[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

libc/config/linux/x86_64/entrypoints.txt

@@ -56,6 +56,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     # stdlib.h entrypoints
     libc.src.stdlib.abs
     libc.src.stdlib.atoi
+    libc.src.stdlib.atof
     libc.src.stdlib.atol
     libc.src.stdlib.atoll
     libc.src.stdlib.bsearch
@@ -65,6 +66,8 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.stdlib.llabs
     libc.src.stdlib.lldiv
     libc.src.stdlib.qsort
+    libc.src.stdlib.strtod
+    libc.src.stdlib.strtof
     libc.src.stdlib.strtol
     libc.src.stdlib.strtoll
     libc.src.stdlib.strtoul

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
+)
+

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

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;
+}

libc/spec/stdc.td

@@ -495,6 +495,7 @@ def StdC : StandardSpec<"stdc"> {
           FunctionSpec<"labs", RetValSpec<LongType>, [ArgSpec<LongType>]>,
           FunctionSpec<"llabs", RetValSpec<LongLongType>, [ArgSpec<LongLongType>]>,
 
+          FunctionSpec<"atof", RetValSpec<DoubleType>, [ArgSpec<ConstCharRestrictedPtr>]>,
           FunctionSpec<"atoi", RetValSpec<IntType>, [ArgSpec<ConstCharPtr>]>,
           FunctionSpec<"atol", RetValSpec<LongType>, [ArgSpec<ConstCharPtr>]>,
           FunctionSpec<"atoll", RetValSpec<LongLongType>, [ArgSpec<ConstCharPtr>]>,
@@ -505,6 +506,8 @@ def StdC : StandardSpec<"stdc"> {
 
           FunctionSpec<"qsort", RetValSpec<VoidType>, [ArgSpec<VoidPtr>, ArgSpec<SizeTType>, ArgSpec<SizeTType>, ArgSpec<QSortCompareT>]>,
 
+          FunctionSpec<"strtod", RetValSpec<DoubleType>, [ArgSpec<ConstCharRestrictedPtr>, ArgSpec<CharRestrictedPtrPtr>]>,
+          FunctionSpec<"strtof", RetValSpec<FloatType>, [ArgSpec<ConstCharRestrictedPtr>, ArgSpec<CharRestrictedPtrPtr>]>,
           FunctionSpec<"strtol", RetValSpec<LongType>, [ArgSpec<ConstCharRestrictedPtr>, ArgSpec<CharRestrictedPtrPtr>, ArgSpec<IntType>]>,
           FunctionSpec<"strtoll", RetValSpec<LongLongType>, [ArgSpec<ConstCharRestrictedPtr>, ArgSpec<CharRestrictedPtrPtr>, ArgSpec<IntType>]>,
           FunctionSpec<"strtoul", RetValSpec<UnsignedLongType>, [ArgSpec<ConstCharRestrictedPtr>, ArgSpec<CharRestrictedPtrPtr>, ArgSpec<IntType>]>,

libc/src/__support/CMakeLists.txt

@@ -23,12 +23,14 @@ add_header_library(
   str_conv_utils
   HDRS
     str_conv_utils.h
+    str_to_float.h
   DEPENDS
     .ctype_utils
     .high_precision_decimal
     libc.include.errno
     libc.src.errno.__errno_location
     libc.utils.CPP.standalone_cpp
+    libc.src.__support.FPUtil.fputil
 )
 
 add_header_library(