[PATCH] x86_64: Add log1p with FMA

[PATCH] x86_64: Add log1p with FMA

[H.J. Lu]

On Skylake, it changes log1p bench performance by:

        Before       After     Improvement
max     63.349       58.347       8%
min     4.448        5.651        -30%
mean    12.0674      10.336       14%

The minimum code path is

 if (hx < 0x3FDA827A)                          /* x < 0.41422  */
    {
      if (__glibc_unlikely (ax >= 0x3ff00000))           /* x <= -1.0 */
        {
	   ...
        }
      if (__glibc_unlikely (ax < 0x3e200000))           /* |x| < 2**-29 */
        {
          math_force_eval (two54 + x);          /* raise inexact */
          if (ax < 0x3c900000)                  /* |x| < 2**-54 */
            {
	      ...
            }
          else
            return x - x * x * 0.5;

FMA and non-FMA code sequences look similar.  Non-FMA version is slightly
faster.  Since log1p is called by asinh and atanh, it improves asinh
performance by:

        Before       After     Improvement
max     75.645       63.135       16%
min     10.074       10.071       0%
mean    15.9483      14.9089      6%

and improves atanh performance by:

        Before       After     Improvement
max     91.768       75.081       18%
min     15.548       13.883       10%
mean    18.3713      16.8011      8%
---
 sysdeps/ieee754/dbl-64/s_log1p.c           |  5 ++++
 sysdeps/x86_64/fpu/multiarch/Makefile      |  2 ++
 sysdeps/x86_64/fpu/multiarch/s_log1p-fma.c |  4 +++
 sysdeps/x86_64/fpu/multiarch/s_log1p.c     | 29 ++++++++++++++++++++++
 4 files changed, 40 insertions(+)
 create mode 100644 sysdeps/x86_64/fpu/multiarch/s_log1p-fma.c
 create mode 100644 sysdeps/x86_64/fpu/multiarch/s_log1p.c

diff --git a/sysdeps/ieee754/dbl-64/s_log1p.c b/sysdeps/ieee754/dbl-64/s_log1p.c
index e6476a8260..eeb0af859f 100644
--- a/sysdeps/ieee754/dbl-64/s_log1p.c
+++ b/sysdeps/ieee754/dbl-64/s_log1p.c
@@ -99,6 +99,11 @@ static const double
 
 static const double zero = 0.0;
 
+#ifndef SECTION
+# define SECTION
+#endif
+
+SECTION
 double
 __log1p (double x)
 {
diff --git a/sysdeps/x86_64/fpu/multiarch/Makefile b/sysdeps/x86_64/fpu/multiarch/Makefile
index add339a876..ea81753b70 100644
--- a/sysdeps/x86_64/fpu/multiarch/Makefile
+++ b/sysdeps/x86_64/fpu/multiarch/Makefile
@@ -38,6 +38,7 @@ libm-sysdep_routines += \
   e_pow-fma \
   s_atan-fma \
   s_expm1-fma \
+  s_log1p-fma \
   s_sin-fma \
   s_sincos-fma \
   s_tan-fma \
@@ -51,6 +52,7 @@ CFLAGS-e_log2-fma.c = -mfma -mavx2
 CFLAGS-e_pow-fma.c = -mfma -mavx2
 CFLAGS-s_atan-fma.c = -mfma -mavx2
 CFLAGS-s_expm1-fma.c = -mfma -mavx2
+CFLAGS-s_log1p-fma.c = -mfma -mavx2
 CFLAGS-s_sin-fma.c = -mfma -mavx2
 CFLAGS-s_tan-fma.c = -mfma -mavx2
 CFLAGS-s_sincos-fma.c = -mfma -mavx2
diff --git a/sysdeps/x86_64/fpu/multiarch/s_log1p-fma.c b/sysdeps/x86_64/fpu/multiarch/s_log1p-fma.c
new file mode 100644
index 0000000000..8952df8f9e
--- /dev/null
+++ b/sysdeps/x86_64/fpu/multiarch/s_log1p-fma.c
@@ -0,0 +1,4 @@
+#define __log1p __log1p_fma
+#define SECTION __attribute__ ((section (".text.fma")))
+
+#include <sysdeps/ieee754/dbl-64/s_log1p.c>
diff --git a/sysdeps/x86_64/fpu/multiarch/s_log1p.c b/sysdeps/x86_64/fpu/multiarch/s_log1p.c
new file mode 100644
index 0000000000..6ce5198d6d
--- /dev/null
+++ b/sysdeps/x86_64/fpu/multiarch/s_log1p.c
@@ -0,0 +1,29 @@
+/* Multiple versions of log1p.
+   Copyright (C) 2023 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 <libm-alias-double.h>
+
+extern double __redirect_log1p (double);
+
+#define SYMBOL_NAME log1p
+#include "ifunc-fma.h"
+
+libc_ifunc_redirected (__redirect_log1p, __log1p, IFUNC_SELECTOR ());
+
+#define __log1p __log1p_sse2
+#include <sysdeps/ieee754/dbl-64/s_log1p.c>
-- 
2.41.0

Re: [PATCH] x86_64: Add log1p with FMA

[Paul Zimmermann]

       Hi H.J.

> On Skylake, it changes log1p bench performance by:
> 
>         Before       After     Improvement
> max     63.349       58.347       8%
> min     4.448        5.651        -30%
> mean    12.0674      10.336       14%

the largest known error decreases from 0.903 ulp:

log1p 0 -1 -0x1.2c10396268852p-2 [1] [0.903] 0.902709 0.9027087876691803

to 0.899 ulp:

log1p 0 -1 -0x1.2bf183e0344b2p-2 [1] [0.899] 0.898172 0.8981716806917975

LGTM.

Paul

PS: the largest error for asinh is unchanged:
asinh 0 -1 -0x1.02657ff36d5f3p-2 [2] [1.92] 1.91356 1.913551236624182
and for atanh it seems to improve.