Open LLVM Projects

- What is this?
- Improving the current system
  1. Port glibc to LLVM
  2. Improving the Nightly Tester
  3. Compile programs with the LLVM Compiler
  4. Extend the LLVM intermediate representation
  5. Miscellaneous Improvements
- Adding new capabilities to LLVM
  1. Pointer and Alias Analysis
  2. Profile Guided Optimization
  3. New Transformations and Analyses
  4. X86 Back-end Improvements
  5. Miscellaneous Additions

What is this?

This document is meant to be a sort of "big TODO list" for LLVM. Each project in this document is something that would be useful for LLVM to have, and would also be a great way to get familiar with the system. Some of these projects are small and self-contained, which may be implemented in a couple of days, others are larger. Several of these projects may lead to interesting research projects in their own right. In any case, we welcome all contributions.

If you are thinking about tackling one of these projects, please send a mail to the LLVM Developer's mailing list, so that we know the project is being worked on. Additionally this is a good way to get more information about a specific project or to suggest other projects to add to this page.

The projects in this page are open ended. More specific projects are filed as unassigned enhancements in our LLVM bug tracker. Here is the current list:

Improving the current system

Improvements to the current infrastructure are always very welcome and tend to be fairly straight-forward to implement. Here are some of the key areas that can use improvement...

Port glibc to LLVM

It would be very useful to port glibc to LLVM. This would allow a variety of interprocedural algorithms to be much more effective in the face of library calls. The most important pieces to port are things like the string library and the `stdio` related functions... low-level system calls like '`read`' should stay unimplemented in LLVM.

Improving the Nightly Tester

The Nightly Tester is a simple perl script (located in `utils/NightlyTest.pl`) which runs every night to generate a daily report. It could use the following improvements:

1. Regression tests - We should run the regression tests in addition to the program tests...

Compile programs with the LLVM Compiler

We are always looking for new testcases and benchmarks for use with LLVM. In particular, it is useful to try compiling your favorite C source code with LLVM. If it doesn't compile, try to figure out why or report it to the llvm-bugs list. If you get the program to compile, it would be extremely useful to convert the build system to be compatible with the LLVM Programs testsuite so that we can check it into CVS and the automated tester can use it to track progress of the compiler.

When testing a code, try running it with a variety of optimizations, and with all the back-ends: CBE, llc, and lli.

Extend the LLVM intermediate representation

1. Add support for platform-independent prefetch support. The GCC prefetch project page has a good survey of the prefetching capabilities of a variety of modern processors.

Miscellaneous Improvements

1. Someone needs to look into getting the `ranlib` tool to index LLVM bytecode files, so that linking in .a files is not hideously slow. They would also then have to implement the reader for this index in `gccld`.
2. Rework the PassManager to be more flexible
3. Some transformations and analyses only work on reducible flow graphs. It would be nice to have a transformation which could be "required" by these passes which makes irreducible graphs reducible. This can easily be accomplished through code duplication. See Making Graphs Reducible with Controlled Node Splitting and perhaps Nesting of Reducible and Irreducible Loops.

Adding new capabilities to LLVM

Sometimes creating new things is more fun than improving existing things. These projects tend to be more involved and perhaps require more work, but can also be very rewarding.

Pointer and Alias Analysis

We have a strong base for development of both pointer analysis based optimizations as well as pointer analyses themselves. It seems natural to want to take advantage of this...

1. Implement a flow-sensitive context-sensitive alias analysis algorithm
   - Pick one of the somewhat efficient algorithms, but strive for maximum precision
2. Implement a flow-sensitive context-insensitive alias analysis algorithm
   - Just an efficient local algorithm perhaps?
3. Implement an interface to update analyses in response to common code motion transformations
4. Implement alias-analysis-based optimizations:
   - Dead store elimination
   - ...

Profile Guided Optimization

We now have a unified infrastructure for writing profile-guided transformations, which will work either at offline-compile-time or in the JIT, but we don't have many transformations. We would welcome new profile-guided transformations as well as improvements to the current profiling system.

Ideas for profile guided transformations:

1. Superblock formation (with many optimizations)
2. Loop unrolling/peeling
3. Profile directed inlining
4. Code layout
5. ...

Improvements to the existing support:

1. The current block and edge profiling code that gets inserted is very simple and inefficient. Through the use of control-dependence information, many fewer counters could be inserted into the code. Also, if the execution count of a loop is known to be a compile-time or runtime constant, all of the counters in the loop could be avoided.
2. You could implement one of the "static profiling" algorithms which analyze a piece of code an make educated guesses about the relative execution frequencies of various parts of the code.
3. You could add path profiling support, or adapt the existing LLVM path profiling code to work with the generic profiling interfaces.

New Transformations and Analyses

1. Implement a Dependence Analysis Infrastructure
   - Design some way to represent and query dep analysis
2. Implement a strength reduction pass
3. Value range propagation pass
4. Implement an unswitching pass
5. Write a loop unroller, with a simple heuristic for when to unroll

X86 Back-end Improvements

1. Implement a better instruction selector
2. Implement support for the "switch" instruction without requiring the lower-switches pass.

Miscellaneous Additions

1. Port the Bigloo Scheme compiler, from Manuel Serrano at INRIA Sophia-Antipolis, to output LLVM bytecode. It seems that it can already output .NET bytecode, JVM bytecode, and C, so LLVM would ostensibly be another good candidate.
2. Write a new frontend for some other language (Java? OCaml? Forth?)
3. Write a new backend for a target (IA64? MIPS? MMIX?)
4. Write a disassembler for machine code that would use TableGen to output `MachineInstr`s for transformations, optimizations, etc.
5. Random test vector generator: Use a C grammar to generate random C code; run it through llvm-gcc, then run a random set of passes on it using opt. Try to crash opt. When opt crashes, use bugpoint to reduce the test case and mail the result to yourself. Repeat ad infinitum.
6. Design a simple, recognizable logo.