[Bug symtab/28159] New: [gdb/symtab] gdb generates .gdb_index with only empty hashtable entries, then fails to recognize it as empty

[Bug symtab/28159] New: [gdb/symtab] gdb generates .gdb_index with only empty hashtable entries, then fails to recognize it as empty

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28159

            Bug ID: 28159
           Summary: [gdb/symtab] gdb generates .gdb_index with only empty
                    hashtable entries, then fails to recognize it as empty
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: symtab
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

While working on the test-case for PR28004, I ran the test-case with target
boards cc-with-debug-names and cc-with-gdb-index.

I noticed a difference in behaviour between the two:
- in the .debug_names case, the index was found to be empty, and
  consequently gdb falls back on partial symbols
- in the .gdb_index case, the index was not found to be empty

I debugged this and found that this was caused by the .gdb_index having a hash
table of 8192 bytes, all 0, in other words, it's filled with empty entries.

This shows two problems:
- gdb generates such an index
- gdb should recognize that such an index is empty

The second problem is fixed by:
...
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index de84c47b626..3f9c017360a 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -2777,6 +2777,15 @@ to use the section anyway."),
   ++i;
   map->constant_pool = buffer.slice (metadata[i]);

+  if (map->constant_pool.empty () && !map->symbol_table.empty ())
+    {
+      map->symbol_table
+       = offset_view (gdb::array_view<const gdb_byte> (symbol_table,
+                                                       symbol_table));
+    }
+
   return 1;
 }

...

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[Bug symtab/28159] [gdb/symtab] gdb generates .gdb_index with only empty hashtable entries, then fails to recognize it as empty

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28159

--- Comment #1 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Tom de Vries <vries@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=cc6b3d766d6694d4c4ee14b75f6529365a443a98

commit cc6b3d766d6694d4c4ee14b75f6529365a443a98
Author: Tom de Vries <tdevries@suse.de>
Date:   Fri Aug 6 21:52:41 2021 +0200

    [gdb/symtab] Recognize .gdb_index symbol table with empty entries as empty

    When reading a .gdb_index that contains a non-empty symbol table with only
    empty entries, gdb doesn't recognize it as empty.

    Fix this by recognizing that the constant pool is empty, and then setting
the
    symbol table to empty.

    Tested on x86_64-linux.

    gdb/ChangeLog:

    2021-08-01  Tom de Vries  <tdevries@suse.de>

            PR symtab/28159
            * dwarf2/read.c (read_gdb_index_from_buffer): Handle symbol table
            filled with empty entries.

    gdb/testsuite/ChangeLog:

    2021-08-01  Tom de Vries  <tdevries@suse.de>

            PR symtab/28159
            * gdb.dwarf2/dw2-zero-range.exp: Remove kfail.

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[Bug symtab/28159] [gdb/symtab] gdb generates .gdb_index with only empty hashtable entries, then fails to recognize it as empty

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28159

--- Comment #2 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #0)
> This shows two problems:
> - gdb generates such an index

Fixed by:
...
diff --git a/gdb/dwarf2/index-write.c b/gdb/dwarf2/index-write.c
index 4e00c716d91..5de07a6c07b 100644
--- a/gdb/dwarf2/index-write.c
+++ b/gdb/dwarf2/index-write.c
@@ -1385,6 +1385,9 @@ write_gdbindex (dwarf2_per_objfile *per_objfile, FILE
*out_file,
   uniquify_cu_indices (&symtab);

   data_buf symtab_vec, constant_pool;
+  if (symtab.n_elements == 0)
+    symtab.data.resize (0);
+
   write_hash_table (&symtab, symtab_vec, constant_pool);

   write_gdbindex_1(out_file, objfile_cu_list, types_cu_list, addr_vec,
...

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[Bug symtab/28159] [gdb/symtab] gdb generates .gdb_index with only empty hashtable entries, then fails to recognize it as empty

[tromey at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28159

Tom Tromey <tromey at sourceware dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |tromey at sourceware dot org

--- Comment #3 from Tom Tromey <tromey at sourceware dot org> ---
I wonder if gdb should refuse to write an empty index.
My psymtab rewrite branch does this, but maybe it
could be done upstream separately.

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[Bug symtab/28159] [gdb/symtab] gdb generates .gdb_index with only empty hashtable entries, then fails to recognize it as empty

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28159

--- Comment #4 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Tom Tromey from comment #3)
> I wonder if gdb should refuse to write an empty index.
> My psymtab rewrite branch does this, but maybe it
> could be done upstream separately.

Conversely,  I start to wonder why gdb refuses to read an empty index.  If
that's an accurate representation of the debug info, why fall back to partial
symbols?

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[Bug symtab/28159] [gdb/symtab] gdb generates .gdb_index with only empty hashtable entries, then fails to recognize it as empty

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28159

--- Comment #5 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #2)
> (In reply to Tom de Vries from comment #0)
> > This shows two problems:
> > - gdb generates such an index
> 
> Fixed by:
> ...
> diff --git a/gdb/dwarf2/index-write.c b/gdb/dwarf2/index-write.c
> index 4e00c716d91..5de07a6c07b 100644
> --- a/gdb/dwarf2/index-write.c
> +++ b/gdb/dwarf2/index-write.c
> @@ -1385,6 +1385,9 @@ write_gdbindex (dwarf2_per_objfile *per_objfile, FILE
> *out_file,
>    uniquify_cu_indices (&symtab);
>  
>    data_buf symtab_vec, constant_pool;
> +  if (symtab.n_elements == 0)
> +    symtab.data.resize (0);
> +
>    write_hash_table (&symtab, symtab_vec, constant_pool);
>  
>    write_gdbindex_1(out_file, objfile_cu_list, types_cu_list, addr_vec,
> ...

Submitted here:
https://sourceware.org/pipermail/gdb-patches/2021-August/181673.html .

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[Bug symtab/28159] [gdb/symtab] gdb generates .gdb_index with only empty hashtable entries, then fails to recognize it as empty

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28159

--- Comment #6 from Tom de Vries <vries at gcc dot gnu.org> ---
Committed:
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=34daac4b169da3cb9df3e8bac6347cae630a99dd
:

[gdb/symtab] Don't write .gdb_index symbol table with empty entries
author  Tom de Vries <tdevries@suse.de> 
        Fri, 27 Aug 2021 15:14:49 +0000 (17:14 +0200)
committer       Tom de Vries <tdevries@suse.de> 
        Fri, 27 Aug 2021 15:14:49 +0000 (17:14 +0200)
commit  34daac4b169da3cb9df3e8bac6347cae630a99dd
tree    f3ce338b4bd3c8cf74e39c5376a0d20399a96ca8        tree
parent  426f7bbfd4179a5b81f87fa324a854c370e4b07a        commit | diff
[gdb/symtab] Don't write .gdb_index symbol table with empty entries

When comparing the sizes of the index files generated for shlib
outputs/gdb.dwarf2/dw2-zero-range/shr1.sl, I noticed a large difference
between .debug_names:
...
$ gdb -q -batch $shlib -ex "save gdb-index -dwarf-5 ."
$ du -b -h shr1.sl.debug_names shr1.sl.debug_str
61      shr1.sl.debug_names
0       shr1.sl.debug_str
...
and .gdb_index:
...
$ gdb -q -batch $shlib -ex "save gdb-index ."
$ du -b -h shr1.sl.gdb-index
8.2K    shr1.sl.gdb-index
...

The problem is that the .gdb_index contains a non-empty symbol table with only
empty entries.

Fix this by making the symbol table empty, such that we have instead:
...
$ du -b -h shr1.sl.gdb-index
184     shr1.sl.gdb-index
...

Tested on x86_64-linux.

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[Bug symtab/28159] [gdb/symtab] gdb generates .gdb_index with only empty hashtable entries, then fails to recognize it as empty

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28159

Tom de Vries <vries at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |12.1

--- Comment #7 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #0)
> This shows two problems:
> - gdb generates such an index
> - gdb should recognize that such an index is empty

Both problems are now fixed, marking resolved-fixed.

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