[PATCH 2/8] x86: Do not use __builtin_fpclassify for _Float64x/long double
H.J. Lu
hjl.tools@gmail.com
Wed Dec 17 23:22:47 GMT 2025
On Thu, Dec 18, 2025 at 1:56 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
> Neither gcc [1] nor clang [2] handles pseudo-normal numbers correctly
> with the __builtin_fpclassify, so disable its usage for _Float64x and
> long double types.
Please add tests to verify this fix.
>
> This only affects x86, so add a new header, fp-builtin-denormal.h, that
> defines whether the architecture requires disabling the optimization
> through a new glibc define (__FP_BUILTIN_FPCLASSIFY_DENORMAL).
>
> It fixes the regression on test-ldouble-fpclassify and
> test-float64x-fpclassify when built with clang:
>
> Failure: fpclassify (pseudo_zero): Exception "Invalid operation" set
> Failure: fpclassify (pseudo_inf): Exception "Invalid operation" set
> Failure: fpclassify (pseudo_qnan): Exception "Invalid operation" set
> Failure: fpclassify (pseudo_snan): Exception "Invalid operation" set
> Failure: fpclassify (pseudo_unnormal): Exception "Invalid operation" set
> Failure: fpclassify_downward (pseudo_zero): Exception "Invalid operation" set
> Failure: fpclassify_downward (pseudo_inf): Exception "Invalid operation" set
> Failure: fpclassify_downward (pseudo_qnan): Exception "Invalid operation" set
> Failure: fpclassify_downward (pseudo_snan): Exception "Invalid operation" set
> Failure: fpclassify_downward (pseudo_unnormal): Exception "Invalid operation" set
> Failure: fpclassify_towardzero (pseudo_zero): Exception "Invalid operation" set
> Failure: fpclassify_towardzero (pseudo_inf): Exception "Invalid operation" set
> Failure: fpclassify_towardzero (pseudo_qnan): Exception "Invalid operation" set
> Failure: fpclassify_towardzero (pseudo_snan): Exception "Invalid operation" set
> Failure: fpclassify_towardzero (pseudo_unnormal): Exception "Invalid operation" set
> Failure: fpclassify_upward (pseudo_zero): Exception "Invalid operation" set
> Failure: fpclassify_upward (pseudo_inf): Exception "Invalid operation" set
> Failure: fpclassify_upward (pseudo_qnan): Exception "Invalid operation" set
> Failure: fpclassify_upward (pseudo_snan): Exception "Invalid operation" set
> Failure: fpclassify_upward (pseudo_unnormal): Exception "Invalid operation" set
>
> Checked on x86_64-linux-gnu with gcc-15 and clang-18.
>
> [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123161
> [2] https://github.com/llvm/llvm-project/issues/172533
> ---
> bits/fp-builtin-denormal.h | 28 ++++++++
> math/Makefile | 1 +
> math/math.h | 89 +++++++++++++++++++++++++-
> sysdeps/x86/bits/fp-builtin-denormal.h | 25 ++++++++
> 4 files changed, 141 insertions(+), 2 deletions(-)
> create mode 100644 bits/fp-builtin-denormal.h
> create mode 100644 sysdeps/x86/bits/fp-builtin-denormal.h
>
> diff --git a/bits/fp-builtin-denormal.h b/bits/fp-builtin-denormal.h
> new file mode 100644
> index 0000000000..77b7184975
> --- /dev/null
> +++ b/bits/fp-builtin-denormal.h
> @@ -0,0 +1,28 @@
> +/* Denormal number definitions.
> + Copyright (C) 2025 Free Software Foundation, Inc.
> + This file is part of the GNU C Library.
> +
> + The GNU C Library is free software; you can redistribute it and/or
> + modify it under the terms of the GNU Lesser General Public
> + License as published by the Free Software Foundation; either
> + version 2.1 of the License, or (at your option) any later version.
> +
> + The GNU C Library is distributed in the hope that it will be useful,
> + but WITHOUT ANY WARRANTY; without even the implied warranty of
> + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> + Lesser General Public License for more details.
> +
> + You should have received a copy of the GNU Lesser General Public
> + License along with the GNU C Library; if not, see
> + <https://www.gnu.org/licenses/>. */
> +
> +#ifndef _MATH_H
> +# error "Never use <bits/fp-builtin-denormal.h> directly; include <math.h> instead."
> +#endif
> +
> +/* __FP_BUILTIN_FPCLASSIFY_DENORMAL is defined to 1 if compiler supports
> + handling pseudo-denormal numbers with fpclassify builtin. Pseudo-denormal
> + is a non-standard denormalized floating-point number only supported by
> + Intel double extended-precision (long double). By default assume 1 to
> + enable the usage of compiler builtin on math.h. */
> +#define __FP_BUILTIN_FPCLASSIFY_DENORMAL 1
> diff --git a/math/Makefile b/math/Makefile
> index f7a1d9016c..8fe2540ce0 100644
> --- a/math/Makefile
> +++ b/math/Makefile
> @@ -28,6 +28,7 @@ headers := \
> bits/floatn-common.h \
> bits/floatn.h \
> bits/flt-eval-method.h \
> + bits/fp-builtin-denormal.h \
> bits/fp-fast.h \
> bits/fp-logb.h \
> bits/iscanonical.h \
> diff --git a/math/math.h b/math/math.h
> index 74b064d96e..cbec2f406a 100644
> --- a/math/math.h
> +++ b/math/math.h
> @@ -1064,6 +1064,86 @@ extern int signgam;
> : FUNC ## l ARGS)
> #endif
>
> +
> +/* Depending on the type of TG_ARG and extra DEFINE to check, either call the
> + BUILTIN with ARGS_B or an appropriately suffixed version of FUNC with
> + arguments (including parentheses) ARGS_B. The function call is used for
> + long double and/or _Float64x is the builtin can not be safely used on all
> + arguments (defined by DEFINE). */
> +
> +#include <bits/fp-builtin-denormal.h>
> +
> +#ifdef __NO_LONG_DOUBLE_MATH
> +# define __MATH_TG_BUILTIN_CLASSIFY(TG_ARG, BUILTIN, ARGS_B, FUNC, ARGS_F, \
> + DEFINE) \
> + BUILTIN ARGS_B
> +#elif __HAVE_DISTINCT_FLOAT128
> +# if __HAVE_GENERIC_SELECTION
> +# if __HAVE_FLOATN_NOT_TYPEDEF && __HAVE_FLOAT32
> +# define __MATH_TG_BUILTIN_CLASSIFY_F32(BUILTIN, ARGS_B, FUNC, ARGS_F, \
> + DEFINE) \
> + _Float32: BUILTIN ARGS_B,
> +# else
> +# define __MATH_TG_BUILTIN_CLASSIFY_F32(BUILTIN, ARGS_B, FUNC, ARGS_F, \
> + DEFINE)
> +# endif
> +# define __MATH_TG_BUILTIN_CLASSIFY_LDOUBLE(BUILTIN, ARGS_B, FUNC, ARGS_F, \
> + DEFINE) \
> + long double: DEFINE ? BUILTIN ARGS_B : __ ## FUNC ## l ARGS_F,
> +# if __HAVE_FLOATN_NOT_TYPEDEF && __HAVE_FLOAT64X
> +# if __HAVE_FLOAT64X_LONG_DOUBLE
> +# define __MATH_TG_BUILTIN_CLASSIFY_F64X(BUILTIN, ARGS_B, FUNC, ARGS_F, \
> + DEFINE) \
> + _Float64x: DEFINE ? BUILTIN ARGS_B : __ ## FUNC ## l ARGS_F,
> +# else
> +# define __MATH_TG_BUILTIN_CLASSIFY_F64X(BUILTIN, ARGS_B, FUNC, ARGS_F, \
> + DEFINE) \
> + _Float64x: DEFINE ? BUILTIN ARGS_B : __ ## FUNC ## f128 ARGS_F,
> +# endif
> +# else
> +# define __MATH_TG_BUILTIN_CLASSIFY_F64X(BUILTIN, ARGS_B, FUNC, ARGS_F, \
> + DEFINE)
> +# endif
> +# define __MATH_TG_BUILTIN_CLASSIFY_F128(BUILTIN, ARGS_B, FUNC, ARGS_F, \
> + DEFINE) \
> + _Float128: BUILTIN ARGS_B
> +# define __MATH_TG_BUILTIN_CLASSIFY(TG_ARG, BUILTIN, ARGS_B, FUNC, ARGS_F, \
> + DEFINE) \
> + _Generic ((TG_ARG), \
> + float: BUILTIN ARGS_B, \
> + __MATH_TG_BUILTIN_CLASSIFY_F32 (BUILTIN, ARGS_B, FUNC, ARGS_F, \
> + DEFINE) \
> + default: BUILTIN ARGS_B, \
> + __MATH_TG_BUILTIN_CLASSIFY_LDOUBLE (BUILTIN, ARGS_B, FUNC, \
> + ARGS_F, DEFINE) \
> + __MATH_TG_BUILTIN_CLASSIFY_F64X (BUILTIN, ARGS_B, FUNC, ARGS_F,\
> + DEFINE) \
> + __MATH_TG_BUILTIN_CLASSIFY_F128 (BUILTIN, ARGS_B, FUNC, ARGS_F,\
> + DEFINE))
> +# else
> +# define __MATH_TG_BUILTIN_CLASSIFY(TG_ARG, BUILTIN, ARGS_B, FUNC, ARGS_F, \
> + DEFINE) \
> + __builtin_choose_expr \
> + (__builtin_types_compatible_p (__typeof (TG_ARG), float), \
> + __builtin ## BUILTIN ARGS_B, \
> + __builtin_choose_expr \
> + (__builtin_types_compatible_p (__typeof (TG_ARG), double), \
> + __builtin ## BUILTIN ARGS_B, \
> + __builtin_choose_expr \
> + (__builtin_types_compatible_p (__typeof (TG_ARG), long double), \
> + DEFINE ? BUILTIN ARGS_B : __ ## FUNC ## l ARGS_F, \
> + BUILTIN ARGS_B)))
> +# endif
> +#else
> +# define __MATH_TG_BUILTIN_CLASSIFY(TG_ARG, BUILTIN, ARGS_B, FUNC, ARGS_F, \
> + DEFINE) \
> + (sizeof (TG_ARG) == sizeof (float) \
> + ? BUILTIN ARGS_B \
> + : sizeof (TG_ARG) == sizeof (double) \
> + ? BUILTIN ARGS_B \
> + : DEFINE ? BUILTIN ARGS_B : __ ## FUNC ## l ARGS_F)
> +#endif
> +
> /* ISO C99 defines some generic macros which work on any data type. */
> #ifdef __USE_ISOC99
>
> @@ -1101,8 +1181,13 @@ enum
> 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) __builtin_fpclassify (FP_NAN, FP_INFINITE, \
> - FP_NORMAL, FP_SUBNORMAL, FP_ZERO, x)
> +# 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)
> # else
> # define fpclassify(x) __MATH_TG ((x), __fpclassify, (x))
> # endif
> diff --git a/sysdeps/x86/bits/fp-builtin-denormal.h b/sysdeps/x86/bits/fp-builtin-denormal.h
> new file mode 100644
> index 0000000000..bfb8f54237
> --- /dev/null
> +++ b/sysdeps/x86/bits/fp-builtin-denormal.h
> @@ -0,0 +1,25 @@
> +/* Define __FP_BUILTIN_DENORMAL.
> + Copyright (C) 2025 Free Software Foundation, Inc.
> + This file is part of the GNU C Library.
> +
> + The GNU C Library is free software; you can redistribute it and/or
> + modify it under the terms of the GNU Lesser General Public
> + License as published by the Free Software Foundation; either
> + version 2.1 of the License, or (at your option) any later version.
> +
> + The GNU C Library is distributed in the hope that it will be useful,
> + but WITHOUT ANY WARRANTY; without even the implied warranty of
> + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> + Lesser General Public License for more details.
> +
> + You should have received a copy of the GNU Lesser General Public
> + License along with the GNU C Library; if not, see
> + <https://www.gnu.org/licenses/>. */
> +
> +#ifndef _MATH_H
> +# error "Never use <bits/fp-builtin-denormal.h> directly; include <math.h> instead."
> +#endif
> +
> +/* Neither GCC (bug 123161) nor clang (issue 172533) handles pseudo-normal
> + numbers correctly with fpclassify builtin. */
> +#define __FP_BUILTIN_FPCLASSIFY_DENORMAL 0
> --
> 2.43.0
>
--
H.J.
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