[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

1 files changed, 20 insertions, 0 deletions

diff --git a/libc/src/stdlib/strtod.cpp b/libc/src/stdlib/strtod.cpp
new file mode 100644
index 000000000000..754918c36789
--- /dev/null
+++ b/libc/src/stdlib/strtod.cpp
@@ -0,0 +1,20 @@
+//===-- Implementation of strtod ------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/stdlib/strtod.h"
+#include "src/__support/common.h"
+#include "src/__support/str_to_float.h"
+
+namespace __llvm_libc {
+
+LLVM_LIBC_FUNCTION(double, strtod,
+                   (const char *__restrict str, char **__restrict str_end)) {
+  return internal::strtofloatingpoint<double>(str, str_end);
+}
+
+} // namespace __llvm_libc