preload: hoist preloads into src/ directory

They will get used from libsysprof-profile, but it's nice to have them
a directory up going forward.

src/preload/backtrace-helper.h

@@ -0,0 +1,79 @@
+/* backtrace-helper.h
+ *
+ * Copyright 2020 Christian Hergert <chergert@redhat.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#pragma once
+
+#ifdef HAVE_EXECINFO_H
+# include <execinfo.h>
+#endif
+#ifdef ENABLE_LIBUNWIND
+# define UNW_LOCAL_ONLY
+# include <libunwind.h>
+#endif
+
+static void
+backtrace_init (void)
+{
+#ifdef ENABLE_LIBUNWIND
+  unw_set_caching_policy (unw_local_addr_space, UNW_CACHE_PER_THREAD);
+# ifdef HAVE_UNW_SET_CACHE_SIZE
+  unw_set_cache_size (unw_local_addr_space, 1024, 0);
+#endif
+#endif
+}
+
+static int
+backtrace_func (SysprofCaptureAddress  *addrs,
+                guint                   n_addrs,
+                G_GNUC_UNUSED gpointer  user_data)
+{
+#if defined(ENABLE_LIBUNWIND)
+# if GLIB_SIZEOF_VOID_P == 8
+  /* We know that collector will overwrite fields *AFTER* it
+   * has called the backtrace function allowing us to cheat
+   * and subtract an offset from addrs to avoid having to
+   * copy frame pointers around.
+   */
+  return unw_backtrace ((void **)addrs - 2, n_addrs) - 2;
+# else
+  static const int skip = 2;
+  void **stack = alloca (n_addrs * sizeof (gpointer));
+  int n = unw_backtrace (stack, n_addrs);
+  for (guint i = skip; i < n; i++)
+    addrs[i-skip] = GPOINTER_TO_SIZE (stack[i]);
+  return MAX (0, n - skip);
+# endif
+#elif defined(HAVE_EXECINFO_H)
+# if GLIB_SIZEOF_VOID_P == 8
+  /* See note on unw_backtrace() */
+  return backtrace ((void **)addrs - 2, n_addrs) - 2;
+# else /* GLIB_SIZEOF_VOID_P != 8 */
+  static const int skip = 2;
+  void **stack = alloca (n_addrs * sizeof (gpointer));
+  int n = backtrace (stack, n_addrs);
+  for (guint i = skip; i < n; i++)
+    addrs[i-skip] = GPOINTER_TO_SIZE (stack[i]);
+  return MAX (0, n - skip);
+# endif /* GLIB_SIZEOF_VOID_P */
+#else
+  return 0;
+#endif
+}
+