[libc] created tan function fuzzer (#101570)

Also edited file header formatting on sin_fuz and cos_fuzz

libc/fuzzing/math/CMakeLists.txt

@@ -79,3 +79,12 @@ add_libc_fuzzer(
   DEPENDS
     libc.src.math.cos
 )
+
+add_libc_fuzzer(
+  tan_fuzz
+  NEED_MPFR
+  SRCS
+    tan_fuzz.cpp
+  DEPENDS
+    libc.src.math.tan
+)

libc/fuzzing/math/cos_fuzz.cpp

@@ -1,4 +1,4 @@
-//===-- cos_fuzz.cpp ----------------------------------------------------===//
+//===-- cos_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.

libc/fuzzing/math/sin_fuzz.cpp

@@ -1,4 +1,4 @@
-//===-- sin_fuzz.cpp ----------------------------------------------------===//
+//===-- sin_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.

libc/fuzzing/math/tan_fuzz.cpp

@@ -0,0 +1,40 @@
+//===-- tan_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 tan implementation.
+///
+//===----------------------------------------------------------------------===//
+
+#include "src/math/tan.h"
+#include <math.h>
+#include <mpfr.h>
+
+extern "C" int LLVMFuzzerTestOneInput(const double x) {
+  // remove NaN and inf as preconditions
+  if (isnan(x))
+    return 0;
+  if (isinf(x))
+    return 0;
+  // signed zeros already tested in unit tests
+  if (signbit(x) && x == 0.0)
+    return 0;
+  mpfr_t input;
+  mpfr_init2(input, 53);
+  mpfr_set_d(input, x, MPFR_RNDN);
+  int output = mpfr_tan(input, input, MPFR_RNDN);
+  mpfr_subnormalize(input, output, MPFR_RNDN);
+  double to_compare = mpfr_get_d(input, MPFR_RNDN);
+
+  double result = LIBC_NAMESPACE::tan(x);
+
+  if (result != to_compare)
+    __builtin_trap();
+
+  mpfr_clear(input);
+  return 0;
+}