[PATCH v4] gdb: Do not add empty sections to the section map

[PATCH v4] gdb: Do not add empty sections to the section map

[Ilya Leoshkevich]

From: Ulrich Weigand <ulrich.weigand@de.ibm.com>

build_objfile_section_table () creates four synthetic sections per
objfile, which are collected by update_section_map () and passed to
std::sort ().  When there are a lot of objfiles, for example, when
debugging JITs, the presence of these sections slows down the sorting
significantly.

The output of update_section_map () is used by find_pc_section (),
which can never return any of these sections: their size is 0, so they
cannot be accepted by bsearch_cmp ().

Filter them (and all the other empty sections) out in
insert_section_p (), which is used only by update_section_map ().
---

v3: https://sourceware.org/pipermail/gdb-patches/2022-June/189692.html
v3 -> v4: Use whether a section is empty or not as the filtering
          criteria (Pedro).

v2: https://sourceware.org/pipermail/gdb-patches/2022-May/189154.html
v2 -> v3: Fix comments and commit message (Andrew).

v1: https://sourceware.org/pipermail/binutils/2022-May/120863.html
v1 -> v2: Fix code style, post to the correct mailing list (Andrew).

 gdb/objfiles.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/gdb/objfiles.c b/gdb/objfiles.c
index 60d8aa5cb78..4fc859f185a 100644
--- a/gdb/objfiles.c
+++ b/gdb/objfiles.c
@@ -986,7 +986,7 @@ preferred_obj_section (struct obj_section *a, struct obj_section *b)
 }
 
 /* Return 1 if SECTION should be inserted into the section map.
-   We want to insert only non-overlay and non-TLS section.  */
+   We want to insert only non-overlay non-TLS non-empty sections.  */
 
 static int
 insert_section_p (const struct bfd *abfd,
@@ -1003,6 +1003,12 @@ insert_section_p (const struct bfd *abfd,
   if ((bfd_section_flags (section) & SEC_THREAD_LOCAL) != 0)
     /* This is a TLS section.  */
     return 0;
+  if (bfd_section_size (section) == 0)
+    {
+      /* This is an empty section.  It has no PCs for find_pc_section (), so
+	 there is no reason to insert it into the section map.  */
+      return 0;
+    }
 
   return 1;
 }
-- 
2.35.3

Re: [PATCH v4] gdb: Do not add empty sections to the section map

[Pedro Alves]

On 2022-06-01 18:07, Ilya Leoshkevich wrote:
> From: Ulrich Weigand <ulrich.weigand@de.ibm.com>
> 
> build_objfile_section_table () creates four synthetic sections per
> objfile, which are collected by update_section_map () and passed to
> std::sort ().  When there are a lot of objfiles, for example, when
> debugging JITs, the presence of these sections slows down the sorting
> significantly.
> 
> The output of update_section_map () is used by find_pc_section (),
> which can never return any of these sections: their size is 0, so they
> cannot be accepted by bsearch_cmp ().
> 
> Filter them (and all the other empty sections) out in
> insert_section_p (), which is used only by update_section_map ().
> ---
> 
> v3: https://sourceware.org/pipermail/gdb-patches/2022-June/189692.html
> v3 -> v4: Use whether a section is empty or not as the filtering
>           criteria (Pedro).

Thanks.  This LGTM.