Re: [PATCH 1/4] Remove architecture specific sched_cpucount optimizations

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 05/05/2021 15:25, Paul Eggert wrote:
> On 5/5/21 10:28 AM, Florian Weimer via Libc-alpha wrote:
>>> diff --git a/posix/sched_cpucount.c b/posix/sched_cpucount.c
>>> index b0ca4ea7bc..529286e777 100644
>>> --- a/posix/sched_cpucount.c
>>> +++ b/posix/sched_cpucount.c
>>> @@ -22,31 +22,11 @@ int
>>>   __sched_cpucount (size_t setsize, const cpu_set_t *setp)
>>>   {
>>>     int s = 0;
>>> +  for (int i = 0; i < setsize / sizeof (__cpu_mask); i++)
>>>       {
>>> +      __cpu_mask si = setp->__bits[i];
>>> +      /* Clear the least significant bit set.  */
>>> +      for (; si != 0; si &= si - 1, s++);
>>>       }
>>> -
>>>     return s;
>>>   }
>> Why “si”?  It think si &= si - 1 clears the*most*  significant bit in
>> si.  If you agree, please update the comment.
> 
> Better yet, define a static function 'popcount' that uses Kernighan's trick and call that function. As things stand it's not obvious what the code is doing, regardless of which bit it's clearing. The function's comment should explain why it's not using __builtin_popcount.

What about the below:

diff --git a/posix/sched_cpucount.c b/posix/sched_cpucount.c
index b0ca4ea7bc..6cb5c4337e 100644
--- a/posix/sched_cpucount.c
+++ b/posix/sched_cpucount.c
@@ -17,36 +17,21 @@
 
 #include <sched.h>
 
+/* Counting bits set, Brian Kernighan's way  */
+static inline unsigned int
+countbits (__cpu_mask v)
+{
+  unsigned int s = 0;
+  for (; v != 0; s++)
+    v &= v - 1;
+  return s;
+}
 
 int
 __sched_cpucount (size_t setsize, const cpu_set_t *setp)
 {
   int s = 0;
-  const __cpu_mask *p = setp->__bits;
-  const __cpu_mask *end = &setp->__bits[setsize / sizeof (__cpu_mask)];
-
-  while (p < end)
-    {
-      __cpu_mask l = *p++;
-
-#ifdef POPCNT
-      s += POPCNT (l);
-#else
-      if (l == 0)
-	continue;
-
-      _Static_assert (sizeof (l) == sizeof (unsigned int)
-		      || sizeof (l) == sizeof (unsigned long)
-		      || sizeof (l) == sizeof (unsigned long long),
-		      "sizeof (__cpu_mask");
-      if (sizeof (__cpu_mask) == sizeof (unsigned int))
-	s += __builtin_popcount (l);
-      else if (sizeof (__cpu_mask) == sizeof (unsigned long))
-	s += __builtin_popcountl (l);
-      else
-	s += __builtin_popcountll (l);
-#endif
-    }
-
+  for (int i = 0; i < setsize / sizeof (__cpu_mask); i++)
+    s += countbits (setp->__bits[i]);
   return s;
 }
diff --git a/sysdeps/i386/i686/multiarch/sched_cpucount.c b/sysdeps/i386/i686/multiarch/sched_cpucount.c
deleted file mode 100644
index 7db31b02f8..0000000000
--- a/sysdeps/i386/i686/multiarch/sched_cpucount.c
+++ /dev/null
@@ -1 +0,0 @@
-#include <sysdeps/x86_64/multiarch/sched_cpucount.c>
diff --git a/sysdeps/ia64/sched_cpucount.c b/sysdeps/ia64/sched_cpucount.c
deleted file mode 100644
index 8440864b02..0000000000
--- a/sysdeps/ia64/sched_cpucount.c
+++ /dev/null
@@ -1,20 +0,0 @@
-/* Copyright (C) 2007-2021 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/>.  */
-
-#define POPCNT(l) __builtin_popcountl (l)
-
-#include <posix/sched_cpucount.c>
diff --git a/sysdeps/powerpc/sched_cpucount.c b/sysdeps/powerpc/sched_cpucount.c
deleted file mode 100644
index 8f00e3dbc8..0000000000
--- a/sysdeps/powerpc/sched_cpucount.c
+++ /dev/null
@@ -1,22 +0,0 @@
-/* Copyright (C) 2007-2021 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/>.  */
-
-#ifdef _ARCH_PWR5
-# define POPCNT(l) __builtin_popcountl (l)
-#endif
-
-#include <posix/sched_cpucount.c>
diff --git a/sysdeps/x86_64/multiarch/sched_cpucount.c b/sysdeps/x86_64/multiarch/sched_cpucount.c
deleted file mode 100644
index 5180a11434..0000000000
--- a/sysdeps/x86_64/multiarch/sched_cpucount.c
+++ /dev/null
@@ -1,36 +0,0 @@
-/* Count bits in CPU set.  x86-64 multi-arch version.
-   This file is part of the GNU C Library.
-   Copyright (C) 2008-2021 Free Software Foundation, Inc.
-   Contributed by Ulrich Drepper <drepper@redhat.com>.
-
-   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 <sched.h>
-#include "init-arch.h"
-
-#define __sched_cpucount static generic_cpucount
-#include <posix/sched_cpucount.c>
-#undef __sched_cpucount
-
-#define POPCNT(l) \
-  ({ __cpu_mask r; \
-     asm ("popcnt %1, %0" : "=r" (r) : "0" (l));\
-     r; })
-#define __sched_cpucount static popcount_cpucount
-#include <posix/sched_cpucount.c>
-#undef __sched_cpucount
-
-libc_ifunc (__sched_cpucount,
-	    CPU_FEATURE_USABLE (POPCNT) ? popcount_cpucount : generic_cpucount);
diff --git a/sysdeps/x86_64/sched_cpucount.c b/sysdeps/x86_64/sched_cpucount.c
deleted file mode 100644
index 5a27336d6d..0000000000
--- a/sysdeps/x86_64/sched_cpucount.c
+++ /dev/null
@@ -1,25 +0,0 @@
-/* Copyright (C) 2007-2021 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/>.  */
-
-#ifdef __amdfam10
-# define POPCNT(l) \
-  ({ __cpu_mask r;							      \
-     asm ("popcntq %1, %0" : "=r" (r) : "0" (l));			      \
-     r; })
-#endif
-
-#include <posix/sched_cpucount.c>