malloc: Optimize small memory clearing for calloc

Add calloc-clear-memory.h to clear memory size up to 36 bytes (72 bytes
on 64-bit targets) for calloc.  Use repeated stores with 1 branch, instead
of up to 3 branches.  On x86-64, it is faster than memset since calling
memset needs 1 indirect branch, 1 broadcast, and up to 4 branches.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>

diff --git a/malloc/malloc-internal.h b/malloc/malloc-internal.h
index cba03433fe3fe61f99c7c205af412dc3d265836e..3349e2d1fef6280eec78b37be7eb15f5db0c8d74 100644 (file)
--- a/malloc/malloc-internal.h
+++ b/malloc/malloc-internal.h
@@ -23,6 +23,7 @@
 #include <malloc-sysdep.h>
 #include <malloc-size.h>
 #include <malloc-hugepages.h>
+#include <calloc-clear-memory.h>
 
 /* Called in the parent process before a fork.  */
 void __malloc_fork_lock_parent (void) attribute_hidden;

diff --git a/malloc/malloc.c b/malloc/malloc.c
index 287fa0904dbab6bbaf6f3963b5f0d5ab67fd46ac..ac3901bdd5e8e9a6977a791a49ce2c78bcc0e45d 100644 (file)
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -3755,8 +3755,6 @@ __libc_calloc (size_t n, size_t elem_size)
   INTERNAL_SIZE_T sz, oldtopsize;
   void *mem;
   unsigned long clearsize;
-  unsigned long nclears;
-  INTERNAL_SIZE_T *d;
   ptrdiff_t bytes;
 
   if (__glibc_unlikely (__builtin_mul_overflow (n, elem_size, &bytes)))
@@ -3853,40 +3851,8 @@ __libc_calloc (size_t n, size_t elem_size)
     }
 #endif
 
-  /* Unroll clear of <= 36 bytes (72 if 8byte sizes).  We know that
-     contents have an odd number of INTERNAL_SIZE_T-sized words;
-     minimally 3.  */
-  d = (INTERNAL_SIZE_T *) mem;
   clearsize = csz - SIZE_SZ;
-  nclears = clearsize / sizeof (INTERNAL_SIZE_T);
-  assert (nclears >= 3);
-
-  if (nclears > 9)
-    return memset (d, 0, clearsize);
-
-  else
-    {
-      *(d + 0) = 0;
-      *(d + 1) = 0;
-      *(d + 2) = 0;
-      if (nclears > 4)
-        {
-          *(d + 3) = 0;
-          *(d + 4) = 0;
-          if (nclears > 6)
-            {
-              *(d + 5) = 0;
-              *(d + 6) = 0;
-              if (nclears > 8)
-                {
-                  *(d + 7) = 0;
-                  *(d + 8) = 0;
-                }
-            }
-        }
-    }
-
-  return mem;
+  return clear_memory ((INTERNAL_SIZE_T *) mem, clearsize);
 }
 #endif /* IS_IN (libc) */
 

diff --git a/sysdeps/generic/calloc-clear-memory.h b/sysdeps/generic/calloc-clear-memory.h
new file mode 100644 (file)
index 0000000..1f9d70d
--- /dev/null
+++ b/sysdeps/generic/calloc-clear-memory.h
@@ -0,0 +1,49 @@
+/* Clear a block of memory for calloc.  Generic version.
+   Copyright (C) 2024 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/>.  */
+
+static __always_inline void *
+clear_memory (INTERNAL_SIZE_T *d, unsigned long clearsize)
+{
+  /* Unroll clear memory size up to 9 * INTERNAL_SIZE_T bytes.  We know
+     that contents have an odd number of INTERNAL_SIZE_T-sized words;
+     minimally 3 words.  */
+  unsigned long nclears = clearsize / sizeof (INTERNAL_SIZE_T);
+
+  if (nclears > 9)
+    return memset (d, 0, clearsize);
+
+  /* NB: The VRP pass in GCC 14.2 will optimize it out.  */
+  if (nclears < 3)
+    __builtin_unreachable ();
+
+  /* Use repeated stores with 1 branch, instead of up to 3.  */
+  *(d + 0) = 0;
+  *(d + 1) = 0;
+  *(d + 2) = 0;
+  *(d + nclears - 2) = 0;
+  *(d + nclears - 2 + 1) = 0;
+  if (nclears > 5)
+    {
+      *(d + 3) = 0;
+      *(d + 3 + 1) = 0;
+      *(d + nclears - 4) = 0;
+      *(d + nclears - 4 + 1) = 0;
+    }
+
+  return d;
+}