LLVM 2.0 is done! Download it here: http://llvm.org/releases/ or view
the release notes: http://llvm.org/releases/2.0/docs/ReleaseNotes.html
LLVM 2.0 is a great release in many ways. It includes a wide range of new
features, new optimizations, better codegen, and new targets. We were
also able to signficantly revise several core aspects of the LLVM IR and
design (such as the type system and bytecode format) based on lessons
learned in the LLVM 1.x series.
One of the most exciting aspects of this release is seeing the strength of
the community and the adoption that LLVM is receiving. LLVM continues to
be used for a broad variety of academic research projects (e.g. see
http://llvm.org/pubs/ ), but is also being used for a wide range of
commercial products and industrial development projects (see
http://llvm.org/Users.html ). We're seeing many different groups using
LLVM for very different purposes, many of which we never envisioned :).
In any case, LLVM 2.0 includes a huge number of new features. I list the
significant changes from February below, but there were also another 42
changes in February that are also a part of LLVM 2.0 (see the link below).
Here we go!:
New Features:
1. Reid and Sheng contributed IR, optimizer, and interpreter support for
arbitrary bitwidth integers which have sizes > 64 bits. This means
that LLVM IR can now express operations on 31337-bit wide integers,
for example (however, for most people, 128-bit wide integers on 64-bit
targets will be the most useful new integer type). Currently neither
llvm-gcc nor the native code generators support non-standard width
integers yet though.
2. The LLVM 1.x "bytecode" format has been replaced with a
completely new
binary representation, named 'bitcode'. Because we plan to
maintain
binary compatibility between LLVM 2.x ".bc" files, this is an
important change to get right. Bitcode brings a number of advantages
to the LLVM over the old bytecode format. It is denser (files are
smaller), more extensible, requires less memory to read, is easier to
keep backwards compatible (so LLVM 2.5 will read 2.0 .bc files), and
has many other nice features. Please see
http://llvm.org/docs/BitCodeFormat.html for more details.
3. Christopher Lamb added support for alignment values on load and store
instructions, finishing off PR400. This allows the IR to express
loads that are not sufficiently aligned (e.g. due to '#pragma
packed')
or to capture extra alignment information.
4. Roman Samoilov contributed a new MSIL backend to LLVM. llc
-march=msil will now turn LLVM into MSIL (".net") bytecode. This
is
still fairly early development with a number of limitations.
5. Lauro implemented support for Thread Local Storage with the __thread
keyword, and added codegen support for Linux on X86 and ARM. Some
front-end pieces will land in LLVM 2.1 though.
6. Anton and Lauro implemented support for 'protected visibility' in
ELF.
7. Anton implemented support for ELF symbol aliases.
8. Reid contributed support for 'polymorphic intrinsics', allowing
things
like llvm.ctpop to work on arbitrary width integers.
llvm-gcc Improvements:
9. Duncan Sands contributed many enhancements to llvm-gcc, some of which
are language independent and others that are aimed towards better Ada
support. He made improvements to NON_LVALUE_EXPR, arrays with
non-zero base, structs with variable sized fields, VIEW_CONVERT_EXPR,
CEIL_DIV_EXPR, and many other things.
10. Devang, Duncan and Andrew all contributed many patches to improve
"attribute packed" support in the CFE, and handle many other
obscure
struct layout cases correctly.
Optimizer Improvements:
11. Devang implemented support for a new LoopPass class, implemented
passmanager support for it, and converted existing loop transforms to
use it. See: http://llvm.org/docs/WritingAnLLVMPass.html#LoopPass
12. Devang contributed a new loop rotation pass, which converts "for
loops" into "do/while loops", where the condition is at the
bottom of
the loop.
13. Devang added support that allows ModulePasses to use the result of
FunctionPasses. This requires holding multiple FunctionPasses (e.g.
dominator info) in memory at a time.
14. Owen and Devang both worked to eliminate the [Post]DominatorSet
classes from LLVM, switching clients to use the far-more-efficient
ETForest class instead. Owen removed the ImmediateDominator class,
switching clients to use DominatorTree instead. These changes reduce
memory usage and speed up the optimizer.
Target-Independent Code Generator Enhancements:
15. Jim, Anton and Duncan contributed many enhancements and improvements
to C++/Ada zero-cost DWARF exception handling support. While it is
not yet solid, it is mostly complete and just in need of continued bug
fixes and optimizations at this point. Jim wrote
http://llvm.org/docs/ExceptionHandling.html to describe the approach.
16. Many bugfixes and other improvements have been made to inline asm
support. The two large missing features are support for 80-bit
floating point stack registers on X86 (PR879), and support for inline
asm in the C backend (PR802). If you run into other issues, please
report them.
17. Evan implemented a new register scavenger, which is useful for finding
free registers after register allocation. This is useful when
rewriting frame references on RISC targets, for example.
18. LLVM now supports describing target calling conventions explicitly in
.td files, reducing the amount of C++ code that needs to be written
for a port.
19. Evan contributed heuristics to avoid coallescing virtregs with very
large live ranges to physregs. This effectively pinned the physreg
for the entire live range of the virtreg, which was very bad for code
quality.
20. Evan implemented support for very simple (but still very useful)
rematerialization in the register allocator, enough to move
instructions like "load immediate" and constant pool loads.
21. Anton significantly improved 'switch' lowering, improving codegen
for
sparse switches that have dense subregions, and implemented support
for the shift/and trick.
22. The code generator now has more accurate and general hooks for
describing addressing modes ("isLegalAddressingMode") to
optimizations
like loop strength reduction and code sinking.
23. Dale and Evan contributed several improvements to the Loop Strength
Reduction pass, and added support for sinking expressions across
blocks to reduce register pressure.
24. Evan added support for tracking physreg sub-registers and
super-registers in the code generator, as well as extensive register
allocator changes to track them.
25. Nate contributed initial support for virtreg sub-registers. See
PR1350 for more information.
Target-Specific Code Generator Enhancements:
26. Nicolas Geoffray contributed support for the Linux/ppc ABI, and the
linux/ppc JIT is fully functional now. llvm-gcc and static
compilation are not fully supported yet though.
27. Bill contributed support for the X86 MMX instruction set.
28. Dale contributed many enhancements to the ARM constant island pass,
making ARM codegen significantly better for large functions.
29. Anton fixed several bugs in DWARF debug emission on linux and
cygwin/mingw. Debugging basically works on these targets now.
30. Lauro contributed support for the ARM AAPCS/EABI ABI and PIC codegen
on arm/linux.
31. Dale implemented more aggressive size analysis for ARM inline asm
strings.
32. Raul Herbster contributed fixes for DWARF debug info generation on
arm/linux.
Other Improvements:
33. Anton and Reid are working to migrate from CVS to SVN in June: See
http://llvm.org/SVNMigration.html This will allow us to host llvm-gcc
and llvm in the same repository again!
34. Lauro contributed support to llvm-test for running on low-memory or
slow machines (make SMALL_PROBLEM_SIZE=1).
35. Jeff contributed many portability fixes to the llvm-test testsuite,
and has done a great job keeping llvm itself building with MS Visual
Studio.
In addition to the features above, this this release also includes
hundreds of bug fixes, minor optimization improvements, compile-time
speedups, etc. LLVM has literally compiled millions of lines of code in
several different environments. For example, Anton has found that LLVM
successfully (and correctly!) builds Qt 4.3rc1, Mozilla/Seamonkey,
koffice, etc out of the box on linux/x86.
Also, be sure to glance through the February update, which includes a
bunch of other new features and big changes that are also included in 2.0:
http://lists.cs.uiuc.edu/pipermail/llvm-announce/2007-February/000021.html
This release wouldn't be possible without many people in the LLVM
community: building new features, reporting bugs, testing the pre-release
bits, and contributing in many other ways. Tanya (our release manager)
deserves a lot of credit for this being the smoothest and best release so
far, as well keeping our releases coming out on time!
If you have any questions or comments, please contact the LLVMdev
mailing list (llvmdev at cs.uiuc.edu)!
-Chris
--
http://nondot.org/sabre/
http://llvm.org/
