LLVM 1.7 Release Notes
- What's New?
- Installation Instructions
- Portability and Supported Platforms
- Known Problems
- Additional Information
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.
For more information about LLVM, including information about the latest
release, please check out the main LLVM
web site. If you have questions or comments, the LLVM developer's mailing
list is a good place to send them.
Note that if you are reading this file from CVS 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.
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.
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.
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
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.
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 includes significantly extended support for SIMD vectors in its
core instruction set. It now includes three new instructions for manipulating
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.
Because llvm-gcc3 does not support GCC generic vectors or vector intrinsics,
llvm-gcc4 must be used.
The LLVM X86 backend now supports Intel SSE 1, 2, and 3, and now uses scalar
SSE operations to implement scalar floating point math when the target supports
SSE1 (for floats) or SSE2 (for doubles). Vector SSE instructions are generated
by llvm-gcc4 when the generic vector mechanism or specific SSE intrinsics are
The LLVM PowerPC backend now supports the Altivec instruction set, including
both GCC -maltivec and -faltivec modes. Altivec instructions are generated
by llvm-gcc4 when the generic vector mechanism or specific Altivec intrinsics
- The Loop Unswitching pass (-loop-unswitch) has had several bugs
fixed, has several new features, and is enabled by default in llvmgcc3
- 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
- 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
- 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).
- The Mac OS/X PowerPC and X86 backends now have initial support for
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
- llvmgcc4 now fully supports C99 Variable Length Arrays, including dynamic
- The official LLVM URL is now
- 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.
LLVM is known to work on the following platforms:
- Intel and AMD machines running Red Hat Linux, Fedora Core and FreeBSD
(and probably other unix-like systems).
- 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.
- Alpha-based machines running Debian GNU/Linux.
- Itanium-based machines running Linux and HP-UX.
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.
This section contains all known problems with the LLVM system, listed by
component. As new problems are discovered, they will be added to these
sections. If you run into a problem, please check the LLVM bug database and submit a bug if
there isn't already one.
The following components of this LLVM release are either untested, known to
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.
- The -cee pass is known to be buggy, and may be removed in in a
- 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.
- In the JIT, dlsym() on a symbol compiled by the JIT will not
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:
- "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
- The following Unix system functionality has not been tested and may not
- sigsetjmp, siglongjmp - These are not turned into the
appropriate invoke/unwind instructions. Note that
setjmp and longjmp are compiled correctly.
- getcontext, setcontext, makecontext
- These functions have not been tested.
- Although many GCC extensions are supported, some are not. In particular,
the following extensions are known to not be supported:
- Local Labels: Labels local to a block.
- Nested Functions: As in Algol and Pascal, lexical scoping of functions.
- Constructing Calls: Dispatching a call to another function.
- Extended Asm: Assembler instructions with C expressions as operands.
- Constraints: Constraints for asm operands.
- Asm Labels: Specifying the assembler name to use for a C symbol.
- Explicit Reg Vars: Defining variables residing in specified registers.
- Vector Extensions: Using vector instructions through built-in functions.
- Target Builtins: Built-in functions specific to particular targets.
- Thread-Local: Per-thread variables.
- Pragmas: Pragmas accepted by GCC.
The following GCC extensions are partially supported. An ignored
attribute means that the LLVM compiler ignores the presence of the attribute,
but the code should still work. An unsupported attribute is one which is
ignored by the LLVM compiler and will cause a different interpretation of
- Variable Length:
Arrays whose length is computed at run time.
Supported, but allocated stack space is not freed until the function returns (noted above).
- Function Attributes:
Declaring that functions have no side effects or that they can never
Supported: format, format_arg, non_null,
noreturn, constructor, destructor,
deprecated, warn_unused_result, weak
always_inline, pure, const, nothrow,
malloc, no_instrument_function, cdecl
Unsupported: section, alias,
visibility, regparm, stdcall,
fastcall, all other target specific attributes
- Variable Attributes:
Specifying attributes of variables.
Supported: cleanup, common, nocommon,
unused, used, weak
Unsupported: aligned, mode, packed,
section, shared, tls_model,
dllexport, all target specific attributes.
- Type Attributes: Specifying attributes of types.
Supported: transparent_union, unused,
Unsupported: aligned, packed,
all target specific attributes.
- Other Builtins:
Other built-in functions.
We support all builtins which have a C language equivalent (e.g.,
__builtin_constant_p, and __builtin_expect
(currently ignored). We also support builtins for ISO C99 floating
point comparison macros (e.g., __builtin_islessequal),
__builtin_clz[ll], and __builtin_ctz[ll].
The following extensions are known to be supported:
- Labels as Values: Getting pointers to labels and computed gotos.
- Statement Exprs: Putting statements and declarations inside expressions.
typeof: referring to the type of an expression.
- Lvalues: Using
," and casts in lvalues.
- Conditionals: Omitting the middle operand of a
- Long Long: Double-word integers.
- Complex: Data types for complex numbers.
- Hex Floats:Hexadecimal floating-point constants.
- Zero Length: Zero-length arrays.
- Empty Structures: Structures with no members.
- Variadic Macros: Macros with a variable number of arguments.
- Escaped Newlines: Slightly looser rules for escaped newlines.
- Subscripting: Any array can be subscripted, even if not an lvalue.
- Pointer Arith: Arithmetic on
void-pointers and function pointers.
- Initializers: Non-constant initializers.
- Compound Literals: Compound literals give structures, unions,
or arrays as values.
- Designated Inits: Labeling elements of initializers.
- Cast to Union: Casting to union type from any member of the union.
- Case Ranges: `case 1 ... 9' and such.
- Mixed Declarations: Mixing declarations and code.
- Function Prototypes: Prototype declarations and old-style definitions.
- C++ Comments: C++ comments are recognized.
- Dollar Signs: Dollar sign is allowed in identifiers.
- Character Escapes:
\e stands for the character <ESC>.
- Alignment: Inquiring about the alignment of a type or variable.
- Inline: Defining inline functions (as fast as macros).
- Alternate Keywords:
__asm__, etc., for header files.
- Incomplete Enums:
enum foo;, with details to follow.
- Function Names: Printable strings which are the name of the current function.
- Return Address: Getting the return or frame address of a function.
- Unnamed Fields: Unnamed struct/union fields within structs/unions.
- Attribute Syntax: Formal syntax for attributes.
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).
For this release, the C++ front-end is considered to be fully
tested and works for a number of non-trivial programs, including LLVM
- The C++ front-end inherits all problems afflicting the C
- Destructors for local objects are not always run when a longjmp is
performed. In particular, destructors for objects in the longjmping
function and in the setjmp receiver function may not be run.
Objects in intervening stack frames will be destroyed, however (which is
better than most compilers).
- The LLVM C++ front-end follows the Itanium C++ ABI.
This document, which is not Itanium specific, specifies a standard for name
mangling, class layout, v-table layout, RTTI formats, and other C++
representation issues. Because we use this API, code generated by the LLVM
compilers should be binary compatible with machine code generated by other
Itanium ABI C++ compilers (such as G++, the Intel and HP compilers, etc).
However, the exception handling mechanism used by LLVM is very
different from the model used in the Itanium ABI, so exceptions will not
- The C back-end produces code that violates the ANSI C Type-Based Alias
Analysis rules. As such, special options may be necessary to compile the code
(for example, GCC requires the -fno-strict-aliasing option). This
problem probably cannot be fixed.
- Zero arg vararg functions are not
supported. This should not affect LLVM produced by the C or C++
- The C backend does not correctly implement the llvm.stacksave or
intrinsics. This means that some code compiled by it can run out of stack
space if they depend on these (e.g. C99 varargs).
- On 21164s, some rare FP arithmetic sequences which may trap do not have the
appropriate nops inserted to ensure restartability.
- 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++
- 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
- Defining vararg functions is not supported (but calling them is ok).
- The SPARC backend only supports the 32-bit SPARC ABI (-m32), it does not
support the 64-bit SPARC ABI (-m64).
A wide variety of additional information is available on the LLVM web page, including documentation and publications describing algorithms and
components implemented in LLVM. The web page also contains versions of the
API documentation which is up-to-date with the CVS version of the source code.
You can access versions of these documents specific to this release by going
into the "llvm/doc/" directory in the LLVM tree.
If you have any questions or comments about LLVM, please feel free to contact
us via the mailing
The LLVM Compiler Infrastructure
Last modified: $Date: 2006/04/20 06:10:32 $