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Alive2

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.

WARNING

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

Prerequisites

To build Alive2 you need recent versions of:

  • cmake
  • gcc/clang
  • re2c
  • Z3
  • LLVM (optional)
  • hiredis (optional, needed for caching)

Building

mkdir build
cd build
cmake -GNinja -DCMAKE_BUILD_TYPE=Release ..
ninja

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/libz3.so 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. LLVM can be built in the following way:

cd llvm
mkdir build
cd build
cmake -GNinja -DLLVM_ENABLE_RTTI=ON -DLLVM_ENABLE_EH=ON -DBUILD_SHARED_LIBS=ON -DCMAKE_BUILD_TYPE=Release -DLLVM_TARGETS_TO_BUILD=X86 -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_ENABLE_PROJECTS="llvm;clang" ../llvm

Alive2 should then be configured as follows:

cmake -GNinja -DCMAKE_PREFIX_PATH=~/llvm/build -DBUILD_TV=1 -DCMAKE_BUILD_TYPE=Release ..

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

~/llvm/build/bin/opt -load $HOME/alive2/build/tv/tv.so -load-pass-plugin $HOME/alive2/build/tv/tv.so -tv -instcombine -tv -o /dev/null foo.ll

On a Mac:

~/llvm/build/bin/opt -load $HOME/alive2/build/tv/tv.dylib -load-pass-plugin $HOME/alive2/build/tv/tv.dylib -tv -instcombine -tv -o /dev/null foo.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:

~/llvm/build/bin/llvm-lit -vv -Dopt=$HOME/alive2/build/opt-alive.sh ~/llvm/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:

~/llvm/build/bin/llvm-lit -vv -Dopt=$HOME/alive2/build/opt-alive.sh ~/llvm/llvm/test/Transforms

We run this command on the main LLVM branch each day, and keep track of the results here.

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 <src.c> -S -emit-llvm \
  -fpass-plugin=$HOME/alive2/build/tv/tv.so \
  -Xclang -load -Xclang $HOME/alive2/build/tv/tv.so

Or, more conveniently:

$ $HOME/alive2/build/alivecc -O3 -c <src.c>
$ $HOME/alive2/build/alive++ -O3 -c <src.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_PARALLEL_UNRESTRICTED=1
ALIVECC_PARALLEL_FIFO=1
ALIVECC_DISABLE_UNDEF_INPUT=1
ALIVECC_DISABLE_POISON_INPUT=1
ALIVECC_SMT_TO=timeout in milliseconds
ALIVECC_SUBPROCESS_TIMEOUT=timeout in seconds
ALIVECC_OVERWRITE_REPORTS=1
ALIVECC_REPORT_DIR=dir

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 a source (original) and target (optimized) IR file. For example, let's prove that removing nsw is correct for addition:

$ 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
}
$ 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

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

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

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

Summary:
  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: https://alive2.llvm.org/ce/

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.

Caching

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.

LLVM Bugs Found by Alive2

BugList.md shows the list of LLVM bugs found by Alive2.