Demangle minsyms in parallel

This patch introduces a simple parallel for_each and changes the
minimal symbol reader to use it when computing the demangled name for
a minimal symbol.  This yields a speedup when reading minimal symbols.

gdb/ChangeLog
2019-09-30  Tom Tromey  <tom@tromey.com>

	* minsyms.c (minimal_symbol_reader::install): Use
	parallel_for_each.
	* gdbsupport/parallel-for.h: New file.
	* gdbsupport/parallel-for.c: New file.
	* Makefile.in (HFILES_NO_SRCDIR): Add gdbsupport/parallel-for.h.
	(COMMON_SFILES): Add gdbsupport/parallel-for.c.

5 files changed, 160 insertions, 10 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 402d5432425..e2b91c613ba 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,14 @@
 2019-09-30  Tom Tromey  <tom@tromey.com>
 
+	* minsyms.c (minimal_symbol_reader::install): Use
+	parallel_for_each.
+	* gdbsupport/parallel-for.h: New file.
+	* gdbsupport/parallel-for.c: New file.
+	* Makefile.in (HFILES_NO_SRCDIR): Add gdbsupport/parallel-for.h.
+	(COMMON_SFILES): Add gdbsupport/parallel-for.c.
+
+2019-09-30  Tom Tromey  <tom@tromey.com>
+
 	* event-top.h (thread_local_segv_handler): Declare.
 	* event-top.c (thread_local_segv_handler): New global.
 	(install_handle_sigsegv, handle_sigsegv): New functions.
diff --git a/gdb/Makefile.in b/gdb/Makefile.in
index d4eca61d06e..c7b9efdbbc6 100644
--- a/gdb/Makefile.in
+++ b/gdb/Makefile.in
@@ -970,6 +970,7 @@ COMMON_SFILES = \
 	gdbsupport/gdb_vecs.c \
 	gdbsupport/netstuff.c \
 	gdbsupport/new-op.c \
+	gdbsupport/parallel-for.c \
 	gdbsupport/pathstuff.c \
 	gdbsupport/print-utils.c \
 	gdbsupport/ptid.c \
@@ -1469,6 +1470,7 @@ HFILES_NO_SRCDIR = \
 	gdbsupport/common-inferior.h \
 	gdbsupport/netstuff.h \
 	gdbsupport/host-defs.h \
+	gdbsupport/parallel-for.h \
 	gdbsupport/pathstuff.h \
 	gdbsupport/print-utils.h \
 	gdbsupport/ptid.h \
diff --git a/gdb/gdbsupport/parallel-for.c b/gdb/gdbsupport/parallel-for.c
new file mode 100644
index 00000000000..0024278bd7f
--- /dev/null
+++ b/gdb/gdbsupport/parallel-for.c
@@ -0,0 +1,27 @@
+/* Parallel for loops
+
+   Copyright (C) 2019 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   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/>.  */
+
+#include "gdbsupport/common-defs.h"
+#include "gdbsupport/parallel-for.h"
+
+namespace gdb
+{
+/* See parallel-for.h.  */
+int max_threads = -1;
+}
diff --git a/gdb/gdbsupport/parallel-for.h b/gdb/gdbsupport/parallel-for.h
new file mode 100644
index 00000000000..60b9472e1fe
--- /dev/null
+++ b/gdb/gdbsupport/parallel-for.h
@@ -0,0 +1,101 @@
+/* Parallel for loops
+
+   Copyright (C) 2019 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   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/>.  */
+
+#ifndef GDBSUPPORT_PARALLEL_FOR_H
+#define GDBSUPPORT_PARALLEL_FOR_H
+
+#include <algorithm>
+#if CXX_STD_THREAD
+#include <system_error>
+#include <thread>
+#endif
+
+#include "gdbsupport/block-signals.h"
+
+namespace gdb
+{
+
+/* True if threading should be enabled.  */
+
+extern int max_threads;
+
+/* A very simple "parallel for".  This splits the range of iterators
+   into subranges, and then passes each subrange to the callback.  The
+   work may or may not be done in separate threads.
+
+   This approach was chosen over having the callback work on single
+   items because it makes it simple for the caller to do
+   once-per-subrange initialization and destruction.  */
+
+template<class RandomIt, class RangeFunction>
+void
+parallel_for_each (RandomIt first, RandomIt last, RangeFunction callback)
+{
+#if CXX_STD_THREAD
+  int n_threads = std::thread::hardware_concurrency ();
+  /* So we can use a local array below.  */
+  const int local_max = 16;
+  /* Be sure to handle the "unlimited" case.  */
+  if (max_threads >= 0 && n_threads > max_threads)
+    n_threads = max_threads;
+  if (n_threads > local_max)
+    n_threads = local_max;
+  int n_actual_threads = 0;
+
+  std::thread threads[local_max];
+  size_t n_elements = last - first;
+  if (n_threads > 1 && 2 * n_threads <= n_elements)
+    {
+      /* Ensure that signals used by gdb are blocked in the new
+	 threads.  */
+      block_signals blocker;
+
+      size_t elts_per_thread = n_elements / n_threads;
+      n_actual_threads = n_threads - 1;
+      for (int i = 0; i < n_actual_threads; ++i)
+	{
+	  RandomIt end = first + elts_per_thread;
+	  try
+	    {
+	      threads[i] = std::thread (callback, first, end);
+	    }
+	  catch (const std::system_error &failure)
+	    {
+	      /* If a thread failed to start, ignore it and fall back
+		 to processing in the main thread. */
+	      n_actual_threads = i;
+	      break;
+	    }
+	  first = end;
+	}
+    }
+#endif /* CXX_STD_THREAD */
+
+  /* Process all the remaining elements in the main thread.  */
+  callback (first, last);
+
+#if CXX_STD_THREAD
+  for (int i = 0; i < n_actual_threads; ++i)
+    threads[i].join ();
+#endif /* CXX_STD_THREAD */
+}
+
+}
+
+#endif /* GDBSUPPORT_PARALLEL_FOR_H */
diff --git a/gdb/minsyms.c b/gdb/minsyms.c
index 2b259d39c11..84bf2bb61e2 100644
--- a/gdb/minsyms.c
+++ b/gdb/minsyms.c
@@ -53,6 +53,8 @@
 #include "gdbsupport/symbol.h"
 #include <algorithm>
 #include "safe-ctype.h"
+#include "gdbsupport/alt-stack.h"
+#include "gdbsupport/parallel-for.h"
 
 /* See minsyms.h.  */
 
@@ -1332,16 +1334,25 @@ minimal_symbol_reader::install ()
       m_objfile->per_bfd->msymbols = std::move (msym_holder);
 
       msymbols = m_objfile->per_bfd->msymbols.get ();
-      for (int i = 0; i < mcount; ++i)
-	{
-	  if (!msymbols[i].name_set)
-	    {
-	      symbol_set_names (&msymbols[i], msymbols[i].name,
-				strlen (msymbols[i].name), 0,
-				m_objfile->per_bfd);
-	      msymbols[i].name_set = 1;
-	    }
-	}
+      gdb::parallel_for_each
+	(&msymbols[0], &msymbols[mcount],
+	 [&] (minimal_symbol *start, minimal_symbol *end)
+	 {
+	   /* Ensure that SIGSEGV is delivered to an alternate signal
+	      stack.  */
+	   gdb::alternate_signal_stack signal_stack;
+
+	   for (minimal_symbol *msym = start; msym < end; ++msym)
+	     {
+	       if (!msym->name_set)
+		 {
+		   symbol_set_names (msym, msym->name,
+				     strlen (msym->name), 0,
+				     m_objfile->per_bfd);
+		   msym->name_set = 1;
+		 }
+	     }
+	 });
 
       build_minimal_symbol_hash_tables (m_objfile);
     }