[PATCH 4/5] libstdc++: Use fast_float in std::from_chars for binary32/64

[Patrick Palka via Gcc-patches]

This makes our std::from_chars implementation use fast_float for parsing
chars_format::scientific/fixed/general parsing into binary32/64 values.
For chars_format::hex (and long double) we still use the fallback
implementation that goes through the strtod family of functions.

libstdc++-v3/ChangeLog:

        * src/c++17/floating_from_chars.cc: (USE_LIB_FAST_FLOAT):
        Conditionally define, and use it to conditionally include
        fast_float.
        (from_chars): Use fast_float for float and double when
        USE_LIB_FAST_FLOAT.
---
 libstdc++-v3/src/c++17/floating_from_chars.cc | 23 +++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/libstdc++-v3/src/c++17/floating_from_chars.cc 
b/libstdc++-v3/src/c++17/floating_from_chars.cc
index aa074869872..13834f54d38 100644
--- a/libstdc++-v3/src/c++17/floating_from_chars.cc
+++ b/libstdc++-v3/src/c++17/floating_from_chars.cc
@@ -34,6 +34,7 @@
 #include <string>
 #include <memory_resource>
 #include <cfenv>
+#include <cfloat>
 #include <cmath>
 #include <cstdlib>
 #include <cstring>
@@ -52,6 +53,18 @@
 extern "C" __ieee128 __strtoieee128(const char*, char**);
 #endif
 
+#if _GLIBCXX_FLOAT_IS_IEEE_BINARY32 && _GLIBCXX_DOUBLE_IS_IEEE_BINARY64
+# define USE_LIB_FAST_FLOAT 1
+#endif
+
+#if USE_LIB_FAST_FLOAT
+# define FASTFLOAT_DEBUG_ASSERT __glibcxx_assert
+namespace
+{
+# include "fast_float/fast_float.h"
+} // anon namespace
+#endif
+
 #if _GLIBCXX_HAVE_USELOCALE
 namespace std _GLIBCXX_VISIBILITY(default)
 {
@@ -406,6 +419,11 @@ from_chars_result
 from_chars(const char* first, const char* last, float& value,
           chars_format fmt) noexcept
 {
+#if USE_LIB_FAST_FLOAT
+  if (fmt != chars_format::hex)
+    return fast_float::from_chars(first, last, value, fmt);
+#endif
+
   errc ec = errc::invalid_argument;
 #if _GLIBCXX_USE_CXX11_ABI
   buffer_resource mr;
@@ -432,6 +450,11 @@ from_chars_result
 from_chars(const char* first, const char* last, double& value,
           chars_format fmt) noexcept
 {
+#if USE_LIB_FAST_FLOAT
+  if (fmt != chars_format::hex)
+    return fast_float::from_chars(first, last, value, fmt);
+#endif
+
   errc ec = errc::invalid_argument;
 #if _GLIBCXX_USE_CXX11_ABI
   buffer_resource mr;
-- 
2.34.0