[PATCH v2 2/4] math: Optimize double ilogb/llogb
Adhemerval Zanella
adhemerval.zanella@linaro.org
Mon Apr 28 17:03:42 GMT 2025
It removes the wrapper by moving the error/EDOM handling to an
out-of-line implementation (__math_invalid_i/__math_invalid_li).
Also, __glibc_unlikely is used on errors case since it helps
code generation on recent gcc.
The code now builds to with gcc-14 on aarch64:
0000000000000000 <__ilogb>:
0: 9e660000 fmov x0, d0
4: d374f801 ubfx x1, x0, #52, #11
8: 340000e1 cbz w1, 24 <__ilogb+0x24>
c: 510ffc20 sub w0, w1, #0x3ff
10: 711ffc3f cmp w1, #0x7ff
14: 54000040 b.eq 1c <__ilogb+0x1c> // b.none
18: d65f03c0 ret
1c: 12b00000 mov w0, #0x7fffffff // #2147483647
20: 14000000 b 0 <__math_invalid_i>
24: d374cc00 lsl x0, x0, #12
28: b40000a0 cbz x0, 3c <__ilogb+0x3c>
2c: dac01000 clz x0, x0
30: 12807fc1 mov w1, #0xfffffc01 // #-1023
34: 4b000020 sub w0, w1, w0
38: d65f03c0 ret
3c: 320107e0 mov w0, #0x80000001 // #-2147483647
40: 14000000 b 0 <__math_invalid_i>
Checked on aarch64-linux-gnu and x86_64-linux-gnu.
---
sysdeps/i386/fpu/math_err.c | 1 -
sysdeps/ieee754/dbl-64/e_ilogb.c | 42 +------------------
sysdeps/ieee754/dbl-64/math_config.h | 2 +
sysdeps/ieee754/dbl-64/math_err.c | 32 ++++++++++++++
sysdeps/ieee754/dbl-64/w_ilogb.c | 63 ++++++++++++++++++++++++++++
sysdeps/ieee754/dbl-64/w_llogb.c | 2 +
6 files changed, 100 insertions(+), 42 deletions(-)
delete mode 100644 sysdeps/i386/fpu/math_err.c
create mode 100644 sysdeps/ieee754/dbl-64/w_ilogb.c
create mode 100644 sysdeps/ieee754/dbl-64/w_llogb.c
diff --git a/sysdeps/i386/fpu/math_err.c b/sysdeps/i386/fpu/math_err.c
deleted file mode 100644
index 1cc8931700..0000000000
--- a/sysdeps/i386/fpu/math_err.c
+++ /dev/null
@@ -1 +0,0 @@
-/* Not needed. */
diff --git a/sysdeps/ieee754/dbl-64/e_ilogb.c b/sysdeps/ieee754/dbl-64/e_ilogb.c
index 89e7498266..1ea2f23e2f 100644
--- a/sysdeps/ieee754/dbl-64/e_ilogb.c
+++ b/sysdeps/ieee754/dbl-64/e_ilogb.c
@@ -1,41 +1 @@
-/* Get integer exponent of a floating-point value.
- Copyright (C) 1999-2025 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/>. */
-
-#include <limits.h>
-#include <math.h>
-#include <stdbit.h>
-#include "math_config.h"
-
-int
-__ieee754_ilogb (double x)
-{
- uint64_t ux = asuint64 (x);
- int ex = (ux & ~SIGN_MASK) >> MANTISSA_WIDTH;
- if (ex == 0) /* zero or subnormal */
- {
- /* Clear sign and exponent */
- ux <<= 12;
- if (ux == 0)
- return FP_ILOGB0;
- /* subnormal */
- return -1023 - stdc_leading_zeros (ux);
- }
- if (ex == EXPONENT_MASK >> MANTISSA_WIDTH) /* NaN or Inf */
- return ux << 12 ? FP_ILOGBNAN : INT_MAX;
- return ex - 1023;
-}
+/* ilogb is implemented at w_ilogb.c */
diff --git a/sysdeps/ieee754/dbl-64/math_config.h b/sysdeps/ieee754/dbl-64/math_config.h
index 3382e385f9..5766f6a1db 100644
--- a/sysdeps/ieee754/dbl-64/math_config.h
+++ b/sysdeps/ieee754/dbl-64/math_config.h
@@ -164,6 +164,8 @@ attribute_hidden double __math_divzero (uint32_t);
/* Invalid input unless it is a quiet NaN. */
attribute_hidden double __math_invalid (double);
+attribute_hidden int __math_invalid_i (int);
+attribute_hidden long int __math_invalid_li (long int);
/* Error handling using output checking, only for errno setting. */
diff --git a/sysdeps/ieee754/dbl-64/math_err.c b/sysdeps/ieee754/dbl-64/math_err.c
index 4a07fd5714..b8c645a3df 100644
--- a/sysdeps/ieee754/dbl-64/math_err.c
+++ b/sysdeps/ieee754/dbl-64/math_err.c
@@ -29,8 +29,24 @@ with_errno (double y, int e)
errno = e;
return y;
}
+
+NOINLINE static int
+with_errno_i (int y, int e)
+{
+ errno = e;
+ return y;
+}
+
+NOINLINE static long int
+with_errno_li (long int y, int e)
+{
+ errno = e;
+ return y;
+}
#else
#define with_errno(x, e) (x)
+#define with_errno_i(x, e) (x)
+#define with_errno_li(x, e) (x)
#endif
attribute_hidden double
@@ -83,6 +99,22 @@ __math_invalid (double x)
return isnan (x) ? y : with_errno (y, EDOM);
}
+attribute_hidden int
+__math_invalid_i (int r)
+{
+ double y = 0.0 / 0.0;
+ math_force_eval (y);
+ return with_errno_i (r, EDOM);
+}
+
+attribute_hidden long int
+__math_invalid_li (long int r)
+{
+ double y = 0.0 / 0.0;
+ math_force_eval (y);
+ return with_errno_li (r, EDOM);
+}
+
/* Check result and set errno if necessary. */
attribute_hidden double
diff --git a/sysdeps/ieee754/dbl-64/w_ilogb.c b/sysdeps/ieee754/dbl-64/w_ilogb.c
new file mode 100644
index 0000000000..8f9641be45
--- /dev/null
+++ b/sysdeps/ieee754/dbl-64/w_ilogb.c
@@ -0,0 +1,63 @@
+/* Get integer exponent of a floating-point value.
+ Copyright (C) 1999-2025 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/>. */
+
+#include <fenv.h>
+#include <errno.h>
+#include <limits.h>
+#include <math.h>
+#include <stdbit.h>
+#include <libm-alias-double.h>
+#include "math_config.h"
+
+#ifdef DEF_AS_LLOGB
+# define IMPL_NAME __llogb
+# define FUNC_NAME llogb
+# define RET_TYPE long int
+# define RET_LOGB0 FP_LLOGB0
+# define RET_LOGBNAN FP_LLOGBNAN
+# define RET_LOGMAX LONG_MAX
+# define RET_INVALID __math_invalid_li
+#else
+# define IMPL_NAME __ilogb
+# define FUNC_NAME ilogb
+# define RET_TYPE int
+# define RET_LOGB0 FP_ILOGB0
+# define RET_LOGBNAN FP_ILOGBNAN
+# define RET_LOGMAX INT_MAX
+# define RET_INVALID __math_invalid_i
+#endif
+
+RET_TYPE
+IMPL_NAME (double x)
+{
+ uint64_t ux = asuint64 (x);
+ int ex = (ux & ~SIGN_MASK) >> MANTISSA_WIDTH;
+ if (__glibc_unlikely (ex == 0)) /* zero or subnormal */
+ {
+ /* Clear sign and exponent */
+ ux <<= 12;
+ if (ux == 0)
+ return RET_INVALID (RET_LOGB0);
+ /* subnormal */
+ return (RET_TYPE)-1023 - stdc_leading_zeros (ux);
+ }
+ if (__glibc_unlikely (ex == EXPONENT_MASK >> MANTISSA_WIDTH)) /* NaN or Inf */
+ return RET_INVALID (ux << 12 ? RET_LOGBNAN : RET_LOGMAX);
+ return ex - 1023;
+}
+libm_alias_double (IMPL_NAME, FUNC_NAME)
diff --git a/sysdeps/ieee754/dbl-64/w_llogb.c b/sysdeps/ieee754/dbl-64/w_llogb.c
new file mode 100644
index 0000000000..c984cd151a
--- /dev/null
+++ b/sysdeps/ieee754/dbl-64/w_llogb.c
@@ -0,0 +1,2 @@
+#define DEF_AS_LLOGB
+#include "w_ilogb.c"
--
2.43.0
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