LLVM 2.3 Release Notes

This document contains the release notes for the LLVM compiler -infrastructure, release 2.3. Here we describe the status of LLVM, including +infrastructure, release 2.4. 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.

@@ -62,12 +61,12 @@ current one. To see the release notes for a specific releases, please see the

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

This is the fifteenth public release of the LLVM Compiler Infrastructure. +It includes a large number of features and refinements from LLVM 2.3.

-Major Changes in LLVM 2.3 +Major Changes in LLVM 2.4

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.

- -

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

....

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.
....

@@ -126,12 +95,12 @@ It includes a large number of features and refinements from LLVM 2.2.

-The core LLVM 2.3 distribution currently consists of code from the core LLVM +The core LLVM 2.4 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. +are the most actively developed are the Clang Project and +vmkit Project.

@@ -142,28 +111,11 @@ and the Clang Project.

-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 +The "vmkit" project 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: -

- -

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

...

@@ -182,12 +134,12 @@ generation support is far enough along to build many C applications. While not yet production quality, it is progressing very nicely. In addition, C++ front-end work has started to make significant progress.

At this point, Clang is most useful if you are interested in source-to-source -transformations (such as refactoring) and other source-level tools for C and -Objective-C. Clang now also includes tools for turning C code into pretty HTML, -and includes a new static -analysis tool in development. This tool focuses on automatically finding -bugs in C and Objective-C code.

Codegen progress/state +

+ +

+static analysis tool +

LLVM 2.3 includes a huge number of bug fixes, performance tweaks and minor +

LLVM 2.4 includes a huge number of bug fixes, performance tweaks and minor improvements. Some of the major improvements and new features are listed in this section.

@@ -213,52 +165,18 @@ this section.

LLVM 2.3 includes several major new capabilities:

LLVM 2.4 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.
-
+
MRVs got generalized to FCAs.
+
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.
fast isel, -O0 compile times
Attrs changes?
The Kaleidoscope tutorial now - includes a "port" of the tutorial that uses the Ocaml bindings to implement - the Kaleidoscope language.
...

@@ -272,19 +190,12 @@ this section.

LLVM 2.3 fully supports the llvm-gcc 4.2 front-end, and includes support +

LLVM 2.4 fully supports the llvm-gcc 4.2 front-end, and includes support for the C, C++, Objective-C, Ada, and Fortran front-ends.

llvm-gcc 4.2 includes numerous fixes to better support the Objective-C -front-end. Objective-C now works very well on Mac OS/X.
Fortran EQUIVALENCEs are now supported by the gfortran -front-end.
llvm-gcc 4.2 includes many other fixes which improve conformance with the -relevant parts of the GCC testsuite.
...

@@ -301,17 +212,7 @@ relevant parts of the GCC testsuite.

LLVM IR now directly represents "common" linkage, instead of representing it -as a form of weak linkage.
LLVM IR now has support for atomic operations, and this functionality can be -accessed through the llvm-gcc "__sync_synchronize", -"__sync_val_compare_and_swap", and related builtins. Support for -atomics are available in the Alpha, X86, X86-64, and PowerPC backends.
The C and Ocaml bindings have extended to cover pass managers, several -transformation passes, iteration over the LLVM IR, target data, and parameter -attribute lists.
...

@@ -324,64 +225,11 @@ attribute lists.

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

+LLVM 2.4 optimizers support a few major 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,
- -
-
```
-for (i = LB; i < UB; ++i)
-  if (i <= NV)
-    LOOP_BODY
-
```
-
- -
is transformed into:
- -
-
```
-NUB = min(NV+1, UB)
-for (i = LB; i < NUB; ++i)
-  LOOP_BODY
-
```
-
-
-
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.
.

@@ -399,45 +247,9 @@ which allows us to implement more aggressive algorithms and make it run faster:

The code generator now has support for carrying information about memory - references throughout the entire code generation process, via the - - MachineMemOperand class. In the future this will be used to improve - both pre-pass and post-pass scheduling, and to improve compiler-debugging - output.
The target-independent code generator infrastructure now uses LLVM's - APInt - class to handle integer values, which allows it to support integer types - larger than 64 bits (for example i128). Note that support for such types is - also dependent on target-specific support. Use of APInt is also a step - toward support for non-power-of-2 integer sizes.
LLVM 2.3 includes several compile time speedups for code with large basic - blocks, particularly in the instruction selection phase, register - allocation, scheduling, and tail merging/jump threading.
Selection dag speedups
...
LLVM 2.3 includes several improvements which make llc's - --view-sunit-dags visualization of scheduling dependency graphs - easier to understand.
The code generator allows targets to write patterns that generate subreg - references directly in .td files now.
memcpy lowering in the backend is more aggressive, particularly for - memcpy calls introduced by the code generator when handling - pass-by-value structure argument copies.
Inline assembly with multiple register results now returns those results - directly in the appropriate registers, rather than going through memory. - Inline assembly that uses constraints like "ir" with immediates now use the - 'i' form when possible instead of always loading the value in a register. - This saves an instruction and reduces register use.
Added support for PIC/GOT style tail calls on X86/32 and initial - support for tail calls on PowerPC 32 (it may also work on PowerPC 64 but is - not thoroughly tested).

@@ -453,48 +265,7 @@ faster:

llvm-gcc's X86-64 ABI conformance is far improved, particularly in the - area of passing and returning structures by value. llvm-gcc compiled code - now interoperates very well on X86-64 systems with other compilers.
Support for Win64 was added. This includes code generation itself, JIT - support, and necessary changes to llvm-gcc.
The LLVM X86 backend now supports the support SSE 4.1 instruction set, and - the llvm-gcc 4.2 front-end supports the SSE 4.1 compiler builtins. Various - generic vector operations (insert/extract/shuffle) are much more efficient - when SSE 4.1 is enabled. The JIT automatically takes advantage of these - instructions, but llvm-gcc must be explicitly told to use them, e.g. with - -march=penryn.
The X86 backend now does a number of optimizations that aim to avoid - converting numbers back and forth from SSE registers to the X87 floating - point stack. This is important because most X86 ABIs require return values - to be on the X87 Floating Point stack, but most CPUs prefer computation in - the SSE units.
The X86 backend supports stack realignment, which is particularly useful for - vector code on OS's without 16-byte aligned stacks, such as Linux and - Windows.
The X86 backend now supports the "sseregparm" options in GCC, which allow - functions to be tagged as passing floating point values in SSE - registers.
Trampolines (taking the address of a nested function) now work on - Linux/X86-64.
__builtin_prefetch is now compiled into the appropriate prefetch - instructions instead of being ignored.
128-bit integers are now supported on X86-64 targets. This can be used - through __attribute__((TImode)) in llvm-gcc.
The register allocator can now rematerialize PIC-base computations, which is - an important optimization for register use.
The "t" and "f" inline assembly constraints for the X87 floating point stack - now work. However, the "u" constraint is still not fully supported.
...

@@ -510,9 +281,7 @@ faster:

The LLVM C backend now supports vector code.
The Cell SPU backend includes a number of improvements. It generates better - code and its stability/completeness is improving.
....

@@ -529,10 +298,7 @@ faster:

LLVM now builds with GCC 4.3.
Bugpoint now supports running custom scripts (with the -run-custom - option) to determine how to execute the command and whether it is making - forward process.
...