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Disable Clinger float fast path under extended FP precision (x87)

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The contiguous number fast path uses a Clinger-style exact algorithm
(significand * 10^scale in double arithmetic), which is only correctly
rounded when double operations are evaluated in true 53-bit precision.
On the x87 FPU used by 32-bit x86 (FLT_EVAL_METHOD == 2) the single
multiply/divide is computed in 80-bit and then double-rounded to double,
so a small fraction of values land 1 ULP off.

This surfaced as test-cbor_cpp11 and test-msgpack_cpp11 failing on the
mingw (x86) job for regression/floats.json: the C++17 builds pass because
they take the correctly-rounded std::from_chars path, while C++11 falls
back to parse_float_fast(). A 5M-sample check over shortest round-trip
decimals reproduces it: 0 divergences with 53-bit doubles, ~1 in 25 000
with 80-bit intermediates; declining to std::strtod fixes all of them.

Guard parse_float_fast() on FLT_EVAL_METHOD so it declines whenever the
platform evaluates doubles in extended precision, letting the caller use
the correctly-rounded std::from_chars / std::strtod path instead. On
mainstream x86-64/ARM64 (FLT_EVAL_METHOD == 0) the fast path is unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AXcDtEma2PjxgmPS9cQGzA
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-07-20 20:49:27 +00:00
parent 0c91bbc4ba
commit 0d5b70a95a
2 changed files with 30 additions and 0 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/nlohmann/detail/input/number_parse.hpp
+15
View File
@@ -9,6 +9,7 @@
#pragma once
#include <array> // array
#include <cfloat> // FLT_EVAL_METHOD
#include <cstddef> // size_t
#include <cstdint> // int64_t, uint64_t
#include <limits> // numeric_limits
@@ -120,6 +121,19 @@ below).
template<typename DecimalPointType>
bool parse_float_fast(const char* first, const char* last, DecimalPointType decimal_point, double& out) noexcept
{
#if defined(FLT_EVAL_METHOD) && FLT_EVAL_METHOD != 0
// Clinger's fast path is only exact when double operations are evaluated in
// true double precision. On platforms that keep intermediates in extended
// precision (e.g. the x87 FPU on 32-bit x86, where FLT_EVAL_METHOD == 2) the
// single significand * 10^scale step is double-rounded and can be 1 ULP off,
// so decline and let the caller fall back to the correctly-rounded
// std::from_chars / std::strtod path.
static_cast<void>(first);
static_cast<void>(last);
static_cast<void>(decimal_point);
static_cast<void>(out);
return false;
#else
static const std::array<double, 23> powers_of_ten =
{
{
@@ -236,6 +250,7 @@ bool parse_float_fast(const char* first, const char* last, DecimalPointType deci
}
out = negative ? -result : result;
return true;
#endif
}
/// fast float path is only exact for `double`; decline for float/long double
single_include/nlohmann/json.hpp
+15
View File
@@ -7791,6 +7791,7 @@ NLOHMANN_JSON_NAMESPACE_END
#include <array> // array
#include <cfloat> // FLT_EVAL_METHOD
#include <cstddef> // size_t
#include <cstdint> // int64_t, uint64_t
#include <limits> // numeric_limits
@@ -7903,6 +7904,19 @@ below).
template<typename DecimalPointType>
bool parse_float_fast(const char* first, const char* last, DecimalPointType decimal_point, double& out) noexcept
{
#if defined(FLT_EVAL_METHOD) && FLT_EVAL_METHOD != 0
// Clinger's fast path is only exact when double operations are evaluated in
// true double precision. On platforms that keep intermediates in extended
// precision (e.g. the x87 FPU on 32-bit x86, where FLT_EVAL_METHOD == 2) the
// single significand * 10^scale step is double-rounded and can be 1 ULP off,
// so decline and let the caller fall back to the correctly-rounded
// std::from_chars / std::strtod path.
static_cast<void>(first);
static_cast<void>(last);
static_cast<void>(decimal_point);
static_cast<void>(out);
return false;
#else
static const std::array<double, 23> powers_of_ten =
{
{
@@ -8019,6 +8033,7 @@ bool parse_float_fast(const char* first, const char* last, DecimalPointType deci
}
out = negative ? -result : result;
return true;
#endif
}
/// fast float path is only exact for `double`; decline for float/long double