LLVM 2.0 Release Notes

Introduction @@ -32,7 +36,7 @@

This document contains the release notes for the LLVM compiler -infrastructure, release 2.0. Here we describe the status of LLVM, including +infrastructure, release 2.3. Here we describe the status of LLVM, including major improvements from the previous release and any known problems. All LLVM releases may be downloaded from the LLVM releases web site.

@@ -44,430 +48,494 @@ href="http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev">LLVM developer's mailing list is a good place to send them.

Note that if you are reading this file from a Subversion checkout or the -main LLVM web page, -this document applies to the next release, not the current one. To see -the release notes for the current or previous releases, see the releases page.

+main LLVM web page, this document applies to the next release, not the +current one. To see the release notes for a specific releases, please see the +releases page.

- What's New? + Major Changes and Sub-project Status

This is the eleventh public release of the LLVM Compiler Infrastructure. -Being the first major release since 1.0, this release is different in several -ways from our previous releases:

- -

We took this as an opportunity to -break backwards compatibility with the LLVM 1.x bytecode and .ll file format. -If you have LLVM 1.9 .ll files that you would like to upgrade to LLVM 2.x, we -recommend the use of the stand alone llvm-upgrade -tool (which is included with 2.0). We intend to keep compatibility with .ll -and .bc formats within the 2.x release series, like we did within the 1.x -series.
There are several significant change to the LLVM IR and internal APIs, such - as a major overhaul of the type system, the completely new bitcode file - format, etc (described below).
We designed the release around a 6 month release cycle instead of the usual - 3-month cycle. This gave us extra time to develop and test some of the - more invasive features in this release.
LLVM 2.0 no longer supports the llvm-gcc3 front-end. Users are required to - upgrade to llvm-gcc4. llvm-gcc4 includes many features over - llvm-gcc3, is faster, and is much easier to - build from source.

- -

Note that while this is a major version bump, this release has been - extensively tested on a wide range of software. It is easy to say that this - is our best release yet, in terms of both features and correctness. This is - the first LLVM release to correctly compile and optimize major software like - LLVM itself, Mozilla/Seamonkey, Qt 4.3rc1, kOffice, etc out of the box on - linux/x86. -

This is the fourteenth public release of the LLVM Compiler Infrastructure. +It includes a large number of features and refinements from LLVM 2.2.

+ +

-New Features in LLVM 2.0 +Major Changes in LLVM 2.3

- -

Major Changes

Changes to the LLVM IR itself:

LLVM 2.3 no longer supports llvm-gcc 4.0, it has been replaced with + llvm-gcc 4.2.

LLVM 2.3 no longer includes the llvm-upgrade tool. It was useful + for upgrading LLVM 1.9 files to LLVM 2.x syntax, but you can always use a + previous LLVM release to do this. One nice impact of this is that the LLVM + regression test suite no longer depends on llvm-upgrade, which makes it run + faster.

Integer types are now completely signless. This means that we - have types like i8/i16/i32 instead of ubyte/sbyte/short/ushort/int - etc. LLVM operations that depend on sign have been split up into - separate instructions (PR950). This - eliminates cast instructions that just change the sign of the operands (e.g. - int -> uint), which reduces the size of the IR and makes optimizers - simpler to write.
Integer types with arbitrary bitwidths (e.g. i13, i36, i42, i1057, etc) are - now supported in the LLVM IR and optimizations (PR1043). However, neither llvm-gcc - (PR1284) nor the native code generators - (PR1270) support non-standard width - integers yet.
'Type planes' have been removed (PR411). - It is no longer possible to have two values with the same name in the - same symbol table. This simplifies LLVM internals, allowing significant - speedups.
Global variables and functions in .ll files are now prefixed with - @ instead of % (PR645).
The LLVM 1.x "bytecode" format has been replaced with a - completely new binary representation, named 'bitcode'. The Bitcode Format 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.
Load and store instructions now track the alignment of their pointer - (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.

The llvm2cpp tool has been folded into llc, use + llc -march=cpp instead of llvm2cpp.

Major new features:

LLVM API Changes:

Several core LLVM IR classes have migrated to use the + 'FOOCLASS::Create(...)' pattern instead of 'new + FOOCLASS(...)' (e.g. where FOOCLASS=BasicBlock). We hope to + standardize on FOOCLASS::Create for all IR classes in the future, + but not all of them have been moved over yet.
LLVM 2.3 renames the LLVMBuilder and LLVMFoldingBuilder classes to + IRBuilder. +
MRegisterInfo was renamed to + + TargetRegisterInfo.
The MappedFile class is gone, please use + + MemoryBuffer instead.
The '-enable-eh' flag to llc has been removed. Now code should + encode whether it is safe to omit unwind information for a function by + tagging the Function object with the 'nounwind' attribute.
The ConstantFP::get method that uses APFloat now takes one argument + instead of two. The type argument has been removed, and the type is + now inferred from the size of the given APFloat value.
A number of ELF features are now supported by LLVM, including 'visibility', - extern weak linkage, Thread Local Storage (TLS) with the __thread - keyword, and symbol aliases. - Among other things, this means that many of the special options needed to - configure llvm-gcc on linux are no longer needed, and special hacks to build - large C++ libraries like Qt are not needed.
LLVM now has a new MSIL backend. llc -march=msil will now turn LLVM - into MSIL (".net") bytecode. This is still fairly early development - with a number of limitations.
A new llvm-upgrade tool - exists to migrates LLVM 1.9 .ll files to LLVM 2.0 syntax.

+ +

+Other LLVM Sub-Projects +

- -

llvm-gcc -Improvements

New features include: +

+The core LLVM 2.3 distribution currently consists of code from the core LLVM +repository (which roughly contains the LLVM optimizer, code generators and +supporting tools) and the llvm-gcc repository. In addition to this code, the +LLVM Project includes other sub-projects that are in development. The two which +are the most actively developed are the new vmkit Project +and the Clang Project.

Precompiled Headers (PCH) are now supported.
"#pragma packed" is now supported, as are the various features - described above (visibility, extern weak linkage, __thread, aliases, - etc).
Tracking function parameter/result attributes is now possible.

+vmkit +

Many internal enhancements have been added, such as improvements to - NON_LVALUE_EXPR, arrays with non-zero base, structs with variable sized - fields, VIEW_CONVERT_EXPR, CEIL_DIV_EXPR, nested functions, and many other - things. This is primarily to supports non-C GCC front-ends, like Ada.

+The "vmkit" project is a new addition to the LLVM family. It is an +implementation of a JVM and a CLI Virtual Machines (Microsoft .NET is an +implementation of the CLI) using the Just-In-Time compiler of LLVM.

+ +

The JVM, called JnJVM, executes real-world applications such as Apache +projects (e.g. Felix and Tomcat) and the SpecJVM98 benchmark. It uses the GNU +Classpath project for the base classes. The CLI implementation, called N3, is +its in early stages but can execute simple applications and the "pnetmark" +benchmark. It uses the pnetlib project as its core library.

+ +

The 'vmkit' VMs compare in performance with industrial and top open-source +VMs on scientific applications. Besides the JIT, the VMs use many features of +the LLVM framework, including the standard set of optimizations, atomic +operations, custom function provider and memory manager for JITed methods, and +specific virtual machine optimizations. vmkit is not an official part of LLVM +2.3 release. It is publicly available under the LLVM license and can be +downloaded from: +

It is simpler to configure llvm-gcc for linux.

svn co http://llvm.org/svn/llvm-project/vmkit/trunk vmkit

-New features include: -

+ +

LLVM 2.3 includes several major new capabilities:

The biggest change in LLVM 2.3 is Multiple Return Value (MRV) support. + MRVs allow LLVM IR to directly represent functions that return multiple + values without having to pass them "by reference" in the LLVM IR. This + allows a front-end to generate more efficient code, as MRVs are generally + returned in registers if a target supports them. See the LLVM IR Reference for more details.
+ +
MRVs are fully supported in the LLVM IR, but are not yet fully supported in + on all targets. However, it is generally safe to return up to 2 values from + a function: most targets should be able to handle at least that. MRV + support is a critical requirement for X86-64 ABI support, as X86-64 requires + the ability to return multiple registers from functions, and we use MRVs to + accomplish this in a direct way.
LLVM 2.3 includes a complete reimplementation of the "llvmc" + tool. It is designed to overcome several problems with the original + llvmc and to provide a superset of the features of the + 'gcc' driver.
+ +
The main features of llvmc2 are: +
- Extended handling of command line options and smart rules for + dispatching them to different tools.
- Flexible (and extensible) rules for defining different tools.
- The different intermediate steps performed by tools are represented + as edges in the abstract graph.
- The 'language' for driver behavior definition is tablegen and thus + it's relatively easy to add new features.
- The definition of driver is transformed into set of C++ classes, thus + no runtime interpretation is needed.
+
LLVM now supports software floating point, which allows LLVM to target - chips that don't have hardware FPUs (e.g. ARM thumb mode).
LLVM 2.3 includes a completely rewritten interface for Link Time Optimization. This interface + is written in C, which allows for easier integration with C code bases, and + incorporates improvements we learned about from the first incarnation of the + interface.
A new register scavenger has been implemented, which is useful for - finding free registers after register allocation. This is useful when - rewriting frame references on RISC targets, for example.
The Kaleidoscope tutorial now + includes a "port" of the tutorial that uses the Ocaml bindings to implement + the Kaleidoscope language.
Heuristics have been added to avoid coalescing vregs with very large live - ranges to physregs. This was bad because it effectively pinned the physical - register for the entire lifetime of the virtual register (PR711).

Support now exists for very simple (but still very useful) - rematerialization the register allocator, enough to move - instructions like "load immediate" and constant pool loads.

X86-specific Code Generator Enhancements: -

In addition to a huge array of bug fixes and minor performance tweaks, the +LLVM 2.3 optimizers support a few major enhancements:

The MMX instruction set is now supported through intrinsics.
The scheduler was improved to better reduce register pressure on - X86 and other targets that are register pressure sensitive.
Linux/x86-64 support is much better.
PIC support for linux/x86 has been added.
The X86 backend now supports the GCC regparm attribute.
LLVM now supports inline asm with multiple constraint letters per operand - (like "mri") which is common in X86 inline asms.

ARM-specific Code Generator Enhancements:

Loop index set splitting on by default. +This transformation hoists conditions from loop bodies and reduces a loop's +iteration space to improve performance. For example,

The ARM code generator is now stable and fully supported.

+for (i = LB; i < UB; ++i)
+  if (i <= NV)
+    LOOP_BODY
+

There are major new features, including support for ARM - v4-v6 chips, vfp support, soft float point support, pre/postinc support, - load/store multiple generation, constant pool entry motion (to support - large functions), inline asm support, weak linkage support, static - ctor/dtor support and many bug fixes.
Added support for Thumb code generation (llc -march=thumb).

is transformed into:

The ARM backend now supports the ARM AAPCS/EABI ABI and PIC codegen on - arm/linux.

+NUB = min(NV+1, UB)
+for (i = LB; i < NUB; ++i)
+  LOOP_BODY
+

Several bugs were fixed for DWARF debug info generation on arm/linux.
LLVM now includes a new memcpy optimization pass which removes +dead memcpy calls, unneeded copies of aggregates, and performs +return slot optimization. The LLVM optimizer now notices long sequences of +consecutive stores and merges them into memcpy's where profitable.
Alignment detection for vector memory references and for memcpy and +memset is now more aggressive.
The Aggressive Dead Code Elimination (ADCE) optimization has been rewritten +to make it both faster and safer in the presence of code containing infinite +loops. Some of its prior functionality has been factored out into the loop +deletion pass, which is safe for infinite loops. The new ADCE pass is +no longer based on control dependence, making it run faster.
The 'SimplifyLibCalls' pass, which optimizes calls to libc and libm + functions for C-based languages, has been rewritten to be a FunctionPass + instead a ModulePass. This allows it to be run more often and to be + included at -O1 in llvm-gcc. It was also extended to include more + optimizations and several corner case bugs were fixed.
LLVM now includes a simple 'Jump Threading' pass, which attempts to simplify + conditional branches using information about predecessor blocks, simplifying + the control flow graph. This pass is pretty basic at this point, but + catches some important cases and provides a foundation to build on.
Several corner case bugs which could lead to deleting volatile memory + accesses have been fixed.
Several optimizations have been sped up, leading to faster code generation + with the same code quality.

PowerPC-specific Code Generator Enhancements:

@@ -480,20 +548,19 @@ following major changes:

LLVM is known to work on the following platforms:

Intel and AMD machines running Red Hat Linux, Fedora Core and FreeBSD +
Intel and AMD machines (IA32) running Red Hat Linux, Fedora Core and FreeBSD (and probably other unix-like systems).
PowerPC and X86-based Mac OS X systems, running 10.2 and above in 32-bit and +
PowerPC and X86-based Mac OS X systems, running 10.3 and above in 32-bit and 64-bit modes.
Intel and AMD machines running on Win32 using MinGW libraries (native)
Intel and AMD machines running on Win32 using MinGW libraries (native).
Intel and AMD machines running on Win32 with the Cygwin libraries (limited support is available for native builds with Visual C++).
Sun UltraSPARC workstations running Solaris 8.
Sun UltraSPARC workstations running Solaris 10.
Alpha-based machines running Debian GNU/Linux.
Itanium-based machines running Linux and HP-UX.
Itanium-based (IA64) machines running Linux and HP-UX.

The core LLVM infrastructure uses -GNU autoconf to adapt itself +

The core LLVM infrastructure uses GNU autoconf to adapt itself to the machine and operating system on which it is built. However, minor porting may be required to get LLVM to work on new platforms. We welcome your portability patches and reports of successful builds or error messages.

@@ -527,17 +594,13 @@ there isn't already one.

be broken or unreliable, or are in early development. These components should not be relied on, and bugs should not be filed against them, but they may be useful to some people. In particular, if you would like to work on one of these -components, please contact us on the LLVMdev list.

+components, please contact us on the LLVMdev list.

The -cee pass is known to be buggy, and may be removed in in a - future release.
C++ EH support is disabled for this release.
The MSIL backend is experimental.
The IA64 code generator is experimental.
The Alpha JIT is experimental.
"-filetype=asm" (the default) is the only supported value for the - -filetype llc option.
The MSIL, IA64, Alpha, SPU, and MIPS backends are experimental.
The llc "-filetype=asm" (the default) is the only supported + value for this option.

"long double" is silently transformed by the front-end into "double". There -is no support for floating point data types of any size other than 32 and 64 -bits.
llvm-gcc does not support __builtin_apply yet. - See Constructing Calls: Dispatching a call to another function.
-
llvm-gcc partially supports these GCC extensions:
-
1. Nested Functions: As in Algol and Pascal, lexical scoping of functions.
  - Nested functions are supported, but llvm-gcc does not support non-local - gotos or taking the address of a nested function.
2. Function Attributes: - - Declaring that functions have no side effects or that they can never - return.
  - - Supported: alias, always_inline, cdecl, - constructor, destructor, - deprecated, fastcall, format, - format_arg, non_null, noreturn, regparm - section, stdcall, unused, used, - visibility, warn_unused_result, weak
  - - Ignored: noinline, pure, const, nothrow, - malloc, no_instrument_function
-
llvm-gcc supports the vast majority of GCC extensions, including:
- -
1. Pragmas: Pragmas accepted by GCC.
2. Local Labels: Labels local to a block.
3. Other Builtins: - Other built-in functions.
4. Variable Attributes: - Specifying attributes of variables.
5. Type Attributes: Specifying attributes of types.
6. Thread-Local: Per-thread variables.
7. Variable Length: - Arrays whose length is computed at run time.
8. Labels as Values: Getting pointers to labels and computed gotos.
9. Statement Exprs: Putting statements and declarations inside expressions.
10. Typeof: typeof: referring to the type of an expression.
11. Lvalues: Using ?:, "," and casts in lvalues.
12. Conditionals: Omitting the middle operand of a ?: expression.
13. Long Long: Double-word integers.
14. Complex: Data types for complex numbers.
15. Hex Floats:Hexadecimal floating-point constants.
16. Zero Length: Zero-length arrays.
17. Empty Structures: Structures with no members.
18. Variadic Macros: Macros with a variable number of arguments.
19. Escaped Newlines: Slightly looser rules for escaped newlines.
20. Extended Asm: Assembler instructions with C expressions as operands.
21. Constraints: Constraints for asm operands.
22. Asm Labels: Specifying the assembler name to use for a C symbol.
23. Explicit Reg Vars: Defining variables residing in specified registers.
24. Vector Extensions: Using vector instructions through built-in functions.
25. Target Builtins: Built-in functions specific to particular targets.
26. Subscripting: Any array can be subscripted, even if not an lvalue.
27. Pointer Arith: Arithmetic on void-pointers and function pointers.
28. Initializers: Non-constant initializers.
29. Compound Literals: Compound literals give structures, unions, -or arrays as values.
30. Designated Inits: Labeling elements of initializers.
31. Cast to Union: Casting to union type from any member of the union.
32. Case Ranges: `case 1 ... 9' and such.
33. Mixed Declarations: Mixing declarations and code.
34. Function Prototypes: Prototype declarations and old-style definitions.
35. C++ Comments: C++ comments are recognized.
36. Dollar Signs: Dollar sign is allowed in identifiers.
37. Character Escapes: \e stands for the character <ESC>.
38. Alignment: Inquiring about the alignment of a type or variable.
39. Inline: Defining inline functions (as fast as macros).
40. Alternate Keywords:__const__, __asm__, etc., for header files.
41. Incomplete Enums: enum foo;, with details to follow.
42. Function Names: Printable strings which are the name of the current function.
43. Return Address: Getting the return or frame address of a function.
44. Unnamed Fields: Unnamed struct/union fields within structs/unions.
45. Attribute Syntax: Formal syntax for attributes.

The only major language feature of GCC not supported by llvm-gcc is + the __builtin_apply family of builtins. However, some extensions + are only supported on some targets. For example, trampolines are only + supported on some targets (these are used when you take the address of a + nested function).

- -

If you run into GCC extensions which have not been included in any of these -lists, please let us know (also including whether or not they work).

If you run into GCC extensions which are not supported, please let us know. +