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Fix isnan so it doesn't cause FP errors #3951

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merged 2 commits into from
May 2, 2024
Merged

Fix isnan so it doesn't cause FP errors #3951

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1aa8489
Fix: isnan() shouldn't cause FP exceptions
alexdewar May 2, 2024
41e75ee
Add tests to check that isnan doesn't cause FP errors
alexdewar May 2, 2024
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2 changes: 1 addition & 1 deletion include/fmt/format.h
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Original file line number Diff line number Diff line change
Expand Up @@ -2695,7 +2695,7 @@ FMT_CONSTEXPR20 auto write_float(OutputIt out, const DecimalFP& f,
}

template <typename T> constexpr auto isnan(T value) -> bool {
return !(value >= value); // std::isnan doesn't support __float128.
return value != value; // std::isnan doesn't support __float128.
}

template <typename T, typename Enable = void>
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20 changes: 20 additions & 0 deletions test/format-test.cc
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Expand Up @@ -15,6 +15,7 @@

#include <stdint.h> // uint32_t

#include <cfenv> // fegetexceptflag and FE_ALL_EXCEPT
#include <climits> // INT_MAX
#include <cmath> // std::signbit
#include <condition_variable> // std::condition_variable
Expand Down Expand Up @@ -109,6 +110,14 @@ TEST(float_test, isfinite) {
#endif
}

void check_no_fp_exception() {
fexcept_t fe;
fegetexceptflag(&fe, FE_ALL_EXCEPT);

// No exception flags should have been set
EXPECT_TRUE(fe == 0);
}

template <typename Float> void check_isnan() {
using fmt::detail::isnan;
EXPECT_FALSE(isnan(Float(0.0)));
Expand All @@ -121,6 +130,17 @@ template <typename Float> void check_isnan() {
EXPECT_FALSE(isnan(Float(-limits::infinity())));
EXPECT_TRUE(isnan(Float(limits::quiet_NaN())));
EXPECT_TRUE(isnan(Float(-limits::quiet_NaN())));

// Sanity check: make sure no error has occurred before we start
check_no_fp_exception();

// Check that no exception is raised for the non-NaN case
isnan(Float(42.0));
check_no_fp_exception();

// Check that no exception is raised for the NaN case
isnan(Float(limits::quiet_NaN()));
check_no_fp_exception();
}

TEST(float_test, isnan) {
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