[PATCH] tilegx: add optimized sched_getcpu() using TILE_COORD SPR

[Chris Metcalf <cmetcalf at tilera dot com>]

We can discover our x,y coordinate in the core mesh with an
mfspr instruction, multiply y by the core mesh width, and have
the core number without needing to ask the kernel.

2012-10-11  Chris Metcalf  <cmetcalf@tilera.com>

	* sysdeps/unix/sysv/linux/tile/tilegx/sched_getcpu.c: New file.

diff --git a/ports/sysdeps/unix/sysv/linux/tile/tilegx/sched_getcpu.c b/ports/sysdeps/unix/sysv/linux/tile/tilegx/sched_getcpu.c
new file mode 100644
index 0000000..617968e
--- /dev/null
+++ b/ports/sysdeps/unix/sysv/linux/tile/tilegx/sched_getcpu.c
@@ -0,0 +1,87 @@
+/* Copyright (C) 2012 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <stddef.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <sched.h>
+#include <sysdep.h>
+#include <arch/spr_def.h>
+
+
+/* The count of cores horizontally (X dimension) on the chip.  */
+static int chip_width;
+
+/* Read the chip "width" from the /sys filesystem.  */
+static int
+initialize_chip_width (void)
+{
+  int w = 0;
+  int fd;
+
+  fd = __open ("/sys/devices/system/cpu/chip_width", O_RDONLY);
+  if (fd >= 0)
+    {
+      char buf[64];
+      ssize_t n;
+      int i;
+      
+      n = __read (fd, buf, sizeof (buf));
+      __close (fd);
+      
+      for (i = 0; i < n; ++i)
+        {
+          if (buf[i] < '0' || buf[i] > '9')
+            break;
+          w = (w * 10) + (buf[i] - '0');
+        }
+    }
+
+  /* Store a negative value so we don't try again.  */
+  if (w == 0)
+    w = -1;
+
+  /* Using an atomic idempotent write here makes this thread-safe.  */
+  chip_width = w;
+  return w;
+}
+
+int
+sched_getcpu (void)
+{
+  unsigned int coord;
+  int w = chip_width;
+
+  if (__builtin_expect (w <= 0, 0))
+    {
+      if (w == 0)
+        w = initialize_chip_width ();
+      if (w < 0)
+        {
+          unsigned int cpu;
+          int r = INLINE_SYSCALL (getcpu, 3, &cpu, NULL, NULL);
+          return r == -1 ? r : cpu;
+        }
+    }
+
+  /* Assign 64-bit value to a 32-bit variable to ensure 32-bit multiply.  */
+  coord = __insn_mfspr (SPR_TILE_COORD);
+
+  /* Extract Y coord from bits 7..10 and X coord from bits 18..21.  */
+  return ((coord >> 7) & 0xf) * w + ((coord >> 18) & 0xf);
+}
-- 
1.7.10.3