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author | Jan Kratochvil <jan.kratochvil@redhat.com> | 2015-01-24 15:44:52 +0100 |
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committer | Jan Kratochvil <jan.kratochvil@redhat.com> | 2015-01-24 15:44:52 +0100 |
commit | 527f3840e1af8bc2e3173922ddae15d0021ed9b1 (patch) | |
tree | e38f8dfe91bce1493992d8fab66a37c40340cfbc /gdb/dwarf2read.c | |
parent | 0c61284400e2a78e25aab1becf2b99465f76970c (diff) | |
download | gdb-527f3840e1af8bc2e3173922ddae15d0021ed9b1.zip gdb-527f3840e1af8bc2e3173922ddae15d0021ed9b1.tar.gz gdb-527f3840e1af8bc2e3173922ddae15d0021ed9b1.tar.bz2 |
Fix 100x slowdown regression on DWZ files
Since Fedora started to use DWZ DWARF compressor:
http://fedoraproject.org/wiki/Features/DwarfCompressor
GDB has slowed down a lot. To make it clear - DWZ is DWARF structure
rearrangement, "compressor" does not mean any zlib style data compression.
This patch reduces LibreOffice backtrace from 5 minutes to 3 seconds (100x)
and it also reduces memory consumption 20x.
[ benchmark is at the bottom of this mail ]
Example of DWZ output:
------------------------------------------------------------------------------
Compilation Unit @ offset 0xc4:
<0><cf>: Abbrev Number: 17 (DW_TAG_partial_unit)
<d0> DW_AT_stmt_list : 0x0
<d4> DW_AT_comp_dir : (indirect string, offset: 0x6f): /usr/src/debug/gdb-7.7.1/build-x86_64-redhat-linux-gnu/gdb
<1><d8>: Abbrev Number: 9 (DW_TAG_typedef)
<d9> DW_AT_name : (indirect string, offset: 0x827dc): size_t
<dd> DW_AT_decl_file : 4
<de> DW_AT_decl_line : 212
<df> DW_AT_type : <0xae>
Compilation Unit @ offset 0xe4:
<0><ef>: Abbrev Number: 13 (DW_TAG_partial_unit)
<f0> DW_AT_stmt_list : 0x0
<f4> DW_AT_comp_dir : (indirect string, offset: 0x6f): /usr/src/debug/gdb-7.7.1/build-x86_64-redhat-linux-gnu/gdb
<1><f8>: Abbrev Number: 45 (DW_TAG_typedef)
<f9> DW_AT_name : (indirect string, offset: 0x251): __off_t
<fd> DW_AT_decl_file : 3
<fe> DW_AT_decl_line : 131
<ff> DW_AT_type : <0x68>
Compilation Unit @ offset 0x62d9f9:
<0><62da04>: Abbrev Number: 20 (DW_TAG_compile_unit)
[...]
<62da12> DW_AT_low_pc : 0x807e10
<62da1a> DW_AT_high_pc : 134
<62da1c> DW_AT_stmt_list : 0xf557e
<1><62da20>: Abbrev Number: 7 (DW_TAG_imported_unit)
<62da21> DW_AT_import : <0xcf> [Abbrev Number: 17]
------------------------------------------------------------------------------
One can see all DW_TAG_partial_unit have DW_AT_stmt_list 0x0 which causes
repeated decoding of that .debug_line unit on each DW_TAG_imported_unit.
This was OK before as each DW_TAG_compile_unit has its own .debug_line unit.
But since the introduction of DW_TAG_partial_unit by DWZ one should cache
read-in DW_AT_stmt_list .debug_line units.
Fortunately one does not need to cache whole
struct linetable *symtab->linetable
and other data from .debug_line mapping PC<->lines
------------------------------------------------------------------------------
Line Number Statements:
Extended opcode 2: set Address to 0x45c880
Advance Line by 25 to 26
Copy
------------------------------------------------------------------------------
as the only part of .debug_line which GDB needs for DW_TAG_partial_unit is:
------------------------------------------------------------------------------
The Directory Table:
../../gdb
/usr/include/bits
[...]
The File Name Table:
Entry Dir Time Size Name
1 1 0 0 gdb.c
2 2 0 0 string3.h
[...]
------------------------------------------------------------------------------
specifically referenced in GDB for DW_AT_decl_file at a single place:
------------------------------------------------------------------------------
fe = &cu->line_header->file_names[file_index - 1];
SYMBOL_SYMTAB (sym) = fe->symtab;
------------------------------------------------------------------------------
This is because for some reason DW_TAG_partial_unit never contains PC-related
DWARF information. I do not know exactly why, the compression ratio is a bit
lower due to it but thanksfully currently it is that way:
dwz.c:
------------------------------------------------------------------------------
/* These attributes reference code, prevent moving
DIEs with them. */
case DW_AT_low_pc:
case DW_AT_high_pc:
case DW_AT_entry_pc:
case DW_AT_ranges:
die->die_ck_state = CK_BAD;
+
/* State of checksum computation. Not computed yet, computed and
suitable for moving into partial units, currently being computed
and finally determined unsuitable for moving into partial units. */
enum { CK_UNKNOWN, CK_KNOWN, CK_BEING_COMPUTED, CK_BAD } die_ck_state : 2;
------------------------------------------------------------------------------
I have also verified also real-world Fedora debuginfo files really comply with
that assumption with dwgrep
https://github.com/pmachata/dwgrep
using:
------------------------------------------------------------------------------
dwgrep -e 'entry ?DW_TAG_partial_unit child* ( ?DW_AT_low_pc , ?DW_AT_high_pc , ?DW_AT_ranges )' /usr/lib/debug/**
------------------------------------------------------------------------------
BTW I think GDB already does not support the whole DW_TAG_imported_unit and
DW_TAG_partial_unit usage possibilities as specified by the DWARF standard.
I think GDB would not work if DW_TAG_imported_unit was used in some inner
level and not at the CU level (readelf -wi level <1>) - this is how DWZ is
using DW_TAG_imported_unit. Therefore I do not think further assumptions
about DW_TAG_imported_unit and DW_TAG_partial_unit usage by DWZ are a problem
for GDB.
One could save the whole .debug_line decoded PC<->lines mapping (and not just
the DW_AT_decl_file table) but:
* there are some problematic corner cases so one could do it incorrectly
* there are no real world data to really test such patch extension
* such extension could be done perfectly incrementally on top of this patch
------------------------------------------------------------------------------
benchmark - on Fedora 20 x86_64 and FSF GDB HEAD:
echo -e 'thread apply all bt\nset confirm no\nq'|./gdb -p `pidof soffice.bin` -ex 'set pagination off' -ex 'maintenance set per-command
space' -ex 'maintenance set per-command symtab' -ex 'maintenance set per-command time'
FSF GDB HEAD ("thread apply all bt"):
Command execution time: 333.693000 (cpu), 335.587539 (wall)
---sec
Space used: 1736404992 (+1477189632 for this command)
----MB
vs.
THIS PATCH ("thread apply all bt"):
Command execution time: 2.595000 (cpu), 2.607573 (wall)
-sec
Space used: 340058112 (+85917696 for this command)
--MB
FSF GDB HEAD ("thread apply all bt full"):
Command execution time: 466.751000 (cpu), 468.345837 (wall)
---sec
Space used: 2330132480 (+2070974464 for this command)
----MB
vs.
THIS PATCH ("thread apply all bt full"):
Command execution time: 18.907000 (cpu), 18.964125 (wall)
--sec
Space used: 364462080 (+110325760 for this command)
---MB
------------------------------------------------------------------------------
gdb/ChangeLog
2015-01-24 Jan Kratochvil <jan.kratochvil@redhat.com>
Fix 100x slowdown regression on DWZ files.
* dwarf2read.c (struct dwarf2_per_objfile): Add line_header_hash.
(struct line_header): Add offset and offset_in_dwz.
(dwarf_decode_lines): Add parameter decode_mapping to the declaration.
(free_line_header_voidp): New declaration.
(line_header_hash, line_header_hash_voidp, line_header_eq_voidp): New
functions.
(dwarf2_build_include_psymtabs): Update dwarf_decode_lines caller.
(handle_DW_AT_stmt_list): Use line_header_hash.
(free_line_header_voidp): New function.
(dwarf_decode_line_header): Initialize offset and offset_in_dwz.
(dwarf_decode_lines): New parameter decode_mapping, use it.
(dwarf2_free_objfile): Free line_header_hash.
Diffstat (limited to 'gdb/dwarf2read.c')
-rw-r--r-- | gdb/dwarf2read.c | 160 |
1 files changed, 147 insertions, 13 deletions
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c index 9d765c5..715b090 100644 --- a/gdb/dwarf2read.c +++ b/gdb/dwarf2read.c @@ -308,6 +308,9 @@ struct dwarf2_per_objfile /* The CUs we recently read. */ VEC (dwarf2_per_cu_ptr) *just_read_cus; + + /* Table containing line_header indexed by offset and offset_in_dwz. */ + htab_t line_header_hash; }; static struct dwarf2_per_objfile *dwarf2_per_objfile; @@ -1024,6 +1027,12 @@ typedef void (die_reader_func_ftype) (const struct die_reader_specs *reader, which contains the following information. */ struct line_header { + /* Offset of line number information in .debug_line section. */ + sect_offset offset; + + /* OFFSET is for struct dwz_file associated with dwarf2_per_objfile. */ + unsigned offset_in_dwz : 1; + unsigned int total_length; unsigned short version; unsigned int header_length; @@ -1512,7 +1521,7 @@ static struct line_header *dwarf_decode_line_header (unsigned int offset, static void dwarf_decode_lines (struct line_header *, const char *, struct dwarf2_cu *, struct partial_symtab *, - CORE_ADDR); + CORE_ADDR, int decode_mapping); static void dwarf2_start_subfile (const char *, const char *); @@ -1844,6 +1853,8 @@ static void free_dwo_file_cleanup (void *); static void process_cu_includes (void); static void check_producer (struct dwarf2_cu *cu); + +static void free_line_header_voidp (void *arg); /* Various complaints about symbol reading that don't abort the process. */ @@ -1910,6 +1921,37 @@ dwarf2_invalid_attrib_class_complaint (const char *arg1, const char *arg2) _("invalid attribute class or form for '%s' in '%s'"), arg1, arg2); } + +/* Hash function for line_header_hash. */ + +static hashval_t +line_header_hash (const struct line_header *ofs) +{ + return ofs->offset.sect_off ^ ofs->offset_in_dwz; +} + +/* Hash function for htab_create_alloc_ex for line_header_hash. */ + +static hashval_t +line_header_hash_voidp (const void *item) +{ + const struct line_header *ofs = item; + + return line_header_hash (ofs); +} + +/* Equality function for line_header_hash. */ + +static int +line_header_eq_voidp (const void *item_lhs, const void *item_rhs) +{ + const struct line_header *ofs_lhs = item_lhs; + const struct line_header *ofs_rhs = item_rhs; + + return (ofs_lhs->offset.sect_off == ofs_rhs->offset.sect_off + && ofs_lhs->offset_in_dwz == ofs_rhs->offset_in_dwz); +} + #if WORDS_BIGENDIAN @@ -4452,7 +4494,7 @@ dwarf2_build_include_psymtabs (struct dwarf2_cu *cu, return; /* No linetable, so no includes. */ /* NOTE: pst->dirname is DW_AT_comp_dir (if present). */ - dwarf_decode_lines (lh, pst->dirname, cu, pst, pst->textlow); + dwarf_decode_lines (lh, pst->dirname, cu, pst, pst->textlow, 1); free_line_header (lh); } @@ -8995,24 +9037,95 @@ static void handle_DW_AT_stmt_list (struct die_info *die, struct dwarf2_cu *cu, const char *comp_dir, CORE_ADDR lowpc) /* ARI: editCase function */ { + struct objfile *objfile = dwarf2_per_objfile->objfile; struct attribute *attr; + unsigned int line_offset; + struct line_header line_header_local; + hashval_t line_header_local_hash; + unsigned u; + void **slot; + int decode_mapping; gdb_assert (! cu->per_cu->is_debug_types); attr = dwarf2_attr (die, DW_AT_stmt_list, cu); - if (attr) + if (attr == NULL) + return; + + line_offset = DW_UNSND (attr); + + /* The line header hash table is only created if needed (it exists to + prevent redundant reading of the line table for partial_units). + If we're given a partial_unit, we'll need it. If we're given a + compile_unit, then use the line header hash table if it's already + created, but don't create one just yet. */ + + if (dwarf2_per_objfile->line_header_hash == NULL + && die->tag == DW_TAG_partial_unit) + { + dwarf2_per_objfile->line_header_hash + = htab_create_alloc_ex (127, line_header_hash_voidp, + line_header_eq_voidp, + free_line_header_voidp, + &objfile->objfile_obstack, + hashtab_obstack_allocate, + dummy_obstack_deallocate); + } + + line_header_local.offset.sect_off = line_offset; + line_header_local.offset_in_dwz = cu->per_cu->is_dwz; + line_header_local_hash = line_header_hash (&line_header_local); + if (dwarf2_per_objfile->line_header_hash != NULL) { - unsigned int line_offset = DW_UNSND (attr); - struct line_header *line_header - = dwarf_decode_line_header (line_offset, cu); + slot = htab_find_slot_with_hash (dwarf2_per_objfile->line_header_hash, + &line_header_local, + line_header_local_hash, NO_INSERT); - if (line_header) + /* For DW_TAG_compile_unit we need info like symtab::linetable which + is not present in *SLOT (since if there is something in *SLOT then + it will be for a partial_unit). */ + if (die->tag == DW_TAG_partial_unit && slot != NULL) { - cu->line_header = line_header; - make_cleanup (free_cu_line_header, cu); - dwarf_decode_lines (line_header, comp_dir, cu, NULL, lowpc); + gdb_assert (*slot != NULL); + cu->line_header = *slot; + return; } } + + /* dwarf_decode_line_header does not yet provide sufficient information. + We always have to call also dwarf_decode_lines for it. */ + cu->line_header = dwarf_decode_line_header (line_offset, cu); + if (cu->line_header == NULL) + return; + + if (dwarf2_per_objfile->line_header_hash == NULL) + slot = NULL; + else + { + slot = htab_find_slot_with_hash (dwarf2_per_objfile->line_header_hash, + &line_header_local, + line_header_local_hash, INSERT); + gdb_assert (slot != NULL); + } + if (slot != NULL && *slot == NULL) + { + /* This newly decoded line number information unit will be owned + by line_header_hash hash table. */ + *slot = cu->line_header; + } + else + { + /* We cannot free any current entry in (*slot) as that struct line_header + may be already used by multiple CUs. Create only temporary decoded + line_header for this CU - it may happen at most once for each line + number information unit. And if we're not using line_header_hash + then this is what we want as well. */ + gdb_assert (die->tag != DW_TAG_partial_unit); + make_cleanup (free_cu_line_header, cu); + } + decode_mapping = (die->tag != DW_TAG_partial_unit); + dwarf_decode_lines (cu->line_header, comp_dir, cu, NULL, lowpc, + decode_mapping); } /* Process DW_TAG_compile_unit or DW_TAG_partial_unit. */ @@ -16915,6 +17028,16 @@ free_line_header (struct line_header *lh) xfree (lh); } +/* Stub for free_line_header to match void * callback types. */ + +static void +free_line_header_voidp (void *arg) +{ + struct line_header *lh = arg; + + free_line_header (lh); +} + /* Add an entry to LH's include directory table. */ static void @@ -17045,6 +17168,9 @@ dwarf_decode_line_header (unsigned int offset, struct dwarf2_cu *cu) back_to = make_cleanup ((make_cleanup_ftype *) free_line_header, (void *) lh); + lh->offset.sect_off = offset; + lh->offset_in_dwz = cu->per_cu->is_dwz; + line_ptr = section->buffer + offset; /* Read in the header. */ @@ -17672,17 +17798,22 @@ dwarf_decode_lines_1 (struct line_header *lh, struct dwarf2_cu *cu, E.g. expand_line_sal requires this when finding psymtabs to expand. A good testcase for this is mb-inline.exp. - LOWPC is the lowest address in CU (or 0 if not known). */ + LOWPC is the lowest address in CU (or 0 if not known). + + Boolean DECODE_MAPPING specifies we need to fully decode .debug_line + for its PC<->lines mapping information. Otherwise only the filename + table is read in. */ static void dwarf_decode_lines (struct line_header *lh, const char *comp_dir, struct dwarf2_cu *cu, struct partial_symtab *pst, - CORE_ADDR lowpc) + CORE_ADDR lowpc, int decode_mapping) { struct objfile *objfile = cu->objfile; const int decode_for_pst_p = (pst != NULL); - dwarf_decode_lines_1 (lh, cu, decode_for_pst_p, lowpc); + if (decode_mapping) + dwarf_decode_lines_1 (lh, cu, decode_for_pst_p, lowpc); if (decode_for_pst_p) { @@ -21779,6 +21910,9 @@ dwarf2_free_objfile (struct objfile *objfile) if (dwarf2_per_objfile->quick_file_names_table) htab_delete (dwarf2_per_objfile->quick_file_names_table); + if (dwarf2_per_objfile->line_header_hash) + htab_delete (dwarf2_per_objfile->line_header_hash); + /* Everything else should be on the objfile obstack. */ } |