[libc] add atof, strtof and strtod

Add the string to floating point conversion functions.
Long doubles aren't supported yet, but floats and doubles are. The
primary algorithm used is the Eisel-Lemire ParseNumberF64 algorithm,
with the Simple Decimal Conversion algorithm as backup.

Links for more information on the algorithms:

Number Parsing at a Gigabyte per Second, Software: Practice and
Experience 51 (8), 2021 (https://arxiv.org/abs/2101.11408)
https://nigeltao.github.io/blog/2020/eisel-lemire.html
https://nigeltao.github.io/blog/2020/parse-number-f64-simple.html

Differential Revision: https://reviews.llvm.org/D109261

3 files changed, 77 insertions, 0 deletions

diff --git a/libc/fuzzing/stdlib/CMakeLists.txt b/libc/fuzzing/stdlib/CMakeLists.txt
index db827f57075f..436125b6cd7a 100644
--- a/libc/fuzzing/stdlib/CMakeLists.txt
+++ b/libc/fuzzing/stdlib/CMakeLists.txt
@@ -6,3 +6,13 @@ add_libc_fuzzer(
     libc.src.stdlib.qsort
 )
 
+add_libc_fuzzer(
+  atof_fuzz
+  SRCS
+    atof_fuzz.cpp
+  HDRS
+    StringParserOutputDiff.h
+  DEPENDS
+    libc.src.stdlib.atof
+)
+
diff --git a/libc/fuzzing/stdlib/StringParserOutputDiff.h b/libc/fuzzing/stdlib/StringParserOutputDiff.h
new file mode 100644
index 000000000000..457da3b5674b
--- /dev/null
+++ b/libc/fuzzing/stdlib/StringParserOutputDiff.h
@@ -0,0 +1,35 @@
+//===-- Template to diff single-input-single-output functions ---*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_FUZZING_STDLIB_STRING_PARSER_OUTPUT_DIFF_H
+#define LLVM_LIBC_FUZZING_STDLIB_STRING_PARSER_OUTPUT_DIFF_H
+
+#include "fuzzing/math/Compare.h"
+
+#include <stddef.h>
+#include <stdint.h>
+
+template <typename T> using StringInputSingleOutputFunc = T (*)(const char *);
+
+template <typename T>
+void StringParserOutputDiff(StringInputSingleOutputFunc<T> func1,
+                            StringInputSingleOutputFunc<T> func2,
+                            const uint8_t *data, size_t size) {
+  if (size < sizeof(T))
+    return;
+
+  const char *x = reinterpret_cast<const char *>(data);
+
+  T result1 = func1(x);
+  T result2 = func2(x);
+
+  if (!ValuesEqual(result1, result2))
+    __builtin_trap();
+}
+
+#endif // LLVM_LIBC_FUZZING_STDLIB_STRING_PARSER_OUTPUT_DIFF_H
diff --git a/libc/fuzzing/stdlib/atof_fuzz.cpp b/libc/fuzzing/stdlib/atof_fuzz.cpp
new file mode 100644
index 000000000000..b368129960d3
--- /dev/null
+++ b/libc/fuzzing/stdlib/atof_fuzz.cpp
@@ -0,0 +1,32 @@
+//===-- atof_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 atof implementation.
+///
+//===----------------------------------------------------------------------===//
+#include "src/stdlib/atof.h"
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#include "fuzzing/stdlib/StringParserOutputDiff.h"
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  uint8_t *container = new uint8_t[size + 1];
+  if (!container)
+    __builtin_trap();
+  size_t i;
+
+  for (i = 0; i < size; ++i)
+    container[i] = data[i];
+  container[size] = '\0'; // Add null terminator to container.
+
+  StringParserOutputDiff<double>(&__llvm_libc::atof, &::atof, container, size);
+  delete[] container;
+  return 0;
+}