| Issue |
165005
|
| Summary |
[libc++] Add missing C++23 <stdfloat> header to libc++
|
| Labels |
libc++
|
| Assignees |
|
| Reporter |
johnplatts
|
The C++23 `<stdfloat>` header is still missing, but this header is easy to implement in libc++.
In the libstdc++ 13 and later implementation of <stdfloat> in C++23 or later mode, std::float16_t is a typedef for _Float16 if `__STDCPP_FLOAT16_T__` is defined, std::float32_t is a typedef for _Float32 if `__STDCPP_FLOAT32_T__` is defined, std::float64_t is a typedef for _Float64 if `__STDCPP_FLOAT64_T__` is defined, and std::float128_t is a typedef for _Float128 if `__STDCPP_FLOAT128_T__` is defined.
`std::is_same_v<std::float32_t, float>` is always false in libstdc++ 13 and later, even though std::float32_t and float both usually have the same format, size, and alignment.
`std::is_same_v<std::float64_t, double>` is always false in libstdc++ 13 and later, even though std::float64_t and double both usually have the same format, size, and alignment.
The `std::numeric_limits<std::float32_t>` and `std::numeric_limits<std::float64_t>` specializations are easy to implement (assuming `std::float32_t` has the same format as `float` and `std::float64_t` has the same format as `double`) as `std::numeric_limits<std::float32_t>` can usually use the same values as `std::numeric_limits<float>` (but with `std::float32_t` used instead of `float` for floating-point values in the `std::numeric_limits<std::float32_t>` specialization) and as `std::numeric_limits<std::float64_t>` can usually use the same values as `std::numeric_limits<double>` (but with `std::float64_t` used instead of `double` for floating-point values in the `std::numeric_limits<std::float64_t>` specialization).
Clang 21 already defines the `__FLT16_MANT_DIG__`, `__FLT16_DIG__`, `__FLT16_MIN__`, `__FLT16_MAX__`, `__FLT16_EPSILON__`, `__FLT16_MIN_EXP__`, `__FLT16_MIN_10_EXP__`, `__FLT16_MAX_EXP__`, `__FLT16_MAX_10_EXP__`, `__FLT16_DENORM_MIN__` macros needed to implement the `std::numeric_limits<std::float16_t>` specialization.
In addition to adding the `<stdfloat>` header to libc++, also need to update the Clang preprocessor to define the `__STDCPP_FLOAT16_T__` (if `_Float16` is available), `__STDCPP_FLOAT32_T__`, `__STDCPP_FLOAT64_T__`, and `__STDCPP_FLOAT128_T__` (if `_Float128` is available) macros in C++23 mode or later.
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