[PATCH] math.h: Do use __MATH_TG_BUILTIN_CLASSIFY for C++

[H.J. Lu]

On Mon, Dec 29, 2025 at 11:12 PM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
> The __MATH_TG_BUILTIN_CLASSIFY macro can not be used in C++ mode due
> it might use __builtin_types_compatible_p, which is a C-only builtin.
>
> Checked with bmg targetting x86_64-linux-gnu and on x86_64-linux-gnu
> and aarch64 with gcc-15 and clang-18.
> ---
>  math/math.h | 43 ++++++++++++++++++++++++++-----------------
>  1 file changed, 26 insertions(+), 17 deletions(-)
>
> diff --git a/math/math.h b/math/math.h
> index 96b64379f7..49489293fc 100644
> --- a/math/math.h
> +++ b/math/math.h
> @@ -1173,21 +1173,25 @@ enum
>
>  /* Return number of classification appropriate for X.  */
>  # if ((__GNUC_PREREQ (4,4) && !defined __SUPPORT_SNAN__)                     \
> -      || __glibc_clang_prereq (2,8))                                         \
> -     && (!defined __OPTIMIZE_SIZE__ || defined __cplusplus)
> -     /* The check for __cplusplus allows the use of the builtin, even
> -       when optimization for size is on.  This is provided for
> -       libstdc++, only to let its configure test work when it is built
> -       with -Os.  No further use of this definition of fpclassify is
> -       expected in C++ mode, since libstdc++ provides its own version
> -       of fpclassify in cmath (which undefines fpclassify).  */
> -#  define fpclassify(x)                                                              \
> -  __MATH_TG_BUILTIN_CLASSIFY ((x),                                           \
> -                             __builtin_fpclassify, (FP_NAN, FP_INFINITE,     \
> -                                                    FP_NORMAL, FP_SUBNORMAL, \
> -                                                    FP_ZERO, x),             \
> -                             fpclassify, (x),                                \
> -                             __FP_BUILTIN_FPCLASSIFY_DENORMAL)
> +       || __glibc_clang_prereq (2,8))                                        \
> +     && !defined __cplusplus
> +  /* The __MATH_TG_BUILTIN_CLASSIFY macro can not be used in C++ mode due
> +     it might use __builtin_types_compatible_p, which is C-only builtin.  */
> +#  define fpclassify(x)                                                       \
> +  __MATH_TG_BUILTIN_CLASSIFY ((x),                                            \
> +                              __builtin_fpclassify, (FP_NAN, FP_INFINITE,     \
> +                                                     FP_NORMAL, FP_SUBNORMAL, \
> +                                                     FP_ZERO, x),             \
> +                              fpclassify, (x),                                \
> +                              __FP_BUILTIN_FPCLASSIFY_DENORMAL)
> +# elif defined __cplusplus
> +   /* The check for __cplusplus allows the use of the builtin, even
> +      when optimization for size is on.  This is provided for
> +      libstdc++ and no further use of this definition of fpclassify is
> +      expected in C++ mode, since libstdc++ provides its own version
> +      of fpclassify in cmath (which undefines fpclassify).  */
> +#  define fpclassify(x) __builtin_fpclassify (FP_NAN, FP_INFINITE,           \
> +     FP_NORMAL, FP_SUBNORMAL, FP_ZERO, x)
>  # else
>  #  define fpclassify(x) __MATH_TG ((x), __fpclassify, (x))
>  # endif
> @@ -1247,12 +1251,17 @@ enum
>  #  define isinf(x) \
>      (__builtin_types_compatible_p (__typeof (x), _Float128) \
>       ? __isinff128 (x) : __builtin_isinf_sign (x))
> -# elif (__GNUC_PREREQ (4,4) && !defined __SUPPORT_SNAN__) \
> -       || __glibc_clang_prereq (3,7)
> +  /* The __MATH_TG_BUILTIN_CLASSIFY macro can not be used in C++ mode due
> +     it might use __builtin_types_compatible_p, which is C-only builtin.  */
> +# elif ((__GNUC_PREREQ (4,4) && !defined __SUPPORT_SNAN__) \
> +        || __glibc_clang_prereq (3,7)) \
> +     && !defined __cplusplus
>  #  define isinf(x) __MATH_TG_BUILTIN_CLASSIFY ((x),                          \
>                                                __builtin_isinf_sign, (x),     \
>                                                isinf, (x),                    \
>                                                __FP_BUILTIN_ISINF_SIGN_DENORMAL)
> +# elif defined __cplusplus
> +#  define isinf(x) __builtin_isinf_sign (x)
>  # else
>  #  define isinf(x) __MATH_TG ((x), __isinf, (x))
>  # endif
> --
> 2.43.0
>

Please add a C++ test for this change.

--
H.J.