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527f3840e1
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. |
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README for GNU development tools This directory contains various GNU compilers, assemblers, linkers, debuggers, etc., plus their support routines, definitions, and documentation. If you are receiving this as part of a GDB release, see the file gdb/README. If with a binutils release, see binutils/README; if with a libg++ release, see libg++/README, etc. That'll give you info about this package -- supported targets, how to use it, how to report bugs, etc. It is now possible to automatically configure and build a variety of tools with one command. To build all of the tools contained herein, run the ``configure'' script here, e.g.: ./configure make To install them (by default in /usr/local/bin, /usr/local/lib, etc), then do: make install (If the configure script can't determine your type of computer, give it the name as an argument, for instance ``./configure sun4''. You can use the script ``config.sub'' to test whether a name is recognized; if it is, config.sub translates it to a triplet specifying CPU, vendor, and OS.) If you have more than one compiler on your system, it is often best to explicitly set CC in the environment before running configure, and to also set CC when running make. For example (assuming sh/bash/ksh): CC=gcc ./configure make A similar example using csh: setenv CC gcc ./configure make Much of the code and documentation enclosed is copyright by the Free Software Foundation, Inc. See the file COPYING or COPYING.LIB in the various directories, for a description of the GNU General Public License terms under which you can copy the files. REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info on where and how to report problems.