[PATCH v2] Cache the text section offset of shared libraries

["Hannes Domani via gdb-patches" <gdb-patches at sourceware dot org>]

Each time a dll is loaded, update_solib_list is called.
This in turn calls deep down xfer_partial -> windows_xfer_shared_libraries,
which calls windows_xfer_shared_library for each loaded dll,
and pe_text_section_offset reads the dll for the text section offset.

Also if the data provided by xfer_partial is bigger than 4K,
then all of this is done for each 4K chunk (see target_read_alloc_1).

Caching of the text section offset improves the startup time of
an application with >300 dynamically loaded plugins from 2m10s to 10s.
And the shutdown time improves from 2m to 2s.

gdb/ChangeLog:

2019-12-21  Hannes Domani  <ssbssa@yahoo.de>

	* i386-cygwin-tdep.c (core_process_module_section): Update.
	* windows-nat.c (struct lm_info_windows): Add text_offset.
	(windows_xfer_shared_libraries): Update.
	* windows-tdep.c (windows_xfer_shared_library):
	Add text_offset_cached argument.
	* windows-tdep.h (windows_xfer_shared_library): Update.
---
 gdb/i386-cygwin-tdep.c |  2 +-
 gdb/windows-nat.c      |  2 ++
 gdb/windows-tdep.c     | 20 ++++++++++++++------
 gdb/windows-tdep.h     |  1 +
 4 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/gdb/i386-cygwin-tdep.c b/gdb/i386-cygwin-tdep.c
index ee04752b54..9f891f4d98 100644
--- a/gdb/i386-cygwin-tdep.c
+++ b/gdb/i386-cygwin-tdep.c
@@ -137,7 +137,7 @@ core_process_module_section (bfd *abfd, asection *sect, void *obj)
   /* The first module is the .exe itself.  */
   if (data->module_count != 0)
     windows_xfer_shared_library (module_name, base_addr,
-				 data->gdbarch, data->obstack);
+				 NULL, data->gdbarch, data->obstack);
   data->module_count++;
 
 out:
diff --git a/gdb/windows-nat.c b/gdb/windows-nat.c
index be5955d379..cd5922a685 100644
--- a/gdb/windows-nat.c
+++ b/gdb/windows-nat.c
@@ -682,6 +682,7 @@ windows_nat_target::store_registers (struct regcache *regcache, int r)
 struct lm_info_windows : public lm_info_base
 {
   LPVOID load_addr = 0;
+  CORE_ADDR text_offset = 0;
 };
 
 static struct so_list solib_start, *solib_end;
@@ -2974,6 +2975,7 @@ windows_xfer_shared_libraries (struct target_ops *ops,
 
       windows_xfer_shared_library (so->so_name, (CORE_ADDR)
 				   (uintptr_t) li->load_addr,
+				   &li->text_offset,
 				   target_gdbarch (), &obstack);
     }
   obstack_grow_str0 (&obstack, "</library-list>\n");
diff --git a/gdb/windows-tdep.c b/gdb/windows-tdep.c
index b6e5b9ff7c..621146bf43 100644
--- a/gdb/windows-tdep.c
+++ b/gdb/windows-tdep.c
@@ -424,19 +424,27 @@ display_tib (const char * args, int from_tty)
 
 void
 windows_xfer_shared_library (const char* so_name, CORE_ADDR load_addr,
+			     CORE_ADDR *text_offset_cached,
 			     struct gdbarch *gdbarch, struct obstack *obstack)
 {
-  CORE_ADDR text_offset;
+  CORE_ADDR text_offset = text_offset_cached ? *text_offset_cached : 0;
 
   obstack_grow_str (obstack, "<library name=\"");
   std::string p = xml_escape_text (so_name);
   obstack_grow_str (obstack, p.c_str ());
   obstack_grow_str (obstack, "\"><segment address=\"");
-  gdb_bfd_ref_ptr dll (gdb_bfd_open (so_name, gnutarget, -1));
-  /* The following calls are OK even if dll is NULL.
-     The default value 0x1000 is returned by pe_text_section_offset
-     in that case.  */
-  text_offset = pe_text_section_offset (dll.get ());
+
+  if (!text_offset)
+    {
+      gdb_bfd_ref_ptr dll (gdb_bfd_open (so_name, gnutarget, -1));
+      /* The following calls are OK even if dll is NULL.
+	 The default value 0x1000 is returned by pe_text_section_offset
+	 in that case.  */
+      text_offset = pe_text_section_offset (dll.get ());
+      if (text_offset_cached)
+	*text_offset_cached = text_offset;
+    }
+
   obstack_grow_str (obstack, paddress (gdbarch, load_addr + text_offset));
   obstack_grow_str (obstack, "\"/></library>");
 }
diff --git a/gdb/windows-tdep.h b/gdb/windows-tdep.h
index ab6c2d6e55..34474f259c 100644
--- a/gdb/windows-tdep.h
+++ b/gdb/windows-tdep.h
@@ -27,6 +27,7 @@ extern void init_w32_command_list (void);
 
 extern void windows_xfer_shared_library (const char* so_name,
 					 CORE_ADDR load_addr,
+					 CORE_ADDR *text_offset_cached,
 					 struct gdbarch *gdbarch,
 					 struct obstack *obstack);
 
-- 
2.24.1