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Alive2 consists of several libraries and tools for analysis and verification of LLVM code and transformations. Alive2 includes the following libraries:

  • Alive2 IR
  • Symbolic executor
  • LLVM → Alive2 IR converter
  • Refinement check (aka optimization verifier)
  • SMT abstraction layer

Included tools:

  • Alive drop-in replacement
  • Translation validation plugins for clang and LLVM's opt
  • Standalone translation validation tool: alive-tv (online)
  • Clang drop-in replacement with translation validation (alivecc and alive++)
  • An LLVM IR interpreter that is UB precise (alive-exec)

For a technical introduction to Alive2, please see our paper from PLDI 2021.


Alive2 does not support inter-procedural transformations. Alive2 may produce spurious counterexamples if run with such passes.


We thank the continuous support of all of our sponsors! Alive2 wouldn't be possible without their support.

Google      NLNet      Woven by Toyota      Matter Labs

If your company has benefitted from Alive2 (including having a less buggy LLVM), please consider sponsoring our research lab.


To build Alive2 you need recent versions of:


export LLVM2_HOME=$PWD/llvm-project
export LLVM2_BUILD=$LLVM2_HOME/build
git clone
cd alive2
mkdir build
cd build
cmake -GNinja -DCMAKE_BUILD_TYPE=Release ..

If CMake cannot find the Z3 include directory (or finds the wrong one) pass the -DZ3_INCLUDE_DIR=/path/to/z3/include and -DZ3_LIBRARIES=/path/to/z3/lib/ arguments to CMake.

Building and Running Translation Validation

Alive2's opt and clang translation validation requires a build of LLVM with RTTI and exceptions turned on. The latest version of Alive2 is always intended to be built against the latest version of LLVM, using the main branch from the LLVM repo on Github. LLVM can be built in the following way.

  • You may prefer to add -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ to the CMake step if your default compiler is gcc.
  • Explicitly setting the target may not be necessary.
  • BUILD_SHARED_LIBS may not be necessary, and for LLVM forks not normally built with the option, may interfere with CMake files’ use of USEDLIBS and LLVMLIBS, and perhaps dd_llvm_target.
  • To build with Xcode rather than Ninja, replace -GNinja with -GXcode in the cmake step below, and append -DLLVM_MAIN_SRC_DIR=$LLVM2_HOME/llvm.
    • It may be necessary to disable warnings for “Implicit Conversion to 32 Bit Type” in the project build settings.
    • Xcode may place tv.dylib in a different location; a symbolic link from the actual location to that in the resultant error message may help.
mkdir build
cd build

Alive2 should then be configured and built as follows:

cd $ALIVE2_HOME/alive2/build

Translation validation of one or more LLVM passes transforming an IR file on Linux:

$LLVM2_BUILD/bin/opt -load $ALIVE2_HOME/alive2/build/tv/ -load-pass-plugin $ALIVE2_HOME/alive2/build/tv/ -tv -instcombine -tv -o /dev/null foo.ll

For the new pass manager:

$LLVM2_BUILD/bin/opt -load $ALIVE2_HOME/alive2/build/tv/ -load-pass-plugin $ALIVE2_HOME/alive2/build/tv/ -passes=tv -passes=instcombine -passes=tv -o /dev/null $LLVM2_HOME/llvm/test/Analysis/AssumptionCache/basic.ll

On a Mac with the old pass manager:

$LLVM2_BUILD/bin/opt -load $ALIVE2_HOME/alive2/build/tv/tv.dylib -load-pass-plugin $ALIVE2_HOME/alive2/build/tv/tv.dylib -tv -instcombine -tv -o /dev/null foo.ll

On a Mac with the new pass manager:

$LLVM2_BUILD/bin/opt -load $ALIVE2_HOME/alive2/build/tv/tv.dylib -load-pass-plugin $ALIVE2_HOME/alive2/build/tv/tv.dylib -passes=tv -passes=instcombine -passes=tv -o /dev/null $LLVM2_HOME/llvm/test/Analysis/AssumptionCache/basic.ll

You can run any pass or combination of passes, but on the command line they must be placed in between the two invocations of the Alive2 -tv pass.

Translation validation of a single LLVM unit test, using lit:

$LLVM2_BUILD/bin/llvm-lit -vv -Dopt=$ALIVE2_HOME/alive2/build/ $LLVM2_HOME/llvm/test/Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-sge-to-icmp-sle.ll

The output should be:

-- Testing: 1 tests, 1 threads --
PASS: LLVM :: Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-sge-to-icmp-sle.ll (1 of 1)
Testing Time: 0.11s
  Expected Passes    : 1

To run translation validation on all the LLVM unit tests for IR-level transformations:

$LLVM2_BUILD/bin/llvm-lit -s -Dopt=$ALIVE2_HOME/alive2/build/ $LLVM2_HOME/llvm/test/Transforms

We run this command on the main LLVM branch each day, and keep track of the results here. To detect unsound transformations in a local run:

fgrep -r "(unsound)" $ALIVE2_HOME/alive2/build/logs/

Running Alive2 as a Clang Plugin

This plugin tries to validate every IR-level transformation performed by LLVM. Invoke the plugin like this:

clang -O3 $LLVM2_HOME/clang/test/C/C99/n505.c -S -emit-llvm \
  -fpass-plugin=$ALIVE2_HOME/alive2/build/tv/ \
  -Xclang -load -Xclang $ALIVE2_HOME/alive2/build/tv/

Or, more conveniently:

$ALIVE2_HOME/alive2/build/alivecc -O3 -c $LLVM2_HOME/clang/test/C/C99/n505.c

$ALIVE2_HOME/alive2/build/alive++ -O3 -c $LLVM2_HOME/clang/test/Analysis/aggrinit-cfg-output.cpp

The Clang plugin can optionally use multiple cores. To enable parallel translation validation, add the -mllvm -tv-parallel=XXX command line options to Clang, where XXX is one of two parallelism managers supported by Alive2. The first (XXX=fifo) uses alive-jobserver: for details about how to use this program, please consult its help output by running it without any command line arguments. The second parallelism manager (XXX=unrestricted) does not restrict parallelism at all, but rather calls fork() freely. This is mainly intended for developer use; it tends to use a lot of RAM.

Use the -mllvm -tv-report-dir=dir to tell Alive2 to place its output files into a specific directory.

The Clang plugin's output can be voluminous. To help control this, it supports an option to reduce the amount of output (-mllvm -tv-quiet).

Our goal is for the alivecc and alive++ compiler drivers to be drop-in replacements for clang and clang++. So, for example, they try to detect when they are being invoked as assemblers or linkers, in which case they do not load the Alive2 plugin. This means that some projects cannot be built if you manually specify command line options to Alive2, for example using -DCMAKE_C_FLAGS=.... Instead, you can tell alivecc and alive++ what to do using a collection of environment variables that generally mirror the plugin's command line interface. For example:

ALIVECC_SMT_TO=timeout in milliseconds

If validating the program takes a long time, you can batch optimizations to verify. Please set ALIVECC_BATCH_OPTS=1 and run alivecc/alive++.

Running the Standalone Translation Validation Tool (alive-tv)

This tool has two modes.

In the first mode, specify either a source (original) and target (optimized) IR file, or a single file containing a function called “src” and also a function called “tgt”. For example, let’s prove that removing nsw is correct for addition:

$ALIVE2_HOME/alive2/build/alive-tv src.ll tgt.ll

define i32 @f(i32 %a, i32 %b) {
  %add = add nsw i32 %b, %a
  ret i32 %add
define i32 @f(i32 %a, i32 %b) {
  %add = add i32 %b, %a
  ret i32 %add

Transformation seems to be correct!

Flipping the inputs yields a counterexample, since it's not correct, in general, to add nsw. If you are not interested in counterexamples using undef, you can use the command-line argument -disable-undef-input.

In the second mode, specify a single unoptimized IR file. alive-tv will optimize it using an optimization pipeline similar to -O2, but without any interprocedural passes, and then attempt to validate the translation.

For example, as of February 6 2020, the release/10.x branch contains an optimizer bug that can be triggered as follows:

cat foo.ll

define i3 @foo(i3) {
  %x1 = sub i3 0, %0
  %x2 = icmp ne i3 %0, 0
  %x3 = zext i1 %x2 to i3
  %x4 = lshr i3 %x1, %x3
  %x5 = lshr i3 %x4, %x3
  ret i3 %x5

$ALIVE2_HOME/alive2/build/alive-tv foo.ll

define i3 @foo(i3 %0) {
  %x1 = sub i3 0, %0
  %x2 = icmp ne i3 %0, 0
  %x3 = zext i1 %x2 to i3
  %x4 = lshr i3 %x1, %x3
  %x5 = lshr i3 %x4, %x3
  ret i3 %x5
define i3 @foo(i3 %0) {
  %x1 = sub i3 0, %0
  ret i3 %x1
Transformation doesn't verify!
ERROR: Value mismatch

i3 %0 = #x5 (5, -3)

i3 %x1 = #x3 (3)
i1 %x2 = #x1 (1)
i3 %x3 = #x1 (1)
i3 %x4 = #x1 (1)
i3 %x5 = #x0 (0)

i3 %x1 = #x3 (3)
Source value: #x0 (0)
Target value: #x3 (3)

  0 correct transformations
  1 incorrect transformations
  0 errors

Please keep in mind that you do not have to compile Alive2 in order to try out alive-tv; it is available online:

Running the Standalone LLVM Execution Tool (alive-exec)

This tool uses Alive2 as an interpreter for an LLVM function. It is currently highly experimental and has many restrictions. For example, the function cannot take inputs, cannot use memory, cannot depend on undefined behaviors, and cannot include loops that execute too many iterations.


The alive-tv tool and the Alive2 translation validation opt plugin support using an external Redis server to avoid performing redundant queries. This feature is not intended for general use, but rather to speed up certain systematic testing workloads that perform a lot of repeated work. When it hits a repeated refinement check, it prints "Skipping repeated query" instead of performing the query.

If you want to use this functionality, you will need to manually start and stop, as appropriate, a Redis server instance on localhost. Alive2 should be the only user of this server.

Diagnosing Unsoundness Reports

  • Select a failing test file. It may be convenient to choose one whose path is given at the beginning of a log file containing the text "(unsound)" as above; this is guaranteed to contain an unsoundness report. Many log files, however, contain only “Source: <stdin>” rather than a file path; the names of these files begin with “in_”.
  • Do a verbose run of Lit for just that file, with the opt option --print-after-all appended. (You may also append other opt options, such as other optimizations.) E.g.:
$LLVM2_BUILD/bin/llvm-lit -vva "-Dopt=$ALIVE2_HOME/alive2/build/ --print-after-all" $LLVM2_HOME/llvm/test/Transforms/InstCombine/insert-const-shuf.ll
  • Collect Lit’s LLVM IR terminal output, for comparison with Alive2’s Alive2 IR output in the log file indicated by “Report written to…”. Sometimes the Lit output may not contain useful LLVM IR, in which case executing the output RUN command separately may give better results.
  • The Alive2 unsoundness report in the corresponding log file will have two versions of the misoptimized function. The Alive2 IR function body may indicate the problem to a human, but for Alive2 translation validation you will need LLVM IR. Search for the function name in the terminal output.
  • Copy the first function definition and necessary declarations and metadata to either a new file or to the Alive2 Compiler Explorer instance, (The -allow-incomplete-ir flag may make copying declarations and metadata unnecessary.) The Alive2 Compiler Explorer instance will run automatically; to check with the standalone alive-tv, see its instructions above. Without a second version of the function to compare, Alive2 just runs the -O2 optimizations; if it reports unsoundness, your fork’s optimizations are not to blame.
  • If there is a second, unsound, function definition in the LLVM IR terminal output, copy it and necessary declarations, and change the second function name.
  • If it now reports a misoptimization, presumably your fork has a bug, demonstrated by the provided examples.
  • To screen out exact duplicate reports when comparing different test runs, move the logs directory out of the way before each run. After each run, copy the relevant logs to a separate destination directory. (Systems with a non-GNU version of cp will need to use coreutils’ gcp instead.)
fgrep --files-with-matches --recursive "(unsound)" $ALIVE2_HOME/alive2/build/logs/ |  xargs cp -p --target-directory=<Destination>

  • Unique unsoundness reports can then be found with a utility such as jdupes --print-unique.
    • If the tests are run on different LLVM directories, the “Source:” line in files whose name does not begin with “in_”, as well as “Command line:” lines on Linux, should be stripped before comparison.


  • Check the “LLVMConfig.cmake” and “CMAKE_PREFIX_PATH” output from CMake in case of build problems. CMake may look for configuration information in old installations of LLVM, e.g., under /opt/, if these are not set properly.
  • Some combinations of Clang and MacOS versions may give link warnings “-undefined dynamic_lookup may not work with chained fixups,” and runtime errors with “symbol not found in flat namespace.” Setting CMAKE_OSX_DEPLOYMENT_TARGET as a cache entry to 11.0 or less at the beginning of CMakeLists.txt may work around this.
  • Building for Translation Validation is tightly coupled to LLVM top of tree source. Building a fork with older source may require reverting to the corresponding Alive2 commit. This in turn may require experimentation with Clang and SDK versions and vendors.
  • Building older source on an up-to-date machine may require adjustments. For example, the now-deleted file scripts/ depended on the deprecated Python 2; update the shebang line to python3.
  • The opt wrapper script build/ accepts a --verbose option, which outputs the command passed to opt. Note that this may interfere with tests which check output.
  • The script also accepts a --no-timeout option, which disables the opt process timeout. This timeout is not supported on Macintosh. To change the SMT timeout, instead pass an -smt-to: option to the alive executable.

LLVM Bugs Found by Alive2 shows the list of LLVM bugs found by Alive2.