From 527f3840e1af8bc2e3173922ddae15d0021ed9b1 Mon Sep 17 00:00:00 2001 From: Jan Kratochvil Date: Sat, 24 Jan 2015 15:44:52 +0100 Subject: [PATCH] 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>: Abbrev Number: 17 (DW_TAG_partial_unit) DW_AT_stmt_list : 0x0 DW_AT_comp_dir : (indirect string, offset: 0x6f): /usr/src/debug/gdb-7.7.1/build-x86_64-redhat-linux-gnu/gdb <1>: Abbrev Number: 9 (DW_TAG_typedef) DW_AT_name : (indirect string, offset: 0x827dc): size_t
DW_AT_decl_file : 4 DW_AT_decl_line : 212 DW_AT_type : <0xae> Compilation Unit @ offset 0xe4: <0>: Abbrev Number: 13 (DW_TAG_partial_unit) DW_AT_stmt_list : 0x0 DW_AT_comp_dir : (indirect string, offset: 0x6f): /usr/src/debug/gdb-7.7.1/build-x86_64-redhat-linux-gnu/gdb <1>: Abbrev Number: 45 (DW_TAG_typedef) DW_AT_name : (indirect string, offset: 0x251): __off_t DW_AT_decl_file : 3 DW_AT_decl_line : 131 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 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. --- gdb/ChangeLog | 16 +++++ gdb/dwarf2read.c | 162 +++++++++++++++++++++++++++++++++++++++++++---- 2 files changed, 164 insertions(+), 14 deletions(-) diff --git a/gdb/ChangeLog b/gdb/ChangeLog index 10c3855d24..2e7e55945f 100644 --- a/gdb/ChangeLog +++ b/gdb/ChangeLog @@ -1,3 +1,19 @@ +2015-01-24 Jan Kratochvil + + 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. + 2015-01-23 Simon Marchi PR gdb/17416 diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c index 9d765c5091..715b090abc 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) - { - unsigned int line_offset = DW_UNSND (attr); - struct line_header *line_header - = dwarf_decode_line_header (line_offset, cu); + if (attr == NULL) + return; - if (line_header) + 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) + { + slot = htab_find_slot_with_hash (dwarf2_per_objfile->line_header_hash, + &line_header_local, + line_header_local_hash, NO_INSERT); + + /* 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. */ }