llvm-tutor

Example LLVM passes - based on LLVM 10

llvm-tutor is a collection of self-contained reference LLVM passes. It's a tutorial that targets novice and aspiring LLVM developers. Key features:

Out-of-tree - builds against a binary LLVM installation (no need to build LLVM from sources)
Complete - includes CMake build scripts, LIT tests, CI set-up and documentation
Modern - based on the latest version of LLVM (and updated with every release)

Overview

LLVM implements a very rich, powerful and popular API. However, like many complex technologies, it can be quite daunting and overwhelming to learn and master. The goal of this LLVM tutorial is to showcase that LLVM can in fact be easy and fun to work with. This is demonstrated through a range self-contained, testable LLVM passes, which are implemented using idiomatic LLVM.

This document explains how to set-up your environment, build and run the examples, and go about debugging. It contains a high-level overview of the implemented examples and contains some background information on writing LLVM passes. The source files, apart from the code itself, contain comments that will guide you through the implementation. All examples are complemented with LIT tests and reference input files.

If you are interested in discovering how to write Clang plugins then visit my other tutorial, clang-tutor, that fallows similar format.

HelloWorld: Your First Pass

The HelloWorld pass from HelloWorld.cpp is a self-contained reference example. The corresponding CMakeLists.txt implements the minimum set-up for an out-of-source pass.

For every function defined in the input module, HelloWord prints its name and the number of arguments that it takes. You can build it like this:

export LLVM_DIR=<installation/dir/of/llvm/10>
mkdir build
cd build
cmake -DLT_LLVM_INSTALL_DIR=$LLVM_DIR <source/dir/llvm/tutor>/HelloWorld/
make

Before you can test it, you need to prepare an input file:

# Generate an LLVM test file
$LLVM_DIR/bin/clang -S -emit-llvm <source/dir/llvm/tutor/>inputs/input_for_hello.c -o input_for_hello.ll

Finally, run HelloWorld with opt (use libHelloWorld.so on Linux and libHelloWorld.dylib on Mac OS):

# Run the pass
$LLVM_DIR/bin/opt -load-pass-plugin ./libHelloWorld.{so|dylib} -passes=hello-world -disable-output input_for_hello.ll
# Expected output
(llvm-tutor) Hello from: foo
(llvm-tutor)   number of arguments: 1
(llvm-tutor) Hello from: bar
(llvm-tutor)   number of arguments: 2
(llvm-tutor) Hello from: fez
(llvm-tutor)   number of arguments: 3
(llvm-tutor) Hello from: main
(llvm-tutor)   number of arguments: 2

The HelloWorld pass doesn't modify the input module. The -disable-output flag is used to prevent opt from printing the output bitcode file.

Development Environment

Platform Support And Requirements

This project has been tested on Linux 18.04 and Mac OS X 10.14.4. In order to build llvm-tutor you will need:

LLVM 10
C++ compiler that supports C++14
CMake 3.4.3 or higher

In order to run the passes, you will need:

clang-10 (to generate input LLVM files)
opt (to run the passes)

There are additional requirements for tests (these will be satisfied by installing LLVM 10):

lit (aka llvm-lit, LLVM tool for executing the tests)
FileCheck (LIT requirement, it's used to check whether tests generate the expected output)

Installing LLVM 10 on Mac OS X

On Darwin you can install LLVM 10 with Homebrew:

brew install llvm@10

If you already have an older version of LLVM installed, you can upgrade it to LLVM 10 like this:

brew upgrade llvm

Once the installation (or upgrade) is complete, all the required header files, libraries and tools will be located in /usr/local/opt/llvm/.

Note for Mac OS Mojave users

If your default linker is older then the one used to build LLVM 10, you may need to pass -mlinker-version=0 to clang-10 for it to work. Otherwise you may see errors like this:

ld: unknown option: -platform_version
clang-10: error: linker command failed with exit code 1 (use -v to see invocation)

This is a known issue that will only affect you if installing with brew.

Installing LLVM 10 on Ubuntu

On Ubuntu Bionic, you can install modern LLVM from the official repository:

wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
sudo apt-add-repository "deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic-10 main"
sudo apt-get update
sudo apt-get install -y llvm-10 llvm-10-dev clang-10 llvm-10-tools

This will install all the required header files, libraries and tools in /usr/lib/llvm-10/.

Building LLVM 10 From Sources

Building from sources can be slow and tricky to debug. It is not necessary, but might be your preferred way of obtaining LLVM 10. The following steps will work on Linux and Mac OS X:

git clone https://github.com/llvm/llvm-project.git
cd llvm-project
git checkout release/10.x
mkdir build
cd build
cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_TARGETS_TO_BUILD=X86 -DLLVM_ENABLE_PROJECTS=clang <llvm-project/root/dir>/llvm/
cmake --build .

For more details read the official documentation.

Building & Testing

Building

You can build llvm-tutor (and all the provided pass plugins) as follows:

cd <build/dir>
cmake -DLT_LLVM_INSTALL_DIR=<installation/dir/of/llvm/10> <source/dir/llvm/tutor>
make

The LT_LLVM_INSTALL_DIR variable should be set to the root of either the installation or build directory of LLVM 10. It is used to locate the corresponding LLVMConfig.cmake script that is used to set the include and library paths.

Testing

In order to run llvm-tutor tests, you need to install llvm-lit (aka lit). It's not bundled with LLVM 10 packages, but you can install it with pip:

# Install lit - note that this installs lit globally
pip install lit

Running the tests is as simple as:

$ lit <build_dir>/test

Voilà! You should see all tests passing.

Plugins (Linux vs Mac OS)

In llvm-tutor every LLVM pass is implemented in a separate shared object (here you can learn more about shared objects). These shared objects are essentially dynamically loadable plugins for opt. All plugins are built in the <build/dir/lib> directory. Note that the extension of dynamically loaded shared objects differs between Linux and Mac OS. For example, for the HelloWorld pass you will get:

libHelloWorld.so on Linux
libHelloWorld.dylib on MacOS.

For the sake of consistency, in this README.md file all examples use the *.so extension. When working on Mac OS, use *.dylib instead.

Overview of The Passes

The available passes are categorised as either Analysis, Transformation or CFG. The difference between Analysis and Transformation passes is rather self-explanatory (here is a more technical breakdown). A CFG pass is simply a Transformation pass that modifies the Control Flow Graph. This is frequently a bit more complex and requires some extra bookkeeping, hence a dedicated category.

In the following table the passes are grouped thematically and ordered by the level of complexity.

Name	Description	Category
HelloWorld	prints the functions encountered in the input module	Analysis
OpcodeCounter	prints the LLVM IR opcodes encountered in the input module	Analysis
InjectFuncCall	instruments the input module by inserting calls to `printf`	Transformation
StaticCallCounter	counts direct function calls at compile-time (static analysis)	Analysis
DynamicCallCounter	counts direct function calls at run-time (dynamic analysis)	Transformation
MBASub	code transformation for integer `sub` instructions	Transformation
MBAAdd	code transformation for 8-bit integer `add` instructions	Transformation
RIV	finds reachable integer values for each basic block	Analysis
DuplicateBB	duplicates basic blocks, requires RIV analysis results	CFG
MergeBB	merges duplicated basic blocks	CFG

Once you've built this project, you can experiment with every pass separately. All passes, except for HelloWorld, are described in more details below.

LLVM passes work with LLVM IR files. You can generate one like this:

export LLVM_DIR=<installation/dir/of/llvm/10>
# Textual form
$LLVM_DIR/bin/clang  -emit-llvm input.c -S -o out.ll
# Binary/bit-code form
$LLVM_DIR/bin/clang  -emit-llvm input.c -o out.bc

It doesn't matter whether you choose the binary, *.bc (default), or textual (.ll, requires the -S flag) form, but obviously the latter is more human-readable. Similar logic applies to opt (by default it generates *.bc files, use -S to generate an *.ll file instead).

As noted earlier, all examples in this file use the *.so extension for pass plugins. When working on Mac OS, use *.dylib instead.

OpcodeCounter

OpcodeCounter prints a summary of the LLVM IR opcodes encountered in every function in the input module. This pass is slightly more complicated than HelloWorld and it can be run automatically. Let's use our tried and tested method first.

Run the pass

We will use input_for_cc.c to test OpcodeCounter:

export LLVM_DIR=<installation/dir/of/llvm/10>
# Generate an LLVM file to analyze
$LLVM_DIR/bin/clang  -emit-llvm -c <source_dir>/inputs/input_for_cc.c -o input_for_cc.bc
# Run the pass through opt
$LLVM_DIR/bin/opt -load <build_dir>/lib/libOpcodeCounter.so -legacy-opcode-counter input_for_cc.bc

For main OpcodeCounter, prints the following summary (note that when running the pass, a summary for other functions defined in input_for_cc.bc is also printed):

=================================================
LLVM-TUTOR: OpcodeCounter results for `main`
=================================================
OPCODE               #N TIMES USED
-------------------------------------------------
load                 2
br                   4
icmp                 1
add                  1
ret                  1
alloca               2
store                4
call                 4
-------------------------------------------------

Run OpcodeCounter Automatically

You can configure llvm-tutor so that OpcodeCounter is run automatically at any optimisation level (i.e. -O{1|2|3|s}). This is achieved through auto-registration with the existing pipelines, which is enabled with the LT_OPT_PIPELINE_REG CMake variable. Simply add -DLT_OPT_PIPELINE_REG=On when building llvm-tutor.

Once built, you can run OpcodeCounter by specifying an optimisation level. Note that you still have to specify the plugin file to be loaded:

$LLVM_DIR/bin/opt -load <build_dir>/lib/libOpcodeCounter.so -O1 input_for_cc.bc

Here I used the Legacy Pass Manager (the plugin file was specified with -load rather than -load-pass-plugin), but the auto-registration also works with the New Pass Manager:

$LLVM_DIR/bin/opt -load-pass-plugin <build_dir>/lib/libOpcodeCounter.so --passes='default<O1>' input_for_cc.bc

This is implemented in OpcodeCounter.cpp, on line 82 for the New PM, and on line 111 for the Legacy PM. This section contains more information about the pass managers in LLVM.

InjectFuncCall

This pass is a HelloWorld example for code instrumentation. For every function defined in the input module, InjectFuncCall will add (inject) the following call to printf:

printf("(llvm-tutor) Hello from: %s\n(llvm-tutor)   number of arguments: %d\n", FuncName, FuncNumArgs)

This call is added at the beginning of each function (i.e. before any other instruction). FuncName is the name of the function and FuncNumArgs is the number of arguments that the function takes.

Run the pass

We will use input_for_hello.c to test InjectFuncCall:

export LLVM_DIR=<installation/dir/of/llvm/10>
# Generate an LLVM file to analyze
$LLVM_DIR/bin/clang  -emit-llvm -c <source_dir>/inputs/input_for_hello.c -o input_for_hello.bc
# Run the pass through opt
$LLVM_DIR/bin/opt -load <build_dir>/lib/libInjectFuncCall.so -legacy-inject-func-call input_for_hello.bc -o instrumented.bin

This generates instrumented.bin, which is the instrumented version of input_for_hello.bc. In order to verify that InjectFuncCall worked as expected, you can either check the output file (and verify that it contains extra calls to printf) or run it:

$LLVM_DIR/bin/lli instrumented.bin
(llvm-tutor) Hello from: main
(llvm-tutor)   number of arguments: 2
(llvm-tutor) Hello from: foo
(llvm-tutor)   number of arguments: 1
(llvm-tutor) Hello from: bar
(llvm-tutor)   number of arguments: 2
(llvm-tutor) Hello from: foo
(llvm-tutor)   number of arguments: 1
(llvm-tutor) Hello from: fez
(llvm-tutor)   number of arguments: 3
(llvm-tutor) Hello from: bar
(llvm-tutor)   number of arguments: 2
(llvm-tutor) Hello from: foo
(llvm-tutor)   number of arguments: 1

InjectFuncCall vs HelloWorld

You might have noticed that InjectFuncCall is somewhat similar to HelloWorld. In both cases the pass visits all functions, prints their names and the number of arguments. The difference between the two passes becomes quite apparent when you compare the output generated for the same input file, e.g. input_for_hello.c. The number of times Hello from is printed is either:

once per every function call in the case of InjectFuncCall, or
once per function definition in the case of HelloWorld.

This makes perfect sense and hints how different the two passes are. Whether to print Hello from is determined at either:

run-time for InjectFuncCall, or
compile-time for HelloWorld.

Also, note that in the case of InjectFuncCall we had to first run the pass with opt and then execute the instrumented IR module in order to see the output. For HelloWorld it was sufficient to run run the pass with opt.

StaticCallCounter

The StaticCallCounter pass counts the number of static function calls in the input LLVM module. Static refers to the fact that these function calls are compile-time calls (i.e. visible during the compilation). This is in contrast to dynamic function calls, i.e. function calls encountered at run-time (when the compiled module is run). The distinction becomes apparent when analysing functions calls within loops, e.g.:

  for (i = 0; i < 10; i++)
    foo();

Although at run-time foo will be executed 10 times, StaticCallCounter will report only 1 function call.

This pass will only consider direct functions calls. Functions calls via function pointers are not taken into account.

StaticCallCounter resembles OpcodeCounter - both passes analyse opcodes in the input module. However, only StaticCallCounter is a genuine analysis pass (in LLVM terms). Indeed, StaticCallCounter has the following additional properties:

it implements the print method (Legacy PM interface)
it inherits from AnalysisInfoMixin (New PM interface)

Run the pass through opt

We will use input_for_cc.c to test StaticCallCounter:

export LLVM_DIR=<installation/dir/of/llvm/10>
# Generate an LLVM file to analyze
$LLVM_DIR/bin/clang  -emit-llvm -c <source_dir>/inputs/input_for_cc.c -o input_for_cc.bc
# Run the pass through opt
$LLVM_DIR/bin/opt -load <build_dir>/lib/libStaticCallCounter.so -legacy-static-cc -analyze input_for_cc.bc

You should see the following output:

=================================================
LLVM-TUTOR: static analysis results
=================================================
NAME                 #N DIRECT CALLS
-------------------------------------------------
foo                  3
bar                  2
fez                  1
-------------------------------------------------

Note the extra command line option above: -analyze. This option is only available when using the Legacy Pass Manager and is used to print the results of the analysis pass that has just been run. It is enabled by implementing the print method mentioned earlier.

Run the pass through `static`

You can run StaticCallCounter through a standalone tool called static. static is an LLVM based tool implemented in StaticMain.cpp. It is a command line wrapper that allows you to run StaticCallCounter without the need for opt:

<build_dir>/bin/static input_for_cc.bc

It is an example of a relatively basic static analysis tool. Its implementation demonstrates how basic pass management in LLVM works (i.e. it handles that for itself instead of relying on opt).

DynamicCallCounter

The DynamicCallCounter pass counts the number of run-time (i.e. encountered during the execution) function calls. It does so by inserting call-counting instructions that are executed every time a function is called. Only calls to functions that are defined in the input module are counted. This pass builds on top of ideas presented in InjectFuncCall. You may want to experiment with that example first.

Run the pass

We will use input_for_cc.c to test DynamicCallCounter:

export LLVM_DIR=<installation/dir/of/llvm/10>
# Generate an LLVM file to analyze
$LLVM_DIR/bin/clang  -emit-llvm -c <source_dir>/inputs/input_for_cc.c -o input_for_cc.bc
# Instrument the input file
$LLVM_DIR/bin/opt -load <build_dir>/lib/libDynamicCallCounter.so -legacy-dynamic-cc input_for_cc.bc -o instrumented_bin

This generates instrumented.bin, which is the instrumented version of input_for_cc.bc. In order to verify that DynamicCallCounter worked as expected, you can either check the output file (and verify that it contains new call-counting instructions) or run it:

# Run the instrumented binary
$LLVM_DIR/bin/lli ./instrumented_bin

You will see the following output:

=================================================
LLVM-TUTOR: dynamic analysis results
=================================================
NAME                 #N DIRECT CALLS
-------------------------------------------------
foo                  13
bar                  2
fez                  1
main                 1

DynamicCallCounter vs StaticCallCounter

The number of function calls reported by DynamicCallCounter and StaticCallCounter are different, but both results are correct. They correspond to run-time and compile-time function calls respectively. Note also that for StaticCallCounter it was sufficient to run the pass through opt to have the summary printed. For DynamicCallCounter we had to run the instrumented binary to see the output. This is similar to what we observed when comparing HelloWorld and InjectFuncCall.

Mixed Boolean Arithmetic Transformations

These passes implement mixed boolean arithmetic transformations. Similar transformation are often used in code obfuscation (you may also know them from Hacker's Delight) and are a great illustration of what and how LLVM passes can be used for.

Similar transformation are possible at the source-code level. The relevant Clang plugins are available in clang-tutor.

MBASub

The MBASub pass implements this rather basic expression:

a - b == (a + ~b) + 1

Basically, it replaces all instances of integer sub according to the above formula. The corresponding LIT tests verify that both the formula and that the implementation are correct.

Run the pass

We will use input_for_mba_sub.c to test MBASub:

export LLVM_DIR=<installation/dir/of/llvm/10>
$LLVM_DIR/bin/clang -emit-llvm -S <source_dir>/inputs/input_for_mba_sub.c -o input_for_sub.ll
$LLVM_DIR/bin/opt -load <build_dir>/lib/libMBASub.so -legacy-mba-sub -S input_for_sub.ll -o out.ll

MBAAdd

The MBAAdd pass implements a slightly more involved formula that is only valid for 8 bit integers:

a + b == (((a ^ b) + 2 * (a & b)) * 39 + 23) * 151 + 111

Similarly to MBASub, it replaces all instances of integer add according to the above identity, but only for 8-bit integers. The LIT tests verify that both the formula and the implementation are correct.

Run the pass

We will use input_for_add.c to test MBAAdd:

export LLVM_DIR=<installation/dir/of/llvm/10>
$LLVM_DIR/bin/clang -O1 -emit-llvm -S <source_dir>/inputs/input_for_mba.c -o input_for_mba.ll
$LLVM_DIR/bin/opt -load <build_dir>/lib/libMBAAdd.so -legacy-mba-add -S input_for_mba.ll -o out.ll

You can also specify the level of obfuscation on a scale of 0.0 to 1.0, with 0 corresponding to no obfuscation and 1 meaning that all add instructions are to be replaced with (((a ^ b) + 2 * (a & b)) * 39 + 23) * 151 + 111, e.g.:

$LLVM_DIR/bin/opt -load <build_dir>/lib/libMBAAdd.so -legacy-mba-add -mba-ratio=0.3 <source_dir>/inputs/input_for_mba.c -o out.ll

RIV

RIV is an analysis pass that for each basic block BB in the input function computes the set reachable integer values, i.e. the integer values that are visible (i.e. can be used) in BB. Since the pass operates on the LLVM IR representation of the input file, it takes into account all values that have integer type in the LLVM IR sense. In particular, since at the LLVM IR level booleans are represented as 1-bit wide integers (i.e. i1), you will notice that booleans are also included in the result.

This pass demonstrates how to request results from other analysis passes in LLVM. In particular, it relies on the Dominator Tree analysis pass from LLVM, which is is used to obtain the dominance tree for the basic blocks in the input function.

Run the pass

We will use input_for_riv.c to test RIV:

export LLVM_DIR=<installation/dir/of/llvm/10>
$LLVM_DIR/bin/clang -emit-llvm -S -O1 <source_dir>/inputs/input_for_riv.c -o input_for_riv.ll
$LLVM_DIR/bin/opt -load <build_dir>/lib/libRIV.so -legacy-riv -analyze input_for_riv.ll

You will see the following output:

=================================================
LLVM-TUTOR: RIV analysis results
=================================================
BB id      Reachable Ineger Values
-------------------------------------------------
BB %entry
             i32 %a
             i32 %b
             i32 %c
BB %if.then
               %add = add nsw i32 %a, 123
               %cmp = icmp sgt i32 %a, 0
             i32 %a
             i32 %b
             i32 %c
BB %if.end8
               %add = add nsw i32 %a, 123
               %cmp = icmp sgt i32 %a, 0
             i32 %a
             i32 %b
             i32 %c
BB %if.then2
               %mul = mul nsw i32 %b, %a
               %div = sdiv i32 %b, %c
               %cmp1 = icmp eq i32 %mul, %div
               %add = add nsw i32 %a, 123
               %cmp = icmp sgt i32 %a, 0
             i32 %a
             i32 %b
             i32 %c
BB %if.else
               %mul = mul nsw i32 %b, %a
               %div = sdiv i32 %b, %c
               %cmp1 = icmp eq i32 %mul, %div
               %add = add nsw i32 %a, 123
               %cmp = icmp sgt i32 %a, 0
             i32 %a
             i32 %b
             i32 %c

Note the extra command line option above: -analyze. It's required to inform opt to print the results of the analysis to stdout.

DuplicateBB

This pass will duplicate all basic blocks in a module, with the exception of basic blocks for which there are no reachable integer values (identified through the RIV pass). An example of such a basic block is the entry block in a function that:

takes no arguments and
is embedded in a module that defines no global values.

Basic blocks are duplicated by first inserting an if-then-else construct and then cloning all the instructions from the original basic block (with the exception of PHI nodes) into two new basic blocks (clones of the original basic block). The if-then-else construct is introduced as a non-trivial mechanism that decides which of the cloned basic blocks to branch to. This condition is equivalent to:

if (var == 0)
  goto clone 1
else
  goto clone 2

in which:

var is a randomly picked variable from the RIV set for the current basic block
clone 1 and clone 2 are labels for the cloned basic blocks.

The complete transformation looks like this:

BEFORE:                     AFTER:
-------                     ------
                              [ if-then-else ]
             DuplicateBB           /  \
[ BB ]      ------------>   [clone 1] [clone 2]
                                   \  /
                                 [ tail ]

LEGEND:
-------
[BB]           - the original basic block
[if-then-else] - a new basic block that contains the if-then-else statement (inserted by DuplicateBB)
[clone 1|2]    - two new basic blocks that are clones of BB (inserted by DuplicateBB)
[tail]         - the new basic block that merges [clone 1] and [clone 2] (inserted by DuplicateBB)

As depicted above, DuplicateBB replaces qualifying basic blocks with 4 new basic blocks. This is implemented through LLVM's SplitBlockAndInsertIfThenElse. DuplicateBB does all the necessary preparation and clean-up. In other words, it's an elaborate wrapper for LLVM's SplitBlockAndInsertIfThenElse.

Run the pass

This pass depends on the RIV pass, which also needs be loaded in order for DuplicateBB to work. Lets use input_for_duplicate_bb.c as our sample input. First, generate the LLVM file:

export LLVM_DIR=<installation/dir/of/llvm/10>
$LLVM_DIR/bin/clang -emit-llvm -S -O1 <source_dir>/inputs/input_for_duplicate_bb.c -o input_for_duplicate_bb.ll

Function foo in input_for_duplicate_bb.ll should look like this (all metadata has been stripped):

define i32 @foo(i32) {
  ret i32 1
}

Note that there's only one basic block (the entry block) and that foo takes one argument (this means that the result from RIV will be a non-empty set). We will now apply DuplicateBB to foo:

$LLVM_DIR/bin/opt -load <build_dir>/lib/libRIV.so -load <build_dir>/lib/libDuplicateBB.so -legacy-duplicate-bb -S input_for_duplicate_bb.ll -o duplicate.ll

After the instrumentation foo will look like this (all metadata has been stripped):

define i32 @foo(i32) {
lt-if-then-else-0:
  %2 = icmp eq i32 %0, 0
  br i1 %2, label %lt-if-then-0, label %lt-else-0

clone-1-0:
  br label %lt-tail-0

clone-2-0:
  br label %lt-tail-0

lt-tail-0:
  ret i32 1
}

There are four basic blocks instead of one. All new basic blocks end with a numeric id of the original basic block (0 in this case). lt-if-then-else-0 contains the new if-then-else condition. clone-1-0 and clone-2-0 are clones of the original basic block in foo. lt-tail-0 is the extra basic block that's required to merge clone-1-0 and clone-2-0.

MergeBB

MergeBB will merge qualifying basic blocks that are identical. To some extent, this pass reverts the transformations introduced by DuplicateBB. This is illustrated below:

BEFORE:                     AFTER DuplicateBB:                 AFTER MergeBB:
-------                     ------------------                 --------------
                              [ if-then-else ]                 [ if-then-else* ]
             DuplicateBB           /  \               MergeBB         |
[ BB ]      ------------>   [clone 1] [clone 2]      -------->    [ clone ]
                                   \  /                               |
                                 [ tail ]                         [ tail* ]

LEGEND:
-------
[BB]           - the original basic block
[if-then-else] - a new basic block that contains the if-then-else statement (**DuplicateBB**)
[clone 1|2]    - two new basic blocks that are clones of BB (**DuplicateBB**)
[tail]         - the new basic block that merges [clone 1] and [clone 2] (**DuplicateBB**)
[clone]        - [clone 1] and [clone 2] after merging, this block should be very similar to [BB] (**MergeBB**)
[label*]       - [label] after being updated by **MergeBB**

Recall that DuplicateBB replaces all qualifying basic block with four new basic blocks, two of which are clones of the original block. MergeBB will merge those two clones back together, but it will not remove the remaining two blocks added by DuplicateBB (it will update them though).

Run the pass

Lets use the following IR implementation of foo as input. Note that basic blocks 3 and 5 are identical and can safely be merged:

define i32 @foo(i32) {
  %2 = icmp eq i32 %0, 19
  br i1 %2, label %3, label %5

; <label>:3:
  %4 = add i32 %0,  13
  br label %7

; <label>:5:
  %6 = add i32 %0,  13
  br label %7

; <label>:7:
  %8 = phi i32 [ %4, %3 ], [ %6, %5 ]
  ret i32 %8
}

We will now apply MergeBB to foo:

$LLVM_DIR/bin/opt -load <build_dir>/lib/libMergeBB.so -legacy-merge-bb -S foo.ll -o merge.ll

After the instrumentation foo will look like this (all metadata has been stripped):

define i32 @foo(i32) {
  %2 = icmp eq i32 %0, 19
  br i1 %2, label %3, label %3

3:
  %4 = add i32 %0, 13
  br label %5

5:
  ret i32 %4
}

As you can see, basic blocks 3 and 5 from the input module have been merged into one basic block.

Run MergeBB on the output from DuplicateBB

It is really interesting to see the effect of MergeBB on the output from DuplicateBB. Lets start with the same input as we used for DuplicateBB:

export LLVM_DIR=<installation/dir/of/llvm/10>
$LLVM_DIR/bin/clang -emit-llvm -S -O1 <source_dir>/inputs/input_for_duplicate_bb.c -o input_for_duplicate_bb.ll

Now we will apply DuplicateBB and MergeBB (in this order) to foo. Recall that DuplicateBB requires RIV, which means that in total we have to load three plugins:

$LLVM_DIR/bin/opt -load-pass-plugin <build_dir>/lib/libRIV.so -load-pass-plugin <build_dir>/lib/libMergeBB.so -load-pass-plugin <build-dir>/lib/libDuplicateBB.so -passes=duplicate-bb,merge-bb -S input_for_duplicate_bb.ll -o merge_after_duplicate.ll

And here's the output:

define i32 @foo(i32) {
lt-if-then-else-0:
  %1 = icmp eq i32 %0, 0
  br i1 %1, label %lt-clone-2-0, label %lt-clone-2-0

lt-clone-2-0:
  br label %lt-tail-0

lt-tail-0:
  ret i32 1
}

Compare this with the output generated by DuplicateBB. Only one of the clones, lt-clone-2-0, has been preserved, and lt-if-then-else-0 has been updated accordingly. Regardless of the value of of the if condition (more precisely, variable %1), the control flow jumps to lt-clone-2-0.

Debugging

Before running a debugger, you may want to analyze the output from LLVM_DEBUG and STATISTIC macros. For example, for MBAAdd:

export LLVM_DIR=<installation/dir/of/llvm/10>
$LLVM_DIR/bin/clang -emit-llvm -S -O1 <source_dir>/inputs/input_for_mba.c -o input_for_mba.ll
$LLVM_DIR/bin/opt -S -load-pass-plugin <build_dir>/lib/libMBAAdd.so -passes=mba-add input_for_mba.ll -debug-only=mba-add -stats -o out.ll

Note the -debug-only=mba-add and -stats flags in the command line - that's what enables the following output:

  %12 = add i8 %1, %0 ->   <badref> = add i8 111, %11
  %20 = add i8 %12, %2 ->   <badref> = add i8 111, %19
  %28 = add i8 %20, %3 ->   <badref> = add i8 111, %27
===-------------------------------------------------------------------------===
                          ... Statistics Collected ...
===-------------------------------------------------------------------------===

3 mba-add - The # of substituted instructions

As you can see, you get a nice summary from MBAAdd. In many cases this will be sufficient to understand what might be going wrong. Note that for these macros to work you need a debug build of LLVM (i.e. opt) and llvm-tutor (i.e. use -DCMAKE_BUILD_TYPE=Debug instead of -DCMAKE_BUILD_TYPE=Release).

For tricker issues just use a debugger. Below I demonstrate how to debug MBAAdd. More specifically, how to set up a breakpoint on entry to MBAAdd::run. Hopefully that will be sufficient for you to start.

Mac OS X

The default debugger on OS X is LLDB. You will normally use it like this:

export LLVM_DIR=<installation/dir/of/llvm/10>
$LLVM_DIR/bin/clang -emit-llvm -S -O1 <source_dir>/inputs/input_for_mba.c -o input_for_mba.ll
lldb -- $LLVM_DIR/bin/opt -S -load-pass-plugin <build_dir>/lib/libMBAAdd.dylib -passes=mba-add input_for_mba.ll -o out.ll
(lldb) breakpoint set --name MBAAdd::run
(lldb) process launch

or, equivalently, by using LLDBs aliases:

export LLVM_DIR=<installation/dir/of/llvm/10>
$LLVM_DIR/bin/clang -emit-llvm -S -O1 <source_dir>/inputs/input_for_mba.c -o input_for_mba.ll
lldb -- $LLVM_DIR/bin/opt -S -load-pass-plugin <build_dir>/lib/libMBAAdd.dylib -passes=mba-add input_for_mba.ll -o out.ll
(lldb) b MBAAdd::run
(lldb) r

At this point, LLDB should break at the entry to MBAAdd::run.

Ubuntu

On most Linux systems, GDB is the most popular debugger. A typical session will look like this:

export LLVM_DIR=<installation/dir/of/llvm/10>
$LLVM_DIR/bin/clang -emit-llvm -S -O1 <source_dir>/inputs/input_for_mba.c -o input_for_mba.ll
gdb --args $LLVM_DIR/bin/opt -S -load-pass-plugin <build_dir>/lib/libMBAAdd.so -passes=mba-add input_for_mba.ll -o out.ll
(gdb) b MBAAdd.cpp:MBAAdd::run
(gdb) r

At this point, GDB should break at the entry to MBAAdd::run.

About Pass Managers in LLVM

LLVM is a quite complex project (to put it mildly) and passes lay at its center - this is true for any multi-pass compiler. In order to manage the passes, a compiler needs a pass manager. LLVM currently enjoys not one, but two pass managers. This is important because depending on which pass manager you decide to use, the implementation of your pass (and in particular how you register it) will look slightly differently.

Overview of Pass Managers in LLVM

As I mentioned earlier, there are two pass managers in LLVM:

Legacy Pass Manager which currently is the default pass manager
- It is implemented in the legacy namespace
- It is very well documented (more specifically, writing and registering a pass withing the Legacy PM is very well documented)
New Pass Manager aka Pass Manager (that's how it's referred to in the code base)
- I understand that it is soon to become the default pass manager in LLVM
- The source code is very throughly commented, but there is no official documentation. Min-Yih Hsu kindly wrote this great blog series that you can refer to instead.

If you are not sure which pass manager to use, it is probably best to make sure that your passes are compatible with both. Fortunately, once you have an implementation that works with one of them, it's relatively straightforward to extend it so that it works with the other one as well.

New vs Legacy PM When Running Opt

MBAAdd implements interface for both pass managers. This is how you will use it with the legacy pass manager:

$LLVM_DIR/bin/opt -S -load <build_dir>/lib/libMBAAdd.so -legacy-mba-add input_for_mba.ll -o out.ll

And this is how you run it with the new pass manager:

$LLVM_DIR/bin/opt -S -load-pass-plugin <build_dir>/lib/libMBAAdd.so -passes=mba-add input_for_mba.ll -o out.ll

There are two differences:

the way you load your plugin: -load vs -load-pass-plugin
the way you specify which pass/plugin to run: -legacy-mba-add vs -passes=mba-add

These differences stem from the fact that in the case of Legacy Pass Manager you register a new command line option for opt, whereas New Pass Manager simply requires you to define a pass pipeline (with -passes=).

Analysis vs Transformation Pass

The implementation of a pass depends on whether it is an Analysis or a Transformation pass. The difference in the API that you will use is often subtle and further differs between the pass managers.

For example, for the New Pass Manager:

a transformation pass will normally inherit from PassInfoMixin,
an analysis pass will inherit from AnalysisInfoMixin.

This is one of the key characteristics of the New Pass Managers - it makes the split into Analysis and Transformation passes very explicit. An Analysis pass requires a bit more bookkeeping and hence a bit more code. For example, you need to add an instance of AnalysisKey so that it can be identified by the New Pass Manager.

In the case of the Legacy Pass Manager, an Analysis pass is required to implement the print method. But otherwise, the API splits passes based on the unit of IR they operate on, e.g. ModulePass vs FunctionPass. This is one of the main differences between the pass managers in LLVM.

Note that for small standalone examples the difference becomes less relevant. HelloWorld is a good example. It does not transform the input module, so in practice it is an Analysis pass. However, in order to keep the implementation as simple as possible, I used the API for Transformation passes.

Within llvm-tutor the following passes can be used as reference Analysis and Transformation examples:

StaticCallCounter - analysis pass
MBASub - transformation pass

Other examples also adhere to LLVM's convention, but sometimes simplicity is favoured over strictness (e.g. OpcodeCounter and HelloWorld). Whenever that is the case, the comments in the code clarify that.

Optimisation Passes Inside LLVM

Apart from writing your own transformations an analyses, you may want to familiarize yourself with the passes available within LLVM. It is a great resource for learning how LLVM works and what makes it so powerful and successful. It is also a great resource for discovering how compilers work in general. Indeed, many of the passes implement general concepts known from the theory of compiler development.

The list of the available passes in LLVM can be a bit daunting. Below is a list of the selected few that are a good starting point. Each entry contains a link to the implementation in LLVM, a short description and a link to test files available within llvm-tutor. These test files contain a collection of annotated test cases for the corresponding pass. The goal of these tests is to demonstrate the functionality of the tested pass through relatively simple examples.

Name	Description	Test files in lllvm-tutor
dce	Dead Code Elimination	dce.ll
memcpyopt	Optimise calls to `memcpy` (e.g. replace them with `memset`)	memcpyopt.ll
reassociate	Reassociate (e.g. 4 + (x + 5) -> x + (4 + 5)). This enables further optimisations, e.g. LICM.	reassociate.ll
always-inline	Always inlines functions decorated with `alwaysinline`	always-inline.ll
loop-deletion	Delete unused loops	loop-deletion.ll
licm	Loop-Invariant Code Motion (a.k.a. LICM)	licm.ll
slp	Superword-level parallelism vectorisation	slp_x86.ll, slp_aarch64.ll

This list focuses on LLVM's transform passes that are relatively easy to demonstrate through small, standalone examples. You can ran an individual test like this:

lit <source/dir/llvm/tutor>/test/llvm/always-inline.ll

To run an individual pass, extract one RUN line from the test file and run it:

$LLVM_DIR/bin/opt -inline-threshold=0 -always-inline -S <source/dir/llvm/tutor>/test/llvm/always-inline.ll

References

Below is a list of LLVM resources available outside the official online documentation that I have found very helpful. Where possible, the items are sorted by date.

LLVM IR
- ”LLVM IR Tutorial-Phis,GEPs and other things, ohmy!”, V.Bridgers, F. Piovezan, EuroLLVM, (slides, video)
- "Mapping High Level Constructs to LLVM IR", M. Rodler (link)
Examples in LLVM
- Control Flow Graph simplifications: llvm/examples/IRTransforms/
- Hello World Pass: llvm/lib/Transforms/Hello/
- Good Bye World Pass: llvm/examples/Bye/
LLVM Pass Development
- "Getting Started With LLVM: Basics ", J. Paquette, F. Hahn, LLVM Dev Meeting 2019 video
- "Writing an LLVM Pass: 101", A. Warzyński, LLVM Dev Meeting 2019 video
- "Writing LLVM Pass in 2018", Min-Yih Hsu blog
- "Building, Testing and Debugging a Simple out-of-tree LLVM Pass" Serge Guelton, Adrien Guinet, LLVM Dev Meeting 2015 (slides, video)
Legacy vs New Pass Manager
- "New PM: taming a custom pipeline of Falcon JIT", F. Sergeev, EuroLLVM 2018 (slides, video)
- "The LLVM Pass Manager Part 2", Ch. Carruth, LLVM Dev Meeting 2014 (slides, video)
- ”Passes in LLVM, Part 1”, Ch. Carruth, EuroLLVM 2014 (slides, video)
LLVM Based Tools Development
- "Introduction to LLVM", M. Shah, Fosdem 2018, link
- llvm-demo, by N Sumner
- "Building an LLVM-based tool. Lessons learned", A. Denisov, blog, video

Credits

This is first and foremost a community effort. This project wouldn't be possible without the amazing LLVM online documentation, the plethora of great comments in the source code, and the llvm-dev mailing list. Thank you!

It goes without saying that there's plenty of great presentations on YouTube, blog posts and GitHub projects that cover similar subjects. I've learnt a great deal from them - thank you all for sharing! There's one presentation/tutorial that has been particularly important in my journey as an aspiring LLVM developer and that helped to democratise out-of-source pass development:

"Building, Testing and Debugging a Simple out-of-tree LLVM Pass" Serge Guelton, Adrien Guinet (slides, video)

Adrien and Serge came up with some great, illustrative and self-contained examples that are great for learning and tutoring LLVM pass development. You'll notice that there are similar transformation and analysis passes available in this project. The implementations available here reflect what I (aka banach-space) found most challenging while studying them.

I also want to thank Min-Yih Hsu for his blog series "Writing LLVM Pass in 2018". It was invaluable in understanding how the new pass manager works and how to use it. Last, but not least I am very grateful to Nick Sunmer (e.g. llvm-demo) and Mike Shah (see Mike's Fosdem 2018 talk) for sharing their knowledge online. I have learnt a great deal from it, thank you! I always look-up to those of us brave and bright enough to work in academia - thank you for driving the education and research forward!

License

The MIT License (MIT)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Name		Name	Last commit message	Last commit date
Latest commit History 139 Commits
.github/workflows		.github/workflows
HelloWorld		HelloWorld
include		include
inputs		inputs
lib		lib
test		test
tools		tools
.clang-format		.clang-format
.gitignore		.gitignore
.travis.yml		.travis.yml
CMakeLists.txt		CMakeLists.txt
Dockerfile		Dockerfile
LICENSE		LICENSE
README.md		README.md

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makeroftools/llvm-tutor

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llvm-tutor

Overview

Table of Contents

HelloWorld: Your First Pass

Development Environment

Platform Support And Requirements

Installing LLVM 10 on Mac OS X

Note for Mac OS Mojave users

Installing LLVM 10 on Ubuntu

Building LLVM 10 From Sources

Building & Testing

Building

Testing

Plugins (Linux vs Mac OS)

Overview of The Passes

OpcodeCounter

Run the pass

Run OpcodeCounter Automatically

InjectFuncCall

Run the pass

InjectFuncCall vs HelloWorld

StaticCallCounter

Run the pass through opt

Run the pass through static

DynamicCallCounter

Run the pass

DynamicCallCounter vs StaticCallCounter

Mixed Boolean Arithmetic Transformations

MBASub

Run the pass

MBAAdd

Run the pass

RIV

Run the pass

DuplicateBB

Run the pass

MergeBB

Run the pass

Run MergeBB on the output from DuplicateBB

Debugging

Mac OS X

Ubuntu

About Pass Managers in LLVM

Overview of Pass Managers in LLVM

New vs Legacy PM When Running Opt

Analysis vs Transformation Pass

Optimisation Passes Inside LLVM

References

Credits

License

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Run the pass through `static`

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