Inspect GCC 13 LTO Objects with lto-dump

You will inspect a GCC link-time optimisation object, list the symbols it contains, examine its LTO sections and ask for details about one symbol. The commands are read-only: they do not rewrite the object or relink your program.

Allow about fifteen minutes if you already have an object compiled with -flto. You need the GCC 13 toolchain and a readable LTO object. The examples use Ubuntu's installed GCC 13.3.0 packages and the command x86_64-linux-gnu-lto-dump-13.

1. Confirm the installed command

Start with ordinary, unprivileged checks. No sudo is needed unless the object is in a directory you cannot read:

$ command -v x86_64-linux-gnu-lto-dump-13
/usr/bin/x86_64-linux-gnu-lto-dump-13
$ dpkg-query -W -f='${Package} ${Version}\n' gcc-13 gcc-13-x86-64-linux-gnu gcc-x86-64-linux-gnu
gcc-13 13.3.0-6ubuntu2~24.04.1
gcc-13-x86-64-linux-gnu 13.3.0-6ubuntu2~24.04.1
gcc-x86-64-linux-gnu 4:13.2.0-7ubuntu1

The version shown above is the package version on the machine used for this guide. The executable does not print a useful version string for --version, so record the package version when you need reproducible diagnostics.

Ask the installed binary for its own option summary:

$ x86_64-linux-gnu-lto-dump-13 -help
Usage: lto-dump [OPTION]... SUB_COMMAND [OPTION]...

LTO dump tool command line options.

Use the target-prefixed command when you want to be explicit about the architecture. The unprefixed alias lto-dump-13 is also installed on this system.

2. Produce a small LTO object for a safe test

If you already have the object you need to inspect, skip to the next step. Otherwise create a temporary source file and compile it without elevated privileges:

$ workdir=$(mktemp -d /tmp/lto-dump-example.XXXXXX)
$ printf '%s\n' 'int answer(void) { return 42; }' > "$workdir/sample.c"
$ gcc-13 -O2 -flto -c "$workdir/sample.c" -o "$workdir/sample.o"
$ file "$workdir/sample.o"
/tmp/lto-dump-example.XXXXXX/sample.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped

The random directory suffix and exact file wording vary. The useful result is a relocatable object produced with -flto. The compiler has not created a service, installed anything or changed a system setting.

3. List the functions and variables

-list is the best first inspection. It prints a table with the symbol type, visibility, size and name:

$ x86_64-linux-gnu-lto-dump-13 -list "$workdir/sample.o"
Type   Visibility  Size  Name
function  default     3  answer

Real objects often contain many more entries. Use -defined-only to omit symbols that are not defined in the object, and use -demangle when the input contains C++ names. The two options are sub-options of -list:

$ x86_64-linux-gnu-lto-dump-13 -list -defined-only -demangle "$workdir/sample.o"
$ x86_64-linux-gnu-lto-dump-13 -list -name-sort "$workdir/sample.o"
$ x86_64-linux-gnu-lto-dump-13 -list -size-sort "$workdir/sample.o"
$ x86_64-linux-gnu-lto-dump-13 -list -reverse-sort "$workdir/sample.o"

Sorting changes presentation only. It does not change the object. -print-value adds initial variable values when those values are available in the dump data.

One version-specific trap is worth recording: the installed GCC 13.3.0 binary rejects -no-sort, although the installed manpage lists it. Trust the executable's help and test the exact command on the host where a script will run; do not build automation around that documented option without checking its availability.

4. Inspect one symbol in detail

Once -list gives you a name, pass it to -symbol=. Keep the value attached to the option:

$ x86_64-linux-gnu-lto-dump-13 -symbol=answer "$workdir/sample.o"
answer/0 (answer)
  Type: function definition analyzed
  Visibility: externally_visible semantic_interposition public
  References:
  Referring:
  Read from file: /tmp/lto-dump-example.XXXXXX/sample.o
  Availability: available
  Unit id: 1
  Function flags: count:1073741824 (estimated locally)
  Called by:
  Calls:
Symbol: answer

Paths, unit identifiers and estimates are input-dependent. Treat the output as compiler analysis data, not as a promise that the function will survive unchanged into the final executable. If the name is absent, rerun -list and copy an exact symbol name rather than guessing a mangled spelling.

5. See the LTO object sections

-objects reports the LTO objects and the sections stored inside them:

$ x86_64-linux-gnu-lto-dump-13 -objects "$workdir/sample.o"
    LTO Object Name: /tmp/lto-dump-example.XXXXXX/sample.o

No.    Offset    Size       Section Name

 1       64       15   .gnu.lto_.profile.983b7ac5fa8de62
 2       79       26   .gnu.lto_.icf.983b7ac5fa8de62
 3      105       22   .gnu.lto_.ipa_sra.983b7ac5fa8de62
 ...

The section names and offsets tell you what LTO data is present and where it sits in the object. They are useful when comparing compiler outputs or investigating a toolchain problem. They are not a replacement for readelf or objdump when you need general ELF headers or machine code.

6. Request statistics or a call graph

The installed command also accepts analysis subcommands for type, tree and GIMPLE statistics. For example:

$ x86_64-linux-gnu-lto-dump-13 -type-stats "$workdir/sample.o"
       Type     Frequency   Percentage

 function_type      1       100.00

On this build, -tree-stats and -gimple-stats can warn that the binary was not configured with detailed memory statistics. That warning describes a build capability; it is not evidence that the object is damaged.

-callgraph is shown by the installed help and emits Graphviz format. It is not listed in the installed manpage, so treat it as an installed-binary feature rather than a portable GCC 13 interface. Save output to a new file if you want to render it:

$ x86_64-linux-gnu-lto-dump-13 -callgraph "$workdir/sample.o" > "$workdir/callgraph.dot"
$ head -n 1 "$workdir/callgraph.dot"
digraph symtab {

Redirection with > truncates an existing destination before the command runs. Use a new path, as above, or make a deliberate backup first. If the command fails, remove only the incomplete new file; the input object remains unchanged.

7. Keep failures in proportion

A normal inspection should need no elevated privileges. If the input cannot be opened, check its path and permissions:

$ ls -l /path/to/object.o
$ test -r /path/to/object.o && echo readable

Use sudo only when that read check identifies an access requirement and your local policy permits it. Root access will not turn a non-LTO object into an LTO object. If the file is an executable or a regular object compiled without -flto, use tools intended for that format instead.

For a repeatable report, capture the package versions, the exact command, the input path and the exit status. Do not treat empty sections, missing symbols or an empty call graph as a failure without checking how the object was compiled and which subcommand you selected.