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[PATCH][gdb] Skip imports of c++ CUs
- From: Tom de Vries <tdevries at suse dot de>
- To: gdb-patches at sourceware dot org
- Date: Mon, 2 Mar 2020 17:13:24 +0100
- Subject: [PATCH][gdb] Skip imports of c++ CUs
Hi,
The DWARF standard appendix E.1 describes techniques that can be used for
compression and deduplication: DIEs can be factored out into a new compilation
unit, and referenced using DW_FORM_ref_addr.
Such a new compilation unit can either use a DW_TAG_compile_unit or
DW_TAG_partial_unit. If a DW_TAG_compile_unit is used, its contents is
evaluated by consumers as though it were an ordinary compilation unit. If a
DW_TAG_partial_unit is used, it's only considered by consumers in the context
of a DW_TAG_imported_unit.
An example of when DW_TAG_partial_unit is required is when the factored out
DIEs are not top-level, f.i. because they were children of a namespace. In
such a case the corresponding DW_TAG_imported_unit will occur as child of the
namespace.
In the case of factoring out DIEs from c++ compilation units, we can factor
out into a new DW_TAG_compile_unit, and no DW_TAG_imported_unit is required.
This begs the question how to interpret a top-level DW_TAG_imported_unit of a
c++ DW_TAG_compile_unit compilation unit. The semantics of
DW_TAG_imported_unit describe that the imported unit logically appears at the
point of the DW_TAG_imported_unit entry. But it's not clear what the effect
should be in this case, since all the imported DIEs are already globally
visible anyway, due to the use of DW_TAG_compile_unit.
So, skip top-level imports of c++ DW_TAG_compile_unit compilation units in
process_imported_unit_die.
Using the cc1 binary from PR23710 comment 1 and setting a breakpoint on do_rpo_vn:
...
$ gdb \
-batch \
-iex "maint set dwarf max-cache-age 316" \
-iex "set language c++" \
-ex "b do_rpo_vn" \
cc1
...
we get a 8.1% reduction in execution time, due to reducing the number of
partial symtabs expanded into full symtabs from 212 to 175.
Build and reg-tested on x86_64-linux.
OK for trunk?
Thanks,
- Tom
[gdb] Skip imports of c++ CUs
gdb/ChangeLog:
2020-03-02 Tom de Vries <tdevries@suse.de>
PR gdb/23710
* dwarf2/read.h (struct dwarf2_per_cu_data): Add unit_type and lang
fields.
* dwarf2/read.c (process_psymtab_comp_unit): Initialize unit_type and lang
fields.
(process_imported_unit_die): Skip import of c++ CUs.
---
gdb/dwarf2/read.c | 22 ++++++++++++++++++++++
gdb/dwarf2/read.h | 6 ++++++
2 files changed, 28 insertions(+)
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 07cee58c1f..fd16ebb3f7 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -7425,6 +7425,18 @@ process_psymtab_comp_unit (struct dwarf2_per_cu_data *this_cu,
cutu_reader reader (this_cu, NULL, 0, false);
+ switch (reader.comp_unit_die->tag)
+ {
+ case DW_TAG_compile_unit:
+ this_cu->unit_type = DW_UT_compile;
+ break;
+ case DW_TAG_partial_unit:
+ this_cu->unit_type = DW_UT_partial;
+ break;
+ default:
+ abort ();
+ }
+
if (reader.dummy_p)
{
/* Nothing. */
@@ -7438,6 +7450,8 @@ process_psymtab_comp_unit (struct dwarf2_per_cu_data *this_cu,
reader.comp_unit_die,
pretend_language);
+ this_cu->lang = this_cu->cu->language;
+
/* Age out any secondary CUs. */
age_cached_comp_units (this_cu->dwarf2_per_objfile);
}
@@ -9760,6 +9774,14 @@ process_imported_unit_die (struct die_info *die, struct dwarf2_cu *cu)
= dwarf2_find_containing_comp_unit (sect_off, is_dwz,
cu->per_cu->dwarf2_per_objfile);
+ /* We're importing a C++ compilation unit with tag DW_TAG_compile_unit
+ into another compilation unit, at root level. Regard this as a hint,
+ and ignore it. */
+ if (die->parent && die->parent->parent == NULL
+ && per_cu->unit_type == DW_UT_compile
+ && per_cu->lang == language_cplus)
+ return;
+
/* If necessary, add it to the queue and load its DIEs. */
if (maybe_queue_comp_unit (cu, per_cu, cu->language))
load_full_comp_unit (per_cu, false, cu->language);
diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index 00652c2b45..f0e3d6bb72 100644
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -323,6 +323,12 @@ struct dwarf2_per_cu_data
dummy CUs (a CU header, but nothing else). */
struct dwarf2_cu *cu;
+ /* The unit type of this CU. */
+ enum dwarf_unit_type unit_type;
+
+ /* The language of this CU. */
+ enum language lang;
+
/* The corresponding dwarf2_per_objfile. */
struct dwarf2_per_objfile *dwarf2_per_objfile;