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.
This commit is contained in:
Jan Kratochvil
parent
0c61284400
commit
527f3840e1
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@ -1,3 +1,19 @@
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2015-01-24 Jan Kratochvil <jan.kratochvil@redhat.com>
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Fix 100x slowdown regression on DWZ files.
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* dwarf2read.c (struct dwarf2_per_objfile): Add line_header_hash.
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(struct line_header): Add offset and offset_in_dwz.
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(dwarf_decode_lines): Add parameter decode_mapping to the declaration.
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(free_line_header_voidp): New declaration.
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(line_header_hash, line_header_hash_voidp, line_header_eq_voidp): New
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functions.
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(dwarf2_build_include_psymtabs): Update dwarf_decode_lines caller.
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(handle_DW_AT_stmt_list): Use line_header_hash.
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(free_line_header_voidp): New function.
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(dwarf_decode_line_header): Initialize offset and offset_in_dwz.
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(dwarf_decode_lines): New parameter decode_mapping, use it.
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(dwarf2_free_objfile): Free line_header_hash.
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2015-01-23 Simon Marchi <simon.marchi@ericsson.com>
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PR gdb/17416
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162
gdb/dwarf2read.c
162
gdb/dwarf2read.c
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@ -308,6 +308,9 @@ struct dwarf2_per_objfile
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/* The CUs we recently read. */
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VEC (dwarf2_per_cu_ptr) *just_read_cus;
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/* Table containing line_header indexed by offset and offset_in_dwz. */
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htab_t line_header_hash;
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};
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static struct dwarf2_per_objfile *dwarf2_per_objfile;
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@ -1024,6 +1027,12 @@ typedef void (die_reader_func_ftype) (const struct die_reader_specs *reader,
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which contains the following information. */
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struct line_header
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{
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/* Offset of line number information in .debug_line section. */
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sect_offset offset;
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/* OFFSET is for struct dwz_file associated with dwarf2_per_objfile. */
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unsigned offset_in_dwz : 1;
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unsigned int total_length;
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unsigned short version;
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unsigned int header_length;
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@ -1512,7 +1521,7 @@ static struct line_header *dwarf_decode_line_header (unsigned int offset,
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static void dwarf_decode_lines (struct line_header *, const char *,
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struct dwarf2_cu *, struct partial_symtab *,
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CORE_ADDR);
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CORE_ADDR, int decode_mapping);
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static void dwarf2_start_subfile (const char *, const char *);
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@ -1844,6 +1853,8 @@ static void free_dwo_file_cleanup (void *);
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static void process_cu_includes (void);
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static void check_producer (struct dwarf2_cu *cu);
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static void free_line_header_voidp (void *arg);
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/* Various complaints about symbol reading that don't abort the process. */
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@ -1910,6 +1921,37 @@ dwarf2_invalid_attrib_class_complaint (const char *arg1, const char *arg2)
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_("invalid attribute class or form for '%s' in '%s'"),
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arg1, arg2);
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}
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/* Hash function for line_header_hash. */
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static hashval_t
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line_header_hash (const struct line_header *ofs)
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{
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return ofs->offset.sect_off ^ ofs->offset_in_dwz;
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}
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/* Hash function for htab_create_alloc_ex for line_header_hash. */
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static hashval_t
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line_header_hash_voidp (const void *item)
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{
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const struct line_header *ofs = item;
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return line_header_hash (ofs);
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}
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/* Equality function for line_header_hash. */
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static int
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line_header_eq_voidp (const void *item_lhs, const void *item_rhs)
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{
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const struct line_header *ofs_lhs = item_lhs;
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const struct line_header *ofs_rhs = item_rhs;
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return (ofs_lhs->offset.sect_off == ofs_rhs->offset.sect_off
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&& ofs_lhs->offset_in_dwz == ofs_rhs->offset_in_dwz);
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}
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#if WORDS_BIGENDIAN
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@ -4452,7 +4494,7 @@ dwarf2_build_include_psymtabs (struct dwarf2_cu *cu,
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return; /* No linetable, so no includes. */
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/* NOTE: pst->dirname is DW_AT_comp_dir (if present). */
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dwarf_decode_lines (lh, pst->dirname, cu, pst, pst->textlow);
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dwarf_decode_lines (lh, pst->dirname, cu, pst, pst->textlow, 1);
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free_line_header (lh);
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}
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@ -8995,24 +9037,95 @@ static void
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handle_DW_AT_stmt_list (struct die_info *die, struct dwarf2_cu *cu,
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const char *comp_dir, CORE_ADDR lowpc) /* ARI: editCase function */
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{
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struct objfile *objfile = dwarf2_per_objfile->objfile;
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struct attribute *attr;
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unsigned int line_offset;
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struct line_header line_header_local;
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hashval_t line_header_local_hash;
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unsigned u;
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void **slot;
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int decode_mapping;
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gdb_assert (! cu->per_cu->is_debug_types);
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attr = dwarf2_attr (die, DW_AT_stmt_list, cu);
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if (attr)
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{
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unsigned int line_offset = DW_UNSND (attr);
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struct line_header *line_header
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= dwarf_decode_line_header (line_offset, cu);
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if (attr == NULL)
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return;
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if (line_header)
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line_offset = DW_UNSND (attr);
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/* The line header hash table is only created if needed (it exists to
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prevent redundant reading of the line table for partial_units).
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If we're given a partial_unit, we'll need it. If we're given a
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compile_unit, then use the line header hash table if it's already
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created, but don't create one just yet. */
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if (dwarf2_per_objfile->line_header_hash == NULL
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&& die->tag == DW_TAG_partial_unit)
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{
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dwarf2_per_objfile->line_header_hash
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= htab_create_alloc_ex (127, line_header_hash_voidp,
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line_header_eq_voidp,
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free_line_header_voidp,
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&objfile->objfile_obstack,
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hashtab_obstack_allocate,
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dummy_obstack_deallocate);
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}
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line_header_local.offset.sect_off = line_offset;
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line_header_local.offset_in_dwz = cu->per_cu->is_dwz;
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line_header_local_hash = line_header_hash (&line_header_local);
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if (dwarf2_per_objfile->line_header_hash != NULL)
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{
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slot = htab_find_slot_with_hash (dwarf2_per_objfile->line_header_hash,
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&line_header_local,
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line_header_local_hash, NO_INSERT);
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/* For DW_TAG_compile_unit we need info like symtab::linetable which
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is not present in *SLOT (since if there is something in *SLOT then
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it will be for a partial_unit). */
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if (die->tag == DW_TAG_partial_unit && slot != NULL)
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{
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cu->line_header = line_header;
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make_cleanup (free_cu_line_header, cu);
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dwarf_decode_lines (line_header, comp_dir, cu, NULL, lowpc);
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gdb_assert (*slot != NULL);
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cu->line_header = *slot;
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return;
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}
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}
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/* dwarf_decode_line_header does not yet provide sufficient information.
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We always have to call also dwarf_decode_lines for it. */
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cu->line_header = dwarf_decode_line_header (line_offset, cu);
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if (cu->line_header == NULL)
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return;
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if (dwarf2_per_objfile->line_header_hash == NULL)
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slot = NULL;
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else
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{
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slot = htab_find_slot_with_hash (dwarf2_per_objfile->line_header_hash,
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&line_header_local,
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line_header_local_hash, INSERT);
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gdb_assert (slot != NULL);
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}
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if (slot != NULL && *slot == NULL)
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{
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/* This newly decoded line number information unit will be owned
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by line_header_hash hash table. */
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*slot = cu->line_header;
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}
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else
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{
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/* We cannot free any current entry in (*slot) as that struct line_header
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may be already used by multiple CUs. Create only temporary decoded
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line_header for this CU - it may happen at most once for each line
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number information unit. And if we're not using line_header_hash
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then this is what we want as well. */
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gdb_assert (die->tag != DW_TAG_partial_unit);
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make_cleanup (free_cu_line_header, cu);
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}
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decode_mapping = (die->tag != DW_TAG_partial_unit);
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dwarf_decode_lines (cu->line_header, comp_dir, cu, NULL, lowpc,
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decode_mapping);
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}
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/* Process DW_TAG_compile_unit or DW_TAG_partial_unit. */
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@ -16915,6 +17028,16 @@ free_line_header (struct line_header *lh)
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xfree (lh);
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}
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/* Stub for free_line_header to match void * callback types. */
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static void
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free_line_header_voidp (void *arg)
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{
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struct line_header *lh = arg;
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free_line_header (lh);
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}
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/* Add an entry to LH's include directory table. */
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static void
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@ -17045,6 +17168,9 @@ dwarf_decode_line_header (unsigned int offset, struct dwarf2_cu *cu)
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back_to = make_cleanup ((make_cleanup_ftype *) free_line_header,
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(void *) lh);
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lh->offset.sect_off = offset;
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lh->offset_in_dwz = cu->per_cu->is_dwz;
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line_ptr = section->buffer + offset;
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/* Read in the header. */
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@ -17672,17 +17798,22 @@ dwarf_decode_lines_1 (struct line_header *lh, struct dwarf2_cu *cu,
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E.g. expand_line_sal requires this when finding psymtabs to expand.
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A good testcase for this is mb-inline.exp.
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LOWPC is the lowest address in CU (or 0 if not known). */
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LOWPC is the lowest address in CU (or 0 if not known).
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Boolean DECODE_MAPPING specifies we need to fully decode .debug_line
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for its PC<->lines mapping information. Otherwise only the filename
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table is read in. */
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static void
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dwarf_decode_lines (struct line_header *lh, const char *comp_dir,
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struct dwarf2_cu *cu, struct partial_symtab *pst,
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CORE_ADDR lowpc)
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CORE_ADDR lowpc, int decode_mapping)
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{
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struct objfile *objfile = cu->objfile;
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const int decode_for_pst_p = (pst != NULL);
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dwarf_decode_lines_1 (lh, cu, decode_for_pst_p, lowpc);
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if (decode_mapping)
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dwarf_decode_lines_1 (lh, cu, decode_for_pst_p, lowpc);
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if (decode_for_pst_p)
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{
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@ -21779,6 +21910,9 @@ dwarf2_free_objfile (struct objfile *objfile)
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if (dwarf2_per_objfile->quick_file_names_table)
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htab_delete (dwarf2_per_objfile->quick_file_names_table);
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if (dwarf2_per_objfile->line_header_hash)
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htab_delete (dwarf2_per_objfile->line_header_hash);
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/* Everything else should be on the objfile obstack. */
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}
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