[PATCH v4 09/11] Demangle minsyms in parallel

Tom Tromey
2019-10-01T20:12:43Z

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
2019-10-01 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.
---
gdb/ChangeLog | 9 +++
gdb/Makefile.in | 2 +
gdb/gdbsupport/parallel-for.c | 27 +++++++++
gdb/gdbsupport/parallel-for.h | 101 ++++++++++++++++++++++++++++++++++
gdb/minsyms.c | 31 +++++++----
5 files changed, 160 insertions(+), 10 deletions(-)
create mode 100644 gdb/gdbsupport/parallel-for.c
create mode 100644 gdb/gdbsupport/parallel-for.h

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);
}
--
2.17.2