LLVM 1.7 Release Notes

This document contains the release notes for the LLVM compiler -infrastructure, release 1.7. Here we describe the status of LLVM, including any -known problems and major improvements from the previous release. The most -up-to-date version of this document can be found on the LLVM releases web site. If you are -not reading this on the LLVM web pages, you should probably go there because -this document may be updated after the release.

+infrastructure, release 2.0. Here we describe the status of LLVM, including any +known problems and major improvements from the previous release. All LLVM +releases may be downloaded from the LLVM +releases web site.

For more information about LLVM, including information about the latest release, please check out the main LLVM @@ -60,218 +58,410 @@ href="http://llvm.org/releases/">releases page.

This is the eighth public release of the LLVM Compiler Infrastructure. This -release incorporates a large number of enhancements and new features, -including vector support (Intel SSE and Altivec), a new GCC4.0-based -C/C++ front-end, Objective C/C++ support, inline assembly support, and many -other big features. -

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.
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.

+ +

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.

-New Features in LLVM 1.7 +New Features in LLVM 2.0

GCC4.0-based llvm-gcc -front-end

- +

Major Changes

- -

LLVM 1.7 includes a brand new llvm-gcc, based on GCC 4.0.1. This version -of llvm-gcc solves many serious long-standing problems with llvm-gcc, including -all of those blocked by the llvm-gcc 4 meta -bug. In addition, llvm-gcc4 implements support for many new features, -including GCC inline assembly, generic vector support, SSE and Altivec -intrinsics, and several new GCC attributes. Finally, llvm-gcc4 is -significantly faster than llvm-gcc3, respects -O options, its -c/-S options -correspond to GCC's (they emit native code), supports Objective C/C++, and -it has debugging support well underway.

- -

If you can use it, llvm-gcc4 offers significant new functionality, and we -hope that it will replace llvm-gcc3 completely in a future release. -Unfortunately, it does not currently support C++ exception handling at all, and -it only works on Apple Mac OS/X machines with X86 or PowerPC processors. +

blah

Inline Assembly -Support

llvm-gcc3 is now officially unsupported. Users are required to + upgrade to llvm-gcc4. llvm-gcc4 includes many features over + llvm-gcc3, is faster, and is much easier to build.

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).

The LLVM IR and llvm-gcc4 front-end now fully support arbitrary GCC inline assembly. The LLVM X86 and PowerPC -code generators have initial support for it, -being able to compile basic statements, but are missing some features. Please -report any inline asm statements that crash the compiler or that are miscompiled -as bugs.

Arbitrary bitwidth integers (e.g. i13, i36, i42, etc) are now + supported in the LLVM IR and optimizations. However, neither llvm-gcc nor + the native code generators support non-standard width integers + (PR1043).

'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.

New SPARC backend

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'. 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.

LLVM 1.7 includes a new, fully functional, SPARC backend built in the -target-independent code generator. This SPARC backend includes support for -SPARC V8 and SPARC V9 subtargets (controlling whether V9 features can be used), -and targets the 32-bit SPARC ABI.

Support was added for alignment values on load and store + instructions (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 LLVM 1.7 release is the last release that will include the LLVM "SparcV9" -backend, which was the very first LLVM native code generator. It will -be removed in LLVM 1.8, being replaced with the new SPARC backend.

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.

Support has been added for 'protected visibility' in ELF.

Generic Vector Support -

Thread Local Storage with the __thread keyword was implemented along + with added codegen support for Linux on X86 and ARM.

ELF symbol aliases supported has been added.

LLVM now includes significantly extended support for SIMD vectors in its -core instruction set. It now includes three new instructions for manipulating -vectors: extractelement, -insertelement, and -shufflevector. Further, -many bugs in vector handling have been fixed, and vectors are now supported by -the target-independent code generator. For example, if a vector operation is -not supported by a particular target, it will be correctly broken down and -executed as scalar operations.

Added support for 'polymorphic intrinsics', allowing things like + llvm.ctpop to work on arbitrary width integers.

Because llvm-gcc3 does not support GCC generic vectors or vector intrinsics, -llvm-gcc4 must be used.

Optimizer +

Optimizer Improvements

New features include: +

The Loop Unswitching pass (-loop-unswitch) has had several bugs - fixed, has several new features, and is enabled by default in llvmgcc3 - now.
The Loop Strength Reduction pass (-loop-reduce) is now enabled for - the X86 and Alpha backends.
The Instruction Combining pass (-instcombine) now includes a - framework and implementation for simplifying code based on whether computed - bits are demanded or not.
The Scalar Replacement of Aggregates pass (-scalarrepl) can now - promote simple unions to registers.
The Reassociation pass (-reassociate) can now - factor expressions, e.g. turning "A*A+A*B" into "A*(A+B)".
Several LLVM passes are significantly -faster.
The pass manager has been entirely rewritten, making it significantly + smaller, simpler, and more extensible. Support has been added to run + FunctionPasses interlaced with CallGraphSCCPasses.
The -scalarrepl pass can now promote unions containing FP values into + a register, it can also handle unions of vectors of the same size.
The predicate simplifier pass has been improved, making it able to do + simple value range propagation and eliminate more conditionals.
There is a new new LoopPass class. The passmanager has been + modified to support it, and all existing loop xforms have been + converted to use it.
There is a new loop rotation pass, which converts "for loops" into + "do/while loops", where the condition is at the bottom of the loop.
ModulePasses may now use the result of FunctionPasses.
The [Post]DominatorSet classes have been removed from LLVM and clients switched to use the far-more-efficient ETForest class instead.
The ImmediateDominator class has also been removed, and clients have been switched to use DominatorTree instead.

Code Generator -Improvements

Code +Generator Enhancements

+New features include: +

LLVM has a new prepass (before register allocation) list scheduler, which - supports bottom-up and top-down scheduling, pluggable priority functions and - pluggable hazard recognizers. The X86 backend uses this to reduce register - pressure and RISC targets schedule based on operation latency.
The tblgen-based target description framework introduced in LLVM 1.6 has - several new features, useful for targets that can fold loads and stores into - operations, and features that make the .td files more expressive.
The instruction selector is significantly faster in 1.7 than in 1.6.
The X86, Alpha and Itanium backends use new DAG-DAG instruction selectors, - making them easier to maintain and generate slightly better code.
The X86 backend now supports generation of Scalar SSE code for scalar FP - expressions. LLVM provides significantly better performance with Scalar SSE - instructions than it does with the Intel floating point stack - instructions.
The Itanium backend now has a bundling pass, which improves performance - by ~10% and reduces code size (previously it unconditionally inserted a stop - bit after every instruction).
Support for Zero-cost DWARF exception handling has been added. It is mostly + complete and just in need of continued bug fixes and optimizations at + this point.
Progress has been made on a direct Mach-o .o file writer. Many small + apps work, but it is not quite complete yet.
Support was added for software floating point routines.
DWARF debug information generation has been improved. LLVM now passes + most of the GDB testsuite on MacOS and debug info is more dense.
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.
Heuristics have been added to avoid coalescing vregs with very large live + ranges to physregs.
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.
Significantly improved 'switch' lowering, improving codegen for + sparse switches that have dense subregions, and implemented support + for the shift/and trick.
The code generator now has more accurate and general hooks for + describing addressing modes ("isLegalAddressingMode") to + optimizations like loop strength reduction and code sinking.
The Loop Strength Reduction pass has been improved, and support added + for sinking expressions across blocks to reduce register pressure.
Added support for tracking physreg sub-registers and super-registers + in the code generator, as well as extensive register + allocator changes to track them.
There is initial support for virtreg sub-registers + (PR1350).

+ +

In addition, the LLVM target description format has itself been extended in + several ways:

+ +

Extended TargetData to support better target parameterization in + the .ll/.bc files, eliminating the 'pointersize/endianness' attributes + in the files (PR761).
TargetData was generalized for finer grained alignment handling, + handling of vector alignment, and handling of preferred alignment
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.

Other New Features

Target-Specific +Improvements

+ +

X86-Specific Code Generator Enhancements: +

The Mac OS/X PowerPC and X86 backends now have initial support for - Darwin DWARF - debugging information, however, debug info generation has been disabled for - the 1.7 release in llvmgcc4.
LLVM includes the new - llvm-config utility, which makes it easier to build and link programs - against the LLVM libraries when not using the LLVM makefiles.
LLVM now supports first class global ctor/dtor initialization lists, no - longer forcing targets to use "__main".
LLVM supports assigning globals and functions to a particular section - in the result executable using the GCC section attribute.
Adding intrinsics to LLVM is now - significantly easier.
llvmgcc4 now fully supports C99 Variable Length Arrays, including dynamic - stack deallocation.
The scheduler was improved to better reduce register pressure on + X86 and other targets that are register pressure sensitive
Linux/x86-64 support has been improved.
PIC support for linux/x86 has been added.
Support now exists for the GCC regparm attribute, and code in the X86 + backend to respect it.
Various improvements have been made for the X86-64 JIT, allowing it to + generate code in the large code model
LLVM now supports inline asm with multiple constraint letters per operand + (like "ri") which is common in X86 inline asms.
Early support has been added for X86 inline asm in the C backend.
Added support for the X86 MMX instruction set.

+ +

ARM-Specific Code Generator Enhancements: +

+ +

Several improvements have been made to the ARM backend, including basic + inline asm support, weak linkage support, static ctor/dtor support and + many bug fixes.
There are major enhancements to the ARM backend, including support for ARM + v4-v6, vfp support, soft float, pre/postinc support, load/store multiple + generation, constant pool entry motion (to support large functions), + and enhancements to ARM constant island pass. +
Added support for Thumb code generation (an ARM subtarget).
More aggressive size analysis for ARM inline asm strings was + implemented.

+ +

Other Target-Specific Code Generator Enhancements: +

+ +

The PowerPC 64 JIT now supports addressing code loaded above the 2G + boundary.
Improved 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.
Many bugs fixed for PowerPC 64.
Support was added for the ARM AAPCS and EABI ABIs and PIC codegen on + arm/linux.
Several bugs in DWARF debug emission on linux and cygwin/mingw were fixed. + Debugging basically works on these targets now.
Support has been added for the X86-64 large code model to the JIT, + which is useful if JIT'd function bodies are more than 2G away from + library functions.
Several bugs were fixed for DWARF debug info generation on arm/linux.

- -

-Significant Changes in LLVM 1.7 + +

Other Improvements

+ +

This release includes many other improvements, including +performance work, specifically designed to tune datastructure +usage. This makes several critical components faster.

+ +

More specific changes include:

+ +

LLVM no longer relies on static destructors to shut itself down. Instead, + it lazily initializes itself and shuts down when llvm_shutdown() is + explicitly called.
LLVM now has significantly fewer static constructors, reducing startup time. +
Several classes have been refactored to reduce the amount of code that + gets linked into apps that use the JIT.
Construction of intrinsic function declarations has been simplified.
The llvm-upgrade tool now exists. This migrates LLVM 1.9 .ll files to + LLVM 2.0 syntax.
The gccas/gccld tools have been removed.
Support has been added to llvm-test for running on low-memory + or slow machines (make SMALL_PROBLEM_SIZE=1).
llvm-test is now more portable and should build with MS Visual Studio.

+ +

API Changes

+ +

LLVM 2.0 contains a revamp of the type system and several other significant +internal changes. If you are programming to the C++ API, be aware of the +following major changes:

The official LLVM URL is now - http://llvm.org/.
The LLVM intrinsics used to be overloaded based on type: for example, - llvm.ctpop could work with any - integer datatype. They are now separated into different intrinsics with - suffixes to denote their argument type (e.g. llvm.ctpop.i32)). Old - LLVM .ll and .bc files that use these intrinsics will continue to work with - new LLVM versions (they are transparently upgraded by the parsers), but will - cause a warning to be emitted.
The llvm.readport, llvm.writeport, llvm.readio, - and llvm.writeio intrinsics have been removed. The first two - were ever only supported by the X86 backend, the last two were never - correctly supported by any target, and none were accessible through the - C front-end. Inline assembly support can now be used to - implement these operations.
The llvm-db tool had basic support for stepping through code, which - used the JIT. This code has been removed, and DWARF emission support added - instead. llvm-db still exists in CVS if someone wanted to write a - ptrace backend for it.
Pass registration is slightly different in LLVM 2.0 (you now needs an + intptr_t in your constructor), as explained in the Writing an LLVM Pass + document.
ConstantBool, ConstantIntegral and ConstantInt + classes have been merged together, we now just have + ConstantInt.
Type::IntTy, Type::UIntTy, Type::SByteTy, ... are + replaced by Type::Int8Ty, Type::Int16Ty, etc. LLVM types + have always corresponded to fixed size types + (e.g. long was always 64-bits), but the type system no longer includes + information about the sign of the type.
Several classes (CallInst, GetElementPtrInst, + ConstantArray, etc), that once took std::vector as + arguments now take ranges instead. For example, you can create a + GetElementPtrInst with code like: + +
```
+      Value *Ops[] = { Op1, Op2, Op3 };
+      GEP = new GetElementPtrInst(BasePtr, Ops, 3);
+    
```
+ + This avoids creation of a temporary vector (and a call to malloc/free). If + you have an std::vector, use code like this: +
```
+      std::vector<Value*> Ops = ...;
+      GEP = new GetElementPtrInst(BasePtr, &Ops[0], Ops.size());
+    
```
+ +
CastInst is now abstract and its functionality is split into several parts, + one for each of the new cast + instructions.
Instruction::getNext()/getPrev() are now private (along with + BasicBlock::getNext, etc), for efficiency reasons (they are now no + longer just simple pointers). Please use BasicBlock::iterator, etc instead. +
Module::getNamedFunction() is now called + Module::getFunction().
SymbolTable.h has been split into ValueSymbolTable.h and +TypeSymbolTable.h.

@@ -289,10 +479,12 @@ Improvements

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

@@ -332,11 +524,12 @@ 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
The IA64 code generator is experimental.
The Alpha JIT is experimental.
"-filetype=asm" (the default) is the only supported value for the @@ -347,163 +540,197 @@ components, please contact us on the llvmdev list.

- Known problems with the Build System + Known problems with the X86 back-end
- The configure script sometimes fails on Solaris/Sparc. A work around is documented in PR656.
- The X86 backend does not yet support inline + assembly that uses the X86 floating point stack.
+
+ +
+ Known problems with the PowerPC back-end +
+ +
+ +
- PowerPC backend does not correctly +implement ordered FP comparisons.
- The Linux PPC32/ABI support needs testing for the interpreter and static +compilation, and lacks Dwarf debugging informations. +
+ +
- Known problems with the LLVM Core + Known problems with the ARM back-end
- In the JIT, dlsym() on a symbol compiled by the JIT will not - work.
- The Thumb mode works only on ARMv6 or higher processors. On sub-ARMv6 +processors, any thumb program compiled with LLVM crashes or produces wrong +results. (PR1388)
- Compilation for ARM Linux OABI (old ABI) is supported, but not fully tested. +
- QEMU-ARM (<= 0.9.0) wrongly executes programs compiled with LLVM. A non-affected QEMU version must be used or this + +patch must be applied on QEMU.
+
- Known problems with the C front-end + Known problems with the SPARC back-end
- -
Bugs
+
+ +
- The SPARC backend only supports the 32-bit SPARC ABI (-m32), it does not + support the 64-bit SPARC ABI (-m64).
+ +
+ + +
+ Known problems with the Alpha back-end +
-
-llvm-gcc3 has many significant problems that are fixed by llvm-gcc4. See - those blocked on the llvm-gcc4 meta bug. -Two major ones include:
+
- On 21164s, some rare FP arithmetic sequences which may trap do not have the +appropriate nops inserted to ensure restartability.
+
+ + +
+ Known problems with the IA64 back-end +
+ +
- With llvm-gcc3, - C99 variable sized arrays do not release stack memory when they go out of - scope. Thus, the following program may run out of stack space: -
```
-    for (i = 0; i != 1000000; ++i) {
-      int X[n];
-      foo(X);
-    }
-
```
- With llvm-gcc3, Initialization of global union variables can only be done with the largest union member.
- C++ programs are likely to fail on IA64, as calls to setjmp are +made where the argument is not 16-byte aligned, as required on IA64. (Strictly +speaking this is not a bug in the IA64 back-end; it will also be encountered +when building C++ programs using the C back-end.)
- The C++ front-end does not use IA64 +ABI compliant layout of v-tables. In particular, it just stores function +pointers instead of function descriptors in the vtable. This bug prevents +mixing C++ code compiled with LLVM with C++ objects compiled by other C++ +compilers.
- There are a few ABI violations which will lead to problems when mixing LLVM +output with code built with other compilers, particularly for floating-point +programs.
- Defining vararg functions is not supported (but calling them is ok).
- The Itanium backend has bitrotted somewhat.
+
- -
- Notes + +
+ Known problems with the C back-end
- The C backend does not support inline + assembly code.
-
"long double" is 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.
The following Unix system functionality has not been tested and may not -work: -
1. sigsetjmp, siglongjmp - These are not turned into the - appropriate invoke/unwind instructions. Note that - setjmp and longjmp are compiled correctly. -
2. getcontext, setcontext, makecontext - - These functions have not been tested. -