[RFC 6/6] Demangle minsyms in parallel

[Tom Tromey <tom@tromey.com>]

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.

2019-03-03  Tom Tromey  <tom@tromey.com>

	* minsyms.c (minimal_symbol_reader::install): Use
	parallel_for_each.
	* common/parallel-for.h: New file.
	* Makefile.in (HFILES_NO_SRCDIR): Add common/parallel-for.h.
---
 gdb/ChangeLog             |  7 ++++
 gdb/Makefile.in           |  1 +
 gdb/common/parallel-for.h | 69 +++++++++++++++++++++++++++++++++++++++
 gdb/minsyms.c             | 23 +++++++------
 4 files changed, 90 insertions(+), 10 deletions(-)
 create mode 100644 gdb/common/parallel-for.h

diff --git a/gdb/Makefile.in b/gdb/Makefile.in
index 5614cc3386c..fec9c4a505c 100644
--- a/gdb/Makefile.in
+++ b/gdb/Makefile.in
@@ -1466,6 +1466,7 @@ HFILES_NO_SRCDIR = \
 	common/common-inferior.h \
 	common/netstuff.h \
 	common/host-defs.h \
+	common/parallel-for.h \
 	common/pathstuff.h \
 	common/print-utils.h \
 	common/ptid.h \
diff --git a/gdb/common/parallel-for.h b/gdb/common/parallel-for.h
new file mode 100644
index 00000000000..a0ae7bebd16
--- /dev/null
+++ b/gdb/common/parallel-for.h
@@ -0,0 +1,69 @@
+/* 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 COMMON_PARALLEL_FOR_H
+#define COMMON_PARALLEL_FOR_H
+
+#include <algorithm>
+#include <thread>
+
+namespace gdb
+{
+
+/* A very simple "parallel for".  This iterates over the elements
+   given by the range of iterators, which must be random access
+   iterators.  For each element, it calls the callback function.  The
+   work may or may not be done by separate threads.  */
+
+template<class RandomIt, class UnaryFunction>
+void parallel_for_each (RandomIt first, RandomIt last, UnaryFunction f)
+{
+  unsigned n_threads = std::thread::hardware_concurrency ();
+  /* So we can use a local array below.  */
+  const unsigned max_threads = 16;
+  if (n_threads > max_threads)
+    n_threads = max_threads;
+
+  if (n_threads == 0 || last - first < 2 * n_threads)
+    {
+      /* Don't bother.  */
+      std::for_each (first, last, f);
+      return;
+    }
+
+  auto body = [&] (RandomIt start)
+    {
+      for (; start < last; start += n_threads)
+	f (*start);
+    };
+
+  std::thread threads[max_threads];
+  for (unsigned i = 0; i < n_threads; ++i)
+    {
+      threads[i] = std::thread (body, first);
+      ++first;
+    }
+
+  for (unsigned i = 0; i < n_threads; ++i)
+    threads[i].join ();
+}
+
+}
+
+#endif /* COMMON_PARALLEL_FOR_H */
diff --git a/gdb/minsyms.c b/gdb/minsyms.c
index 7872b7e2588..75c1bb9daf4 100644
--- a/gdb/minsyms.c
+++ b/gdb/minsyms.c
@@ -53,6 +53,7 @@
 #include "common/symbol.h"
 #include <algorithm>
 #include "safe-ctype.h"
+#include "common/parallel-for.h"
 
 /* See minsyms.h.  */
 
@@ -1409,16 +1410,18 @@ 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 &msym)
+	 {
+	   if (!msym.name_set)
+	     {
+	       symbol_set_names (&msym, msym.name,
+				 strlen (msym.name), 0,
+				 m_objfile->per_bfd);
+	       msym.name_set = 1;
+	     }
+	 });
 
       /* Now build the hash tables; we can't do this incrementally
          at an earlier point since we weren't finished with the obstack
-- 
2.17.2