Re: [PATCH 3/5] libstdc++: Adjust fast_float's over/underflow behavior for conformnace

[Patrick Palka <ppalka@redhat.com>]

On Mon, 15 Nov 2021, Patrick Palka wrote:

> This makes fast_float handle the situation where std::from_chars is
> specified to return result_out_of_range, i.e. when the parsed value
> is outside the representable range of the floating-point type.
> 
> libstdc++-v3/ChangeLog:
> 
> 	* src/c++17/fast_float/LOCAL_PATCHES: Update.
> 	* src/c++17/fast_float/parse_number.h (from_chars_advanced): In
> 	case of over/underflow, return errc::result_out_of_range and don't
> 	modify 'value'.

v2: Now in terms of the amalgamated version of the library.

-- >8 --

Subject: [PATCH 3/5] libstdc++: Adjust fast_float's over/underflow behavior
 for conformance

This adjusts fast_float's behavior in case of over/underflow: rather than
setting value to +-0 or +-infinity, return result_out_of_range and don't
modify value, as per [charconv.from.chars]/1.

libstdc++-v3/ChangeLog:

	* src/c++17/fast_float/LOCAL_PATCHES: Update.
	* src/c++17/fast_float/fast_float.h (from_chars_advanced): In
	case of over/underflow, return errc::result_out_of_range and don't
	modify 'value'.
---
 libstdc++-v3/src/c++17/fast_float/LOCAL_PATCHES | 1 +
 libstdc++-v3/src/c++17/fast_float/fast_float.h  | 9 +++++++++
 2 files changed, 10 insertions(+)

diff --git a/libstdc++-v3/src/c++17/fast_float/LOCAL_PATCHES b/libstdc++-v3/src/c++17/fast_float/LOCAL_PATCHES
index e9d7bba6195..1f90f9d1d85 100644
--- a/libstdc++-v3/src/c++17/fast_float/LOCAL_PATCHES
+++ b/libstdc++-v3/src/c++17/fast_float/LOCAL_PATCHES
@@ -1 +1,2 @@
 r12-????
+r12-????
diff --git a/libstdc++-v3/src/c++17/fast_float/fast_float.h b/libstdc++-v3/src/c++17/fast_float/fast_float.h
index c908719ec3a..97d28940944 100644
--- a/libstdc++-v3/src/c++17/fast_float/fast_float.h
+++ b/libstdc++-v3/src/c++17/fast_float/fast_float.h
@@ -2884,6 +2884,15 @@ from_chars_result from_chars_advanced(const char *first, const char *last,
   // If we called compute_float<binary_format<T>>(pns.exponent, pns.mantissa) and we have an invalid power (am.power2 < 0),
   // then we need to go the long way around again. This is very uncommon.
   if(am.power2 < 0) { am = digit_comp<T>(pns, am); }
+
+  if((pns.mantissa != 0 && am.mantissa == 0 && am.power2 == 0) || am.power2 == binary_format<T>::infinite_power()) {
+    // In case of over/underflow, return result_out_of_range and don't modify value,
+    // as per [charconv.from.chars]/1.  Note that LWG 3081 wants to modify value in
+    // this case too.
+    answer.ec = std::errc::result_out_of_range;
+    return answer;
+  }
+
   to_float(pns.negative, am, value);
   return answer;
 }
-- 
2.34.0