Add DWARF index cache

New in v3:

- Remove things related to the dwarf-5 format.
- Fix compilation on mingw (scoped_mmap.c).

GDB can generate indexes for DWARF debug information, which, when
integrated in the original binary, can speed up loading object files.
This can be done using the gdb-add-index script or directly by the
linker itself.  However, not many people know about this.  And even
among those who do, because it requires additional steps, I don't know a
lot of people who actually go through that trouble.

To help make using the DWARF index more transparent, this patch
introduces a DWARF index cache.  When enabled, loading an index-less
binary in GDB will automatically save an index file in ~/.cache/gdb.
When loading that same object file again, the index file will be looked
up and used to load the DWARF index.  You therefore get the benefit of
the DWARF index without having to do additional manual steps or
modifying your build system.  When an index section is already present
in the file, GDB will prefer that one over looking up the cache.

When doing my edit-compile-debug cycle, I often debug multiple times the
same build, so the cache helps reducing the load time of the debug
sessions after the first one.

- The saved index file is exactly the same as the output of the "save
  gdb-index" command.  It is therefore the exact same content that would
  be found in the .gdb_index or .debug_names section.  We just leave it
  as a standalone file instead of merging it in the binary.

- The cache is just a directory with files named after the object
  file's build-id.  It is not possible to save/load the index for an
  object file without build-id in the cache.

- The cache uses the gdb index format.  The problem with the dwarf-5
  format is that we can generate an addendum to the .debug_str section
  that you're supposed to integrate to the original binary.  This
  complicates a little bit loading the data from the cached index files,
  so I would leave this for later.

- The size taken up by ~/.cache/gdb is not limited.  I was thinking we
  could add configurable limit (like ccache does), but that would come
  after.  Also, maybe a command to flush the cache.

- The cache is disabled by default.  I think once it's been out there
  and tested for a while, it could be turned on by default, so that
  everybody can enjoy it.

- The code was made to follow the XDG specification: if the
  XDG_CACHE_HOME environment variable, it is used, otherwise it falls
  back to ~/.cache/gdb.  It is possible to change it using "set
  index-cache directory".  On other OSes than GNU/Linux, ~/.cache may
  not be the best place to put such data.  On macOS it should probably
  default to ~/Library/Caches/...  On Windows, %LocalAppData%/...  I
  don't intend to do this part, but further patches are welcome.

- I think that we need to be careful that multiple instances of GDB
  don't interfere with each other (not far fetched at all if you run GDB
  in some automated script) and the cache is always coherent (either the
  file is not found, or it is found and entirely valid).  Writing the
  file directly to its final location seems like a recipe for failure.
  One GDB could read a file in the index while it is being written by
  another GDB.  To mitigate this, I made write_psymtabs_to_index write
  to temporary files and rename them once it's done.  Two GDB instances
  writing the index for the same file should not step on each other's
  toes (the last file to be renamed will stay).  A GDB looking up a file
  will only see a complete file or no file.  Also, if GDB crashes while
  generating the index file, it will leave a work-in-progress file, but
  it won't be picked up by other instances looking up in the cache.

gdb/ChangeLog:

	* common/pathstuff.h (get_standard_cache_dir): New.
	* common/pathstuff.c (get_standard_cache_dir): New.
	* build-id.h (build_id_to_string): New.
	* dwarf-index-common.h (INDEX4_SUFFIX, INDEX5_SUFFIX,
	DEBUG_STR_SUFFIX): Move to here.
	* dwarf-index-write.c (INDEX4_SUFFIX, INDEX5_SUFFIX,
	DEBUG_STR_SUFFIX): Move from there.
	(write_psymtabs_to_index): Make non-static, add basename
	parameter.  Write to temporary files, rename when done.
	(save_gdb_index_command): Adjust call to
	write_psymtabs_to_index.
	* dwarf2read.h (dwarf2_per_objfile) <index_cache_res>: New
	field.
	* dwarf2read.c (dwz_file) <index_cache_res>: New field.
	(get_gdb_index_contents_from_cache): New.
	(get_gdb_index_contents_from_cache_dwz): New.
	(dwarf2_initialize_objfile): Read index from cache.
	(dwarf2_build_psymtabs): Save to index.
	* dwarf-index-cache.h: New file.
	* dwarf-index-cache.c: New file.
	* dwarf-index-write.h: New file.

gdb/testsuite/ChangeLog:

	* boards/index-cache-gdb.exp: New file.
	* gdb.dwarf2/index-cache.exp: New file.
	* gdb.dwarf2/index-cache.c: New file.
	* gdb.base/maint.exp: Check if we are using the index cache.

4 files changed, 247 insertions, 0 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index f86e8fb..349aac9 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,10 @@
+2018-08-07  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* boards/index-cache-gdb.exp: New file.
+	* gdb.dwarf2/index-cache.exp: New file.
+	* gdb.dwarf2/index-cache.c: New file.
+	* gdb.base/maint.exp: Check if we are using the index cache.
+
 2018-08-07  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>
 
 	* Makefile.in (saw_dash_j): Allow for GNU make 4.2+ passing -jN in
diff --git a/gdb/testsuite/gdb.base/index-cache.c b/gdb/testsuite/gdb.base/index-cache.c
new file mode 100644
index 0000000..460d70e
--- /dev/null
+++ b/gdb/testsuite/gdb.base/index-cache.c
@@ -0,0 +1,23 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 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/>.  */
+
+int
+main ()
+{
+  return 0;
+}
+
diff --git a/gdb/testsuite/gdb.base/index-cache.exp b/gdb/testsuite/gdb.base/index-cache.exp
new file mode 100644
index 0000000..87f3c91
--- /dev/null
+++ b/gdb/testsuite/gdb.base/index-cache.exp
@@ -0,0 +1,193 @@
+#   Copyright 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 checks that the index-cache feature generates the expected files at
+# the expected location.
+
+standard_testfile
+
+if { [prepare_for_testing "failed to prepare" $testfile $srcfile] } {
+    return
+}
+
+# List the files in DIR on the host (where GDB-under-test runs).
+# Return a list of two elements:
+#   - 0 on success, -1 on failure
+#   - the list of files on success, empty on failure
+
+proc ls_host { dir } {
+    lassign [remote_exec host ls "-1 $dir"] ret output
+
+    if { $ret != 0 } {
+	fail "failed to list files on host in $dir"
+	return -1
+    }
+
+    # ls -1 returns a list separated by \r\n.  split will return a bunch of
+    # empty entries (it treats a sequence of split characters as separate
+    # fields, plus there is a \r\n at the end of the result).  Ignore empty
+    # list elements.
+    set filtered {}
+    set files [split $output \r\n]
+
+    foreach file $files {
+	if { $file != "" } {
+	    lappend filtered $file
+	}
+    }
+
+    return "0 $filtered"
+}
+
+# Execute "show index-cache stats" and verify the output against expected
+# values.
+
+proc check_cache_stats { expected_hits expected_misses } {
+    set re [multi_line \
+	"  Cache hits .this session.: $expected_hits" \
+	"Cache misses .this session.: $expected_misses" \
+    ]
+
+    gdb_test "show index-cache stats" $re "check index-cache stats"
+}
+
+# Run CODE using a fresh GDB configured based on the other parameters.
+
+proc run_test_with_flags { cache_dir cache_enabled code } {
+    global GDBFLAGS testfile
+
+    save_vars { GDBFLAGS } {
+	set GDBFLAGS "$GDBFLAGS -iex \"set index-cache directory $cache_dir\""
+	set GDBFLAGS "$GDBFLAGS -iex \"set index-cache $cache_enabled\""
+
+	clean_restart ${testfile}
+
+	uplevel 1 $code
+    }
+}
+
+# Test administrative stuff.
+
+proc_with_prefix test_basic_stuff { } {
+    global testfile
+
+    clean_restart ${testfile}
+
+    # Check that the index cache is disabled by default.
+    gdb_test \
+	"show index-cache" \
+	" is currently disabled." \
+	"index-cache is disabled by default"
+
+    # Test that we can enable it and "show index-cache" reflects that.
+    gdb_test_no_output "set index-cache on" "enable index cache"
+    gdb_test \
+	"show index-cache" \
+	" is currently enabled." \
+	"index-cache is now enabled"
+
+    # Test the "set/show index-cache directory" commands.
+    gdb_test "set index-cache directory" "Argument required.*" "set index-cache directory without arg"
+    gdb_test_no_output "set index-cache directory /tmp" "change the index cache directory"
+    gdb_test \
+	"show index-cache directory" \
+	"The directory of the index cache is \"/tmp\"."  \
+	"show index cache directory"
+}
+
+# Test loading a binary with the cache disabled.  No file should be created.
+
+proc_with_prefix test_cache_disabled { cache_dir } {
+    lassign [ls_host $cache_dir] ret files_before
+
+    run_test_with_flags $cache_dir off {
+	lassign [ls_host $cache_dir] ret files_after
+
+	set nfiles_created [expr [llength $files_after] - [llength $files_before]]
+	gdb_assert "$nfiles_created == 0" "no files were created"
+
+	check_cache_stats 0 0
+    }
+}
+
+# Test with the cache enabled, we expect to have exactly one file created.
+
+proc_with_prefix test_cache_enabled_miss { cache_dir } {
+    global testfile
+
+    lassign [ls_host $cache_dir] ret files_before
+
+    run_test_with_flags $cache_dir on {
+
+	lassign [ls_host $cache_dir] ret files_after
+	set nfiles_created [expr [llength $files_after] - [llength $files_before]]
+	gdb_assert "$nfiles_created > 0" "at least one file was created"
+
+	set build_id [get_build_id  [standard_output_file ${testfile}]]
+	if { $build_id == "" } {
+	    fail "couldn't get executable build id"
+	    return
+	}
+
+	set expected_created_file [list "${build_id}.gdb-index"]
+	set found_idx [lsearch -exact $files_after $expected_created_file]
+	gdb_assert "$found_idx >= 0" "expected file is there"
+
+	remote_exec host rm "-f $cache_dir/$expected_created_file"
+
+	check_cache_stats 0 1
+    }
+}
+
+
+# Test with the cache enabled, this time we should have one file (the
+# same), but one cache read hit.
+
+proc_with_prefix test_cache_enabled_hit { cache_dir } {
+    # Just to populate the cache.
+    run_test_with_flags $cache_dir on {}
+
+    lassign [ls_host $cache_dir] ret files_before
+
+    run_test_with_flags $cache_dir on {
+	lassign [ls_host $cache_dir] ret files_after
+	set nfiles_created [expr [llength $files_after] - [llength $files_before]]
+	gdb_assert "$nfiles_created == 0" "no files were created"
+
+	check_cache_stats 1 0
+    }
+}
+
+test_basic_stuff
+
+# The cache dir should be on the host (possibly remote), so we can't use the
+# standard output directory for that (it's on the build machine).
+lassign [remote_exec host mktemp -d] ret cache_dir
+
+if { $ret != 0 } {
+    fail "couldn't create temporary cache dir"
+    return
+}
+
+# The ouput of mktemp contains an end of line, remove it.
+set cache_dir [string trimright $cache_dir \r\n]
+
+test_cache_disabled $cache_dir
+test_cache_enabled_miss $cache_dir
+test_cache_enabled_hit $cache_dir
+
+# Test again with the cache disabled, now that it is populated.
+test_cache_disabled $cache_dir
+
diff --git a/gdb/testsuite/gdb.base/maint.exp b/gdb/testsuite/gdb.base/maint.exp
index ee0d089..746081a 100644
--- a/gdb/testsuite/gdb.base/maint.exp
+++ b/gdb/testsuite/gdb.base/maint.exp
@@ -123,6 +123,30 @@ gdb_test_multiple "maint info sections .gdb_index .debug_names" "check for .gdb_
     }
 }
 
+# There also won't be any psymtabs if we read the index from the index cache.
+# We can detect this by looking if the index-cache is enabled and if the number
+# of cache misses is 0.
+set index_cache_misses -1
+gdb_test_multiple "show index-cache stats" "check index cache stats" {
+    -re ".*Cache misses \\(this session\\): (\\d+)\r\n.*$gdb_prompt $" {
+	set index_cache_misses $expect_out(1,string)
+    }
+}
+
+set using_index_cache 0
+gdb_test_multiple "show index-cache" "check index cache status" {
+    -re ".*is currently disabled.\r\n$gdb_prompt $" {
+	set using_index_cache 0
+    }
+    -re ".*is currently enabled.\r\n$gdb_prompt $" {
+	set using_index_cache 1
+    }
+}
+
+if { $index_cache_misses == 0 && $using_index_cache } {
+    set have_gdb_index 1
+}
+
 #
 # this command does not produce any output
 # unless there is some problem with the symtabs and psymtabs