dwz

Section: User Commands (1)
Updated: 15 Feb 2021
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NAME

dwz - DWARF optimization and duplicate removal tool

SYNOPSIS

dwz [OPTION...] [FILES]

DESCRIPTION

dwz is a program that attempts to optimize DWARF debugging information contained in ELF shared libraries and ELF executables for size, by replacing DWARF information representation with equivalent smaller representation where possible and by reducing the amount of duplication using techniques from DWARF standard appendix E - creating DW_TAG_partial_unit compilation units (CUs) for duplicated information and using DW_TAG_imported_unit to import it into each CU that needs it.

The tool handles DWARF 32-bit format debugging sections of versions 2, 3, 4, most of version 5 and GNU extensions on top of those. It is strongly recommended to use at least DWARF 3, but using DWARF 4 or higher will work much better.

While most of DWARF 5 is supported dwz doesn't yet generate spec compliant DWARF Supplementary Object Files (DWARF 5, section 7.3.6) unless the --dwarf-5 option is used. Instead of a .debug_sup section it will generate by default a .gnu_debugaltlink section. And it will use the DW_FORM_GNU_strp_alt and DW_FORM_GNU_reg_alt, instead of DW_FORM_strp_sup and DW_FORM_ref_sup to keep compatibility with existing DWARF consumers.

DWARF 4 .debug_types are supported, but DWARF 5 DW_UT_type units are not. Likewise .gdb_index is supported, but the DWARF 5 .debug_names is not. Also some forms and sections that are only emitted by GCC when generating Split DWARF, DW_FORM_strx and .debug_str_offsets, DW_FORM_addrx and .debug_addr, DW_FORM_rnglistx and DW_FORM_loclistsx, are not supported yet.

The tool has two main modes of operation, without the -m option it attempts to optimize DWARF debugging information in each given object (executable or shared library) individually, with the -m option it afterwards attempts to optimize even more by moving DWARF debugging information entries (DIEs), strings and macro descriptions duplicated in more than one object into a newly created ELF ET_REL object whose filename is given as -m option argument. The debug sections in the executables and shared libraries specified on the command line are then modified again, referring to the entities in the newly created object.

OPTIONS

-m FILE --multifile FILE: Multifile mode. After processing all named executables and shared libraries, attempt to create ELF object FILE and put debugging information duplicated in more than one object there, afterwards optimize each named executable or shared library even further if possible.
-h --hardlink: Look for executables or shared libraries hardlinked together, instead of rewriting them individually rewrite just one of them and hardlink the rest to the first one again.
-M NAME --multifile-name NAME: Specify the name of the common file that should be put into the .gnu_debugaltlink section alongside with its build ID. By default dwz puts there the argument of the -m option.
-r --relative: Specify that the name of the common file to be put into the .gnu_debugaltlink section is supposed to be relative path from the directory containing the executable or shared library to the file named in the argument of the -m option. Either -M or -r option can be specified, but not both.
-q --quiet: Silence up some of the most common messages.
-o FILE --output FILE: This option instructs dwz not to overwrite the specified file, but instead store the new content into FILE. Nothing is written if dwz exits with non-zero exit code. Can be used only with a single executable or shared library (if there are no arguments at all, a.out is assumed).
-l <COUNT|none> --low-mem-die-limit <COUNT|none>: Handle executables or shared libraries containing more than COUNT debugging information entries in their .debug_info section using a slower and more memory usage friendly mode and don't attempt to optimize that object in multifile mode. The default is 10 million DIEs. There is a risk that for very large amounts of debugging information in a single shared library or executable there might not be enough memory (especially when dwz tool is 32-bit binary, it might run out of available virtual address space even sooner). Specifying none as argument disables the limit.
-L <COUNT|none> --max-die-limit <COUNT|none>: Don't attempt to optimize executables or shared libraries containing more than COUNT DIEs at all. The default is 50 million DIEs. Specifying none as argument disables the limit.
-5 --dwarf-5: Emit standard DWARF 5 Supplementary Object Files with .debug_sup and corresponding forms, instead of the GNU extension .gnu_debugaltlink and corresponding forms.
--odr / --no-odr: Experimental. Enable/disable One-Definition-Rule optimization for C++ compilation units. This optimization causes struct/union/class DIEs with the same name to be considered equal. This has the effect that DIEs referring to distinct DIEs representing the same type (like f.i. pointer type DIEs) are considered equal, and may be deduplicated. The status of this optimization is experimental. It's disabled in low-mem mode. Disabled by default.
--odr-mode=<basic|link>: Set the One-Definition-Rule optimization aggressiveness: basic or link. When using the link setting, the optimization will attempt to replace declarations of a struct/union/class with a corresponding definition. When using the basic setting, that part of the optimization is disabled. In normal operation, the link setting should be used. The basic setting is provided only as fallback in case of problems with the link setting. Set to link by default.
--import-optimize / --no-import-optimize: Enable/disable optimization that reduces the number of DW_TAG_imported_unit DIEs generated to import the partial units created by dwz. Disabling the optimization can be used to work around problems in the optimization, or to make it easier to observe which CU imports which PU. Enabled by default.
-? --help: Print short help and exit.
-v --version: Print version number and short licensing notice and exit.

ARGUMENTS

Command-line arguments should be the executables, shared libraries or their stripped to file separate debug information objects.

EXAMPLES

: $ dwz -m .dwz/foobar-1.2.debug -rh \
bin/foo.debug bin/foo2.debug foo/lib/libbar.so.debug

will attempt to optimize debugging information in bin/foo.debug, bin/foo2.debug and lib/libbar.so.debug (by modifying the files in place) and when beneficial also will create .dwz/foobar-1.2.debug file. .gnu_debugaltlink section in the first two files will refer to ../.dwz/foobar-1.2.debug and in the last file to ../../.dwz/foobar-1.2.debug. If e.g. bin/foo.debug and bin/foo2.debug were hardlinked together initially, they will be hardlinked again and for multifile optimizations considered just as a single file rather than two.

: $ dwz -o foo.dwz foo

will not modify foo but instead store the ELF object with optimized debugging information if successful into foo.dwz file it creates.

: $ dwz *.debug foo/*.debug

will attempt to optimize debugging information in *.debug and foo/*.debug files, optimizing each file individually in place.

: $ dwz

is equivalent to dwz a.out command.

AUTHORS

Jakub Jelinek <jakub@redhat.com>, Tom de Vries <tdevries@suse.de>, Mark Wielaard <mark@klomp.org>

BUGS

Use the Bugzilla link of the project web page or our mailing list. https://sourceware.org/dwz/, <dwz@sourceware.org>.

dwz