DEAD: Dead Code Elimination based Automatic Differential Testing

DEAD is a tool to find and process compiler regressions and other missed optimizations automatically to produce reports.

It is based on the paper Finding missed optimizations through the lens of dead code elimination and was first written during a master thesis at the AST Lab.

For a list of reported bugs look at bugs.md.

Setup

Clone the latest release with, for example, git clone -b v0.0.2 https://github.com/DeadCodeProductions/dead.

After navigating into the cloned repository, choose if you want to run DEAD locally or in a Docker container.

Setup with Docker

./build_docker.sh 

# Enter the container
docker run -it -v $(realpath ./docker_storage):/persistent deaddocker

Continue by reading the Run Section.

Local Setup

The following programs or libraries must be installed:

• python >= 3.10
• gcc
• clang
• csmith
• creduce
• cmake
• ccomp (CompCert)
• llvm 13.0.0 or llvm 14.0.0 (for the include files)
• compiler-rt (for the sanitization libraries. It's also part of LLVM)
• boost
• ninja

Optional programs:

• entr

We are running on Arch Linux and have not (yet) tested any other distribution.

To achieve this in Arch with yay as AUR overlay helper, you can run:

yay -Sy --noconfirm python\
                    python-pip\
                    gcc\
                    clang\
                    llvm\
                    compiler-rt\
                    cmake\
                    boost\
                    ninja\
                    csmith\
                    creduce-git\
                    compcert-git

Then run:

# Create python environment
python3 -m venv ./deadenv
source ./deadenv/bin/activate
pip install -r requirements.txt

# Initialize DEAD
./init.py

init.py will:

• create a config file located at ~/.config/dead/config.json
• Compile the callchain-checker: ccc
• Clone repositories of gcc and llvm into the local directory
• Create the compiler_cache and logs directory
• Check if it can find the programs and paths required in the prerequisite-section and complain if not.

Run

As DEAD is based on differential testing, it requires two informations to be able to run:

• Which compilers to find missed optimizations in. These are called target compilers. This is typically the current trunk.
• Which compilers to use as a comparison to find missed optimizations in the target compilers. These are called additional or attacking compilers.

A compiler on the CLI is specified by writing PROJECT REVISION [OPT_LEVEL ...]. For example, to get gcc 11.2.0 with all optimizations, write gcc releases/gcc-11.2.0 1 2 3 s. This can be repeated to specify more compilers.

# Don't run it yet
./main.py run --targets gcc trunk 1 2 3 s\
              --additional-compilers\
                   gcc releases/gcc-11.2.0 1 2 3 s\
                   gcc releases/gcc-10.3.0 1 2 3 s

To not have to repeat oneself, it is possible to specify default optimization levels. The resulting used optimizations for a specified compiler is the union of the default levels and the specifically specified optimization levels. The flags are --additional_compilers_default_opt_levels and --targets_default_opt_levels or -acdol and -tdol respectively.

# Don't run it yet
./main.py run --targets gcc trunk 1 2 3 s\
              --additional-compilers\
                   gcc releases/gcc-11.2.0 \ # Opt levels: 3,s
                   gcc releases/gcc-10.3.0 1\ # Opt levels: 1,3,s
              -acdol 3 s # Additional compilers 

DEAD consists of three parts which are:

• Generator, which finds missed optimizations from the given target and attacking compilers. We call such a missed optimization and any additional information related to it a case.
• Bisector, which finds the introducing commit of the found case.
• Reducer, which extracts a small part of the code, which still exhibits the missed optimization found.

By default, the Reducer is only enabled for cases which have a new bisection commit, as reducing takes long and is often not necessary. It can be enabled for all cases with --reducer and completely disabled with --no-reducer.

The last two important options are --cores POSITIVE_INT and --log-level debug|info|warning|error|critical. When not specified, --cores will equal to the amount of logical cores on the machine. The default verbosity level is warning. However, to have a sense of progress, we suggest setting it to info.

Finally, to find missed optimizations in trunk, run

# For GCC
./main.py -ll info\
          --cores $CORES\
          run --targets gcc trunk 1 2 3 s\ 
              --additional-compilers\
                   gcc releases/gcc-11.2.0\
                   gcc releases/gcc-10.3.0\
                   gcc releases/gcc-9.4.0\
                   gcc releases/gcc-8.5.0\
                   gcc releases/gcc-7.5.0\
              -acdol 1 2 3 s
              #--amount N # Terminate after finding N cases

# For LLVM
./main.py -ll info\
          --cores $CORES\
          run --targets llvm trunk 1 2 3 s z\ 
              --additional-compilers\
                   llvm llvmorg-13.0.1\
                   llvm llvmorg-12.0.1\
                   llvm llvmorg-11.1.0\
                   llvm llvmorg-10.0.1\
                   llvm llvmorg-9.0.1\
                   llvm llvmorg-8.0.1\
                   llvm llvmorg-7.1.0\
                   llvm llvmorg-6.0.1\
                   llvm llvmorg-5.0.2\
                   llvm llvmorg-4.0.1\
              -acdol 1 2 3 s z
              #--amount N # Terminate after finding N cases

Please run ./main.py run -h and ./main.py -h to see more options.

Performance considerations

Assigning all cores of the machine to just one instance of DEAD can lead to less than optimal machine utilization. Some parts of the pipeline can not always use all cores.

• The Bisector is written in a single threaded way and only requires multiple cores when building a new compiler. As the cache grows and many regression have already been found, the cache hit rate increases drastically, making the Bisector essentially a single threaded part.
• GCC compilation includes several single-threaded parts. Compiling with sufficiently many cores will make it look like a mostly single-threaded task due to Amdahl's law. LLVM compilation also includes some single-threaded parts, but these are way less noticeable.
• The Reducer uses creduce to shrink the case. creduce also does not perfectly utilize the machine all the time, when using a lot of threads.

Just oversubscribing the machine is not an option, as some checks are time dependent. Failing these checks will especially impact the throughput of the Reducer.

One fairly good solution is to run multiple smaller instances in parallel.

For the Reducer, 8 logical cores per pipeline did yield good results.

Finding new cases in parallel has the big caveat that the instances wait on each other when one is building a compiler that the other needs. This dependence is very common when the cache is not populated enough. Running multiple instances in parallel too early is detrimental to machine utilization!

Pinpointing when the switch to multiple instances is beneficial is difficult. For this reason we provide run_parallel.sh which spawns multiple instances with the appropriate amount of cores assigned.

./run_parallel.sh llvm|gcc TOTAL_CORES AMOUNT_JOBS

Generating a report

Imagine DEAD ran for some time and it is now time to create a bug report.

Not-yet-reported cases can be explored with the unreported sub-command.

$ ./main.py unreported
ID       Bisection                                     Count
----------------------------------------------------------------
2        0b92cf305dcf34387a8e2564e55ca8948df3b47a      45
...
39       008e7397dad971c03c08fc1b0a4a98fddccaaed8      1
----------------------------------------------------------------
ID       Bisection                                     Count

On the left you see an ID for a case that has the bisection commit shown in the bisection column. Often times, many cases are found which bisect to the same commit. How many cases that bisected to this particular commit have been found is displayed in the 'Count' column. Note that a fix for a reported case may not fix all cases of the bisection!

Select one of the IDs and check if there is already a bug report which includes its bisection commit.

If this is not the case, run

./main.py report $ID > report.txt

It will pull the compiler project of the case, build trunk and test if the missed optimization can still be observed. You can disable pulling with --no-pull. If so, it will output a copy-and-pasteable report into report.txt (don't forget to remove the title if there is one) and case.txt¹, a copy of the reported code.

When you have submitted the bug report, you can save the link to the report via

./main.py set link $ID $LINK

so that the bisection isn't displayed anymore.

Hopefully, the missed optimization gets fixed. When this is the case, you can extract the case ID from the bug report and note down the fixing commit. Then save it with

./main.py set fixed $ID $COMMIT

Inspecting reported cases can be done via

./main.py reported

Massaging workflow

Sometimes it is possible to further reduce the automatically reduced code manually. We call this step massaging, the product of which is massaged code.

Instead of directly generating the report after having selected an ID and checked if the bisection commit was already reported, get the reduced code and try to make it smaller.

./main.py get rcode $ID > rcode.c

To continuously check if the changes still exhibit the missed optimization, open a separate terminal in the same directory and run

echo rcode.c | entr -c ./main.py checkreduced $ID ./rcode.c

This will rerun some checks whenever rcode.c is saved.

When the massaging is done, save it into DEAD with

./main.py set mcode $ID ./rcode.c

DEAD will check if the massaged code still bisects to the same commit as before and will reject the change if not. Empirically, changes to cases who's bisection is rarely found often don't allow any further massaging.

Subcommand overview of main.py

• run: Find new regressions/missed optimizations.
• tofile ID: Save a case into a tar-file.
• absorb PATH: Read tar-files into the database of DEAD.
• report ID: Generate a report for a given case.
• rereduce ID FILE: Reduce a file (again) w.r.t. a case.
• diagnose: Run a set of tests when something seems odd with a case.
• checkreduced ID FILE: Run some lightweight tests based on a case on a piece of code.
• cache: Cache related functionality.
• asm ID: Generate assembly for all code of a case.
• set | get {link,fixed,mcode,rcode,ocode,bisection}: Set or get the specified field of a case.
• build PROJECT REV: Build REV of compiler project PROJECT.
• reduce ID: Reduce case ID.
• edit: Open DEADs configuration in $EDITOR
• unreported: List unreported cases grouped by bisection commit.
• reported: List reported cases.
• findby: Find case ID given some part of the case.

Overview of important files

• bisector.py: Bisects a given interesting case.
• builder.py: Builds the compiler.
• checker.py: Checks if a given case is interesting.
• generator.py: Finds new interesting cases.
• patcher.py: Automatically finds the region in the history where a patch needs to be applied.
• reducer.py: Reduce the code of a given.

Q&A for potential issues

I set flag X which I found in the help, but DEAD says the option does not exist!

Sadly, flags are position dependent. You have to put it after the command whose help you found the flag in and before any other subcommand.

I want to do XYZ. How?

Maybe there's already an option for it. Consult the program with --help for all the options.

Why don't I see anything?

Are you running with -ll info?

DEAD wants to work with a commit that doesn't exist!

If you are checking things manually: Are you sure you are looking in the right repository?

If you are processing a case and git throws an exception, try pulling llvm-project and gcc so you are sure to have all the commits.

Why does this case fail?

Maybe ./main.py diagnose -ci $ID can illuminate the situation.

This case does not reduce but diagnose says everything is fine!

Try throwing your whole machine at it (./main.py reduce ID).

The compilers should already be built and the logs just say INFO:root: [...] is currently building; need to wait.

Stop DEAD, run ./main.py cache clean, restart. What happened? The most likely scenario is that DEAD was interrupted while building a compiler and unable to run the clean-up procedure, confusing DEAD the next time the compiler has to be built. Do not run cache clean while DEAD is running.

A compiler I want to build has a build issue; where do I find the build-logs?

The logs can be found in the path specified by the logdir entry of the $HOME/.config/dead/config.json. For an installation with init.py, this is $PROJECTDIR/logs. For a docker installation this is /persistent/logs in the container.

Footnotes

1. It is .txt instead of .c because GitHub does not allow .c files to be attached to issues. ↩