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From: "vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>
To: gdb-prs@sourceware.org
Subject: [Bug symtab/27922] [gdb/symtab] Don't create two dwarf2_cu's for the
 same CU
Date: Fri, 28 May 2021 07:44:20 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D27922

--- Comment #1 from Tom de Vries <vries at gcc dot gnu.org> ---
Let's try to investigate the size of the problem.

There are 17292 CUs:
...
$ readelf -wi --dwarf-depth 0 libLLVM.so.10-10.0.1-lp152.30.4.x86_64.debug |
grep -c "Compilation Unit @"
17292
...

Now let's instrument the partial DIE reader with some tracing:
...
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 4cc5b4f32d7..40531b7b804 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -18823,6 +18823,10 @@ load_partial_dies (const struct die_reader_specs
*reader,
   if (cu->load_all_dies)
     load_all =3D 1;

+  fprintf (stderr, "Loading%spartial DIEs for CU@%s\n",
+          load_all ? " all " : " ",
+          sect_offset_str (cu->header.sect_off));
+
   cu->partial_dies
     =3D htab_create_alloc_ex (cu->header.length / 12,
                            partial_die_hash,
...

Now do a run and set the max-cache-age such that nothing is evicted from the
cache (also double checked this with assert):
...
$ ./gdb.sh -q -batch -ex "maint set dwarf max-cache-age 100000" -ex
"symbol-file libLLVM.so.10-10.0.1-lp152.30.4.x86_64.debug" 2>&1 | tee LOG
...

So, we end up reading 50% more that the total number of CUs:
...
$ grep -c "Loading partial DIEs" LOG
26178
...
and as for loading all partial DIEs we have:
...
$ grep -c "Loading all partial DIEs" LOG=20
781
...

For contrast, with "maint set dwarf max-cache-age 0" we have:
...
$ grep -c "Loading partial DIEs" LOG
37977
$ grep -c "Loading all partial DIEs" LOG
781
...

--=20
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