x86: Do not use __builtin_fpclassify on clang

clang does not handle pseudo normal numbers, so disable the fpclassify
and isnormal builtin optimization if clang is used.  This only affect
x86, so add a new header, fp-builtin-denormal.h, which defines whether
the architecture required to disable the optimization through a new
glibc define (__FP_BUILTIN_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

4 files changed, 61 insertions, 1 deletions

diff --git a/bits/fp-builtin-denormal.h b/bits/fp-builtin-denormal.h
new file mode 100644
index 0000000000..08df8758f6
--- /dev/null
+++ b/bits/fp-builtin-denormal.h
@@ -0,0 +1,27 @@
+/* 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
+
+/*  __FP_BUILTIN_DENORMAL is define to 1 if compiler supports handling
+    pseudo-normal numbers with builtins.  Pseudo-normal are defined onlyi
+    Intel double extended-precision floating point (long double), so
+    assume 1 to enable the math optimization.  */
+#define __FP_BUILTIN_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 8e904e5d93..2676896c64 100644
--- a/math/math.h
+++ b/math/math.h
@@ -1083,6 +1083,8 @@ enum
       FP_NORMAL
   };
 
+#include <bits/fp-builtin-denormal.h>
+
 /* GCC bug 66462 means we cannot use the math builtins with -fsignaling-nan,
    so disable builtins if this is enabled.  When fixed in a newer GCC,
    the __SUPPORT_SNAN__ check may be skipped for those versions.  */
@@ -1090,6 +1092,7 @@ enum
 /* Return number of classification appropriate for X.  */
 # if ((__GNUC_PREREQ (4,4) && !defined __SUPPORT_SNAN__)		      \
       || __glibc_clang_prereq (2,8))					      \
+      && __FP_BUILTIN_DENORMAL						      \
      && (!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
@@ -1131,7 +1134,8 @@ enum
 
 /* Return nonzero value if X is neither zero, subnormal, Inf, nor NaN.  */
 # if (__GNUC_PREREQ (4,4) && !defined __SUPPORT_SNAN__) \
-     || __glibc_clang_prereq (2,8)
+     || __glibc_clang_prereq (2,8) \
+     && __FP_BUILTIN_DENORMAL
 #  define isnormal(x) __builtin_isnormal (x)
 # else
 #  define isnormal(x) (fpclassify (x) == FP_NORMAL)
diff --git a/sysdeps/x86/bits/fp-builtin-denormal.h b/sysdeps/x86/bits/fp-builtin-denormal.h
new file mode 100644
index 0000000000..a8c1ca0251
--- /dev/null
+++ b/sysdeps/x86/bits/fp-builtin-denormal.h
@@ -0,0 +1,28 @@
+/* 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
+
+/* clang does not handle pseudo-normal with builtins.  */
+#ifdef __clang__
+# define __FP_BUILTIN_DENORMAL 0
+#else
+# define __FP_BUILTIN_DENORMAL 1
+#endif