraits.h"
+#include "utils/FPUtil/FPBits.h"
 
 template <typename T>
 __llvm_libc::cpp::EnableIfType<__llvm_libc::cpp::IsFloatingPointType<T>::Value,
diff --git a/libc/fuzzing/math/nextafter_differential_fuzz.cpp b/libc/fuzzing/math/nextafter_differential_fuzz.cpp
new file mode 100644
index 000000000000..f4a7891df2aa
--- /dev/null
+++ b/libc/fuzzing/math/nextafter_differential_fuzz.cpp
@@ -0,0 +1,26 @@
+//===-- nextafter_differential_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
+//
+//===----------------------------------------------------------------------===//
+///
+/// Differential fuzz test for llvm-libc nextafter implementation.
+///
+//===----------------------------------------------------------------------===//
+
+#include "fuzzing/math/TwoInputSingleOutputDiff.h"
+
+#include "src/math/nextafter.h"
+#include "src/math/nextafterf.h"
+#include "src/math/nextafterl.h"
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  TwoInputSingleOutputDiff<float, float>(&__llvm_libc::nextafterf,
+                                         &::nextafterf, data, size);
+  TwoInputSingleOutputDiff<double, double>(&__llvm_libc::nextafter,
+                                           &::nextafter, data, size);
+  return 0;
+}