Replace the linear search in find_pc_sect_line with a binary search.

This patch addresses slowness when setting breakpoints, especially in
heavily templatized code. Profiling showed that find_pc_sect_line in
symtab.c was the performance bottleneck.  The original logic performed a
linear search over ordered data. This patch uses a binary search, as
suggested by comments around the function.  There are no behavioural
changes, but gdb is now faster at setting breakpoints in template code.
Tested using on make check on an x86 target. The optimisation speeds up
the included template-breakpoints.py performance test by a factor of 7
on my machine.

ChangeLog:

2018-03-20  Stephen Roberts  <stephen.roberts@arm.com>

        * gdb/symtab.c (find_pc_sect_line): now uses binary search.

gdb/testsuite/

        * gdb.perf/template-breakpoints.cc: New file.
        * gdb.perf/template-breakpoints.exp: New file.
        * gdb.perf/template-breakpoints.py: New file.

6 files changed, 216 insertions, 11 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index ac52a9a666e..712e2d932a2 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2018-03-20  Stephen Roberts  <stephen.roberts@arm.com>
+
+	* gdb/symtab.c (find_pc_sect_line): now uses binary search.
+
 2018-03-19  Tom Tromey  <tom@tromey.com>
 
 	* rust-exp.y (struct_expr_tail, struct_expr_list): Add plain
diff --git a/gdb/symtab.c b/gdb/symtab.c
index 894a2537d67..41aaef7582d 100644
--- a/gdb/symtab.c
+++ b/gdb/symtab.c
@@ -3046,8 +3046,6 @@ find_symbol_at_address (CORE_ADDR address)
    find the one whose first PC is closer than that of the next line in this
    symtab.  */
 
-/* If it's worth the effort, we could be using a binary search.  */
-
 struct symtab_and_line
 find_pc_sect_line (CORE_ADDR pc, struct obj_section *section, int notcurrent)
 {
@@ -3214,15 +3212,17 @@ find_pc_sect_line (CORE_ADDR pc, struct obj_section *section, int notcurrent)
       if (item->pc > pc && (!alt || item->pc < alt->pc))
 	alt = item;
 
-      for (i = 0; i < len; i++, item++)
-	{
-	  /* Leave prev pointing to the linetable entry for the last line
-	     that started at or before PC.  */
-	  if (item->pc > pc)
-	    break;
+    auto pc_compare = [](const CORE_ADDR & pc,
+			const struct linetable_entry & lhs)->bool
+    {
+      return pc < lhs.pc;
+    };
 
-	  prev = item;
-	}
+    struct linetable_entry *first = item;
+    struct linetable_entry *last = item + len;
+    item = std::upper_bound (first, last, pc, pc_compare);
+    if (item != first)
+      prev = item - 1;		/* Found a matching item.  */
 
       /* At this point, prev points at the line whose start addr is <= pc, and
          item points at the next line.  If we ran off the end of the linetable
@@ -3247,7 +3247,7 @@ find_pc_sect_line (CORE_ADDR pc, struct obj_section *section, int notcurrent)
       /* If another line (denoted by ITEM) is in the linetable and its
          PC is after BEST's PC, but before the current BEST_END, then
 	 use ITEM's PC as the new best_end.  */
-      if (best && i < len && item->pc > best->pc
+      if (best && item < last && item->pc > best->pc
           && (best_end == 0 || best_end > item->pc))
 	best_end = item->pc;
     }
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index f155ffee716..ca0dadf36b4 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2018-03-20  Stephen Roberts  <stephen.roberts@arm.com>
+
+	* gdb.perf/template-breakpoints.cc: New file.
+	* gdb.perf/template-breakpoints.exp: New file.
+	* gdb.perf/template-breakpoints.py: New file.
+
 2018-03-19  Tom Tromey  <tom@tromey.com>
 
 	* gdb.rust/simple.rs (main): Add local variables field1, field2,
diff --git a/gdb/testsuite/gdb.perf/template-breakpoints.cc b/gdb/testsuite/gdb.perf/template-breakpoints.cc
new file mode 100644
index 00000000000..726270a3a8c
--- /dev/null
+++ b/gdb/testsuite/gdb.perf/template-breakpoints.cc
@@ -0,0 +1,97 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright (C) 2018 Free Software Foundation, Inc.
+
+   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 <iostream>
+
+template <int I, int J, int K, int VAL>
+struct ThirdDimension
+{
+  int
+  value () const
+  {
+    ThirdDimension<I, J, K - 1, VAL> d3;
+    return d3.value();
+  }
+};
+
+template <int I, int J, int VAL>
+struct ThirdDimension<I, J, 0, VAL>
+{
+  int
+  value () const
+  {
+    // Please note - this testcase sets a breakpoint on the following line.
+    // It is therefore sensitive to line numbers. If any changes are made to
+    // this file, please ensure that the testcase is updated to reflect this.
+    std::cout << "Value: " << VAL << std::endl;
+    return VAL;
+  }
+};
+
+template <int I, int J, int K, int VAL>
+struct SecondDimension
+{
+  int
+  value () const
+  {
+    SecondDimension<I, J - 1, K, VAL> d1;
+    ThirdDimension<I, J, K, VAL> d2;
+    return d1.value() + d2.value();
+  }
+};
+
+template <int I, int K, int VAL>
+struct SecondDimension<I, 0, K, VAL>
+{
+  int
+  value () const
+  {
+    ThirdDimension<I, 0, K, VAL> d2;
+    return d2.value();
+  }
+};
+
+template <int I, int J, int K, int VAL>
+struct FirstDimension
+{
+  int
+  value () const
+  {
+    FirstDimension<I - 1, J, K, VAL> d1;
+    SecondDimension<I, J, K, VAL> d2;
+    return d1.value() + d2.value();
+  }
+};
+
+template <int J, int K, int VAL>
+struct FirstDimension<0, J, K, VAL>
+{
+  int
+  value () const
+  {
+    SecondDimension<0, J, K, VAL> d2;
+    return d2.value();
+  }
+};
+
+int
+main (int argc, char *argv[])
+{
+  FirstDimension<EXPANSION_DEPTH, EXPANSION_DEPTH, EXPANSION_DEPTH, 1> product;
+  std::cout << product.value() << std::endl;
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.perf/template-breakpoints.exp b/gdb/testsuite/gdb.perf/template-breakpoints.exp
new file mode 100644
index 00000000000..d69553d8adc
--- /dev/null
+++ b/gdb/testsuite/gdb.perf/template-breakpoints.exp
@@ -0,0 +1,65 @@
+# Copyright (C) 2018 Free Software Foundation, Inc.
+
+# 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/>.
+
+# This test case is to test the performance of GDB when setting breakpoints
+# on heavily temlatized C++ code.
+
+# Parameters:
+# EXPANSION_DEPTH: knob to control how many times template expansions occur
+
+load_lib perftest.exp
+
+if [skip_perf_tests] {
+	return 0
+}
+
+standard_testfile .cc
+set executable $testfile
+set expfile $testfile.exp
+
+# make check-perf RUNTESTFLAGS='template-breakpoints.exp EXPANSION_DEPTH=40'
+if ![info exists EXPANSION_DEPTH] {
+	set EXPANSION_DEPTH 40
+}
+
+PerfTest::assemble {
+	global EXPANSION_DEPTH
+	global srcdir subdir srcfile
+
+	set compile_flags {c++ debug}
+	lappend compile_flags "additional_flags=-DEXPANSION_DEPTH=${EXPANSION_DEPTH}"
+
+	if { [gdb_compile "$srcdir/$subdir/$srcfile" ${binfile} executable $compile_flags] != ""} {
+		return -1
+	}
+
+	return 0
+} {
+	global binfile
+
+	clean_restart $binfile
+
+	if ![runto_main] {
+		fail "can't run to main"
+		return -1
+	}
+
+	return 0
+} {
+
+	gdb_test "python TemplateBreakpoints().run()"
+
+	return 0
+}
diff --git a/gdb/testsuite/gdb.perf/template-breakpoints.py b/gdb/testsuite/gdb.perf/template-breakpoints.py
new file mode 100644
index 00000000000..d85941bf339
--- /dev/null
+++ b/gdb/testsuite/gdb.perf/template-breakpoints.py
@@ -0,0 +1,33 @@
+# Copyright (C) 2018 Free Software Foundation, Inc.
+
+# 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/>.
+
+from perftest import perftest
+
+class TemplateBreakpoints (perftest.TestCaseWithBasicMeasurements):
+    def __init__(self):
+        super (TemplateBreakpoints, self).__init__ ("template-breakpoints")
+
+    def warm_up(self):
+        for _ in range(0, 2):
+            gdb.Breakpoint("template-breakpoints.cc:38").delete()
+
+    def _do_test(self, bpcount):
+        for _ in range(1, bpcount):
+            gdb.Breakpoint("template-breakpoints.cc:38").delete()
+
+    def execute_test(self):
+        for bpcount in range(1, 10):
+            tfunc = lambda bound_bpcount=bpcount: self._do_test(bound_bpcount)
+            self.measure.measure(tfunc, bpcount)