Re: [PATCH] libstdc++: Ensure that std::from_chars is declared when supported

[Jonathan Wakely <jwakely@redhat.com>]

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On Mon, 14 Mar 2022 at 14:15, Patrick Palka wrote:
> I think __floating_from_chars_hex should work fine on 16 bit targets,
> so I suppose we could use it in the !USE_LIB_FAST_FLOAT branch as well.

Good point, and even for SIZE_WIDTH >= 32, it might be faster to use
your __floating_from_chars_hex function than to change locale twice
and allocate memory and call strto{f,d,ld}. It certainly avoids the
non-standard errc::out_of_memory result.

So maybe something like the attached semi-tested patch.

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commit 57b9e2227c7b14f04f2593ec41e177631b6f3fc2
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Mar 16 16:46:07 2022

    libstdc++: Prefer our own __floating_from_chars_hex if supported
    
    This adjust the floating-point overloads of std::from_chars so that they
    prefer to use __floating_from_chars_hex if possible, instead of using
    strtof/strtod/strtold. This avoids changing the per-thread C locale and
    allocating memory for a null-terminated string when we can use the
    hex-specific function instead.
    
    libstdc++-v3/ChangeLog:
    
            * src/c++17/floating_from_chars.cc (from_chars): Always use
            __floating_from_chars_hex for hex format if possible.

diff --git a/libstdc++-v3/src/c++17/floating_from_chars.cc b/libstdc++-v3/src/c++17/floating_from_chars.cc
index 4aa2483bc28..ce30752ee95 100644
--- a/libstdc++-v3/src/c++17/floating_from_chars.cc
+++ b/libstdc++-v3/src/c++17/floating_from_chars.cc
@@ -821,13 +821,12 @@ from_chars_result
 from_chars(const char* first, const char* last, float& value,
 	   chars_format fmt) noexcept
 {
-#if USE_LIB_FAST_FLOAT
+#if _GLIBCXX_FLOAT_IS_IEEE_BINARY32 && _GLIBCXX_DOUBLE_IS_IEEE_BINARY64
   if (fmt == chars_format::hex)
     return __floating_from_chars_hex(first, last, value);
-  else
-    {
-      return fast_float::from_chars(first, last, value, fmt);
-    }
+#endif
+#if USE_LIB_FAST_FLOAT
+  return fast_float::from_chars(first, last, value, fmt);
 #else
   return from_chars_strtod(first, last, value, fmt);
 #endif
@@ -837,13 +836,12 @@ from_chars_result
 from_chars(const char* first, const char* last, double& value,
 	   chars_format fmt) noexcept
 {
-#if USE_LIB_FAST_FLOAT
+#if _GLIBCXX_FLOAT_IS_IEEE_BINARY32 && _GLIBCXX_DOUBLE_IS_IEEE_BINARY64
   if (fmt == chars_format::hex)
     return __floating_from_chars_hex(first, last, value);
-  else
-    {
-      return fast_float::from_chars(first, last, value, fmt);
-    }
+#endif
+#if USE_LIB_FAST_FLOAT
+  return fast_float::from_chars(first, last, value, fmt);
 #else
   return from_chars_strtod(first, last, value, fmt);
 #endif
@@ -853,18 +851,12 @@ from_chars_result
 from_chars(const char* first, const char* last, long double& value,
 	   chars_format fmt) noexcept
 {
-#if ! USE_STRTOD_FOR_FROM_CHARS
+#if ! USE_STRTOD_FOR_FROM_CHARS || __LDBL_MANT_DIG__ == __DBL_MANT_DIG__
   // Either long double is the same as double, or we can't use strtold.
   // In the latter case, this might give an incorrect result (e.g. values
   // out of range of double give an error, even if they fit in long double).
   double dbl_value;
-  from_chars_result result;
-  if (fmt == chars_format::hex)
-    result = __floating_from_chars_hex(first, last, dbl_value);
-  else
-    {
-      result = fast_float::from_chars(first, last, dbl_value, fmt);
-    }
+  from_chars_result result = std::from_chars(first, last, dbl_value, fmt);
   if (result.ec == errc{})
     value = dbl_value;
   return result;
@@ -888,7 +880,8 @@ from_chars_result
 from_chars(const char* first, const char* last, __ieee128& value,
 	   chars_format fmt) noexcept
 {
-  // fast_float doesn't support IEEE binary128 format, but we can use strtold.
+  // fast_float doesn't support IEEE binary128 format, and neither does our
+  // __floating_from_chars_hex, but we can use __strtoieee128 on this target.
   return from_chars_strtod(first, last, value, fmt);
 }
 #endif