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LLVM 3.6 Release Notes

Warning

These are in-progress notes for the upcoming LLVM 3.6 release. You may prefer the LLVM 3.5 Release Notes.

Introduction

This document contains the release notes for the LLVM Compiler Infrastructure, release 3.6. Here we describe the status of LLVM, including major improvements from the previous release, improvements in various subprojects of LLVM, and some of the current users of the code. 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 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 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 a specific release, please see the releases page.

Non-comprehensive list of changes in this release

Prefix data rework

The semantics of the prefix attribute have been changed. Users that want the previous prefix semantics should instead use prologue. To motivate this change, let's examine the primary usecases that these attributes aim to serve,

  1. Code sanitization metadata (e.g. Clang's undefined behavior sanitizer)
  2. Function hot-patching: Enable the user to insert nop operations at the beginning of the function which can later be safely replaced with a call to some instrumentation facility.
  3. Language runtime metadata: Allow a compiler to insert data for use by the runtime during execution. GHC is one example of a compiler that needs this functionality for its tables-next-to-code functionality.

Previously prefix served cases (1) and (2) quite well by allowing the user to introduce arbitrary data at the entrypoint but before the function body. Case (3), however, was poorly handled by this approach as it required that prefix data was valid executable code.

In this release the concept of prefix data has been redefined to be data which occurs immediately before the function entrypoint (i.e. the symbol address). Since prefix data now occurs before the function entrypoint, there is no need for the data to be valid code.

The previous notion of prefix data now goes under the name "prologue data" to emphasize its duality with the function epilogue.

The intention here is to handle cases (1) and (2) with prologue data and case (3) with prefix data. See the language reference for further details on the semantics of these attributes.

This refactoring arose out of discussions with Reid Kleckner in response to a proposal to introduce the notion of symbol offsets to enable handling of case (3).

Metadata is not a Value

Metadata nodes (!{...}) and strings (!"...") are no longer values. They have no use-lists, no type, cannot RAUW, and cannot be function-local.

Bridges between Value and Metadata

LLVM intrinsics can reference metadata using the metadata type, and metadata nodes can reference constant values.

Function-local metadata is limited to direct arguments to LLVM intrinsics.

Metadata is typeless

The following old IR:

@g = global i32 0

define void @foo(i32 %v) {
entry:
  call void @llvm.md(metadata !{i32 %v})
  call void @llvm.md(metadata !{i32* @global})
  call void @llvm.md(metadata !0)
  call void @llvm.md(metadata !{metadata !"string"})
  call void @llvm.md(metadata !{metadata !{metadata !1, metadata !"string"}})
  ret void, !bar !1, !baz !2
}

declare void @llvm.md(metadata)

!0 = metadata !{metadata !1, metadata !2, metadata !3, metadata !"some string"}
!1 = metadata !{metadata !2, null, metadata !"other", i32* @global, i32 7}
!2 = metadata !{}

should now be written as:

@g = global i32 0

define void @foo(i32 %v) {
entry:
  call void @llvm.md(metadata i32 %v) ; The only legal place for function-local
                                      ; metadata.
  call void @llvm.md(metadata i32* @global)
  call void @llvm.md(metadata !0)
  call void @llvm.md(metadata !{!"string"})
  call void @llvm.md(metadata !{!{!1, !"string"}})
  ret void, !bar !1, !baz !2
}

declare void @llvm.md(metadata)

!0 = !{!1, !2, !3, !"some string"}
!1 = !{!2, null, !"other", i32* @global, i32 7}
!2 = !{}

Distinct metadata nodes

Metadata nodes can opt-out of uniquing, using the keyword distinct. Distinct nodes are still owned by the context, but are stored in a side table, and not uniqued.

In LLVM 3.5, metadata nodes would drop uniquing if an operand changed to null during optimizations. This is no longer true. However, if an operand change causes a uniquing collision, they become distinct. Unlike LLVM 3.5, where serializing to assembly or bitcode would re-unique the nodes, they now remain distinct.

The following IR:

!named = !{!0, !1, !2, !3, !4, !5, !6, !7, !8}

!0 = !{}
!1 = !{}
!2 = distinct !{}
!3 = distinct !{}
!4 = !{!0}
!5 = distinct !{!0}
!6 = !{!4, !{}, !5}
!7 = !{!{!0}, !0, !5}
!8 = distinct !{!{!0}, !0, !5}

is equivalent to the following:

!named = !{!0, !0, !1, !2, !3, !4, !5, !5, !6}

!0 = !{}
!1 = distinct !{}
!2 = distinct !{}
!3 = !{!0}
!4 = distinct !{!0}
!5 = !{!3, !0, !4}
!6 = distinct !{!3, !0, !4}

Constructing cyclic graphs

During graph construction, if a metadata node transitively references a forward declaration, the node itself is considered "unresolved" until the forward declaration resolves. An unresolved node can RAUW itself to support uniquing. Nodes automatically resolve once all their operands have resolved.

However, cyclic graphs prevent the nodes from resolving. An API client that constructs a cyclic graph must call resolveCycles() to resolve nodes in the cycle.

To save self-references from that burden, self-referencing nodes are implicitly distinct. So the following IR:

!named = !{!0, !1, !2, !3, !4}

!0 = !{!0}
!1 = !{!1}
!2 = !{!2, !1}
!3 = !{!2, !1}
!4 = !{!2, !1}

is equivalent to:

!named = !{!0, !1, !2, !3, !3}

!0 = distinct !{!0}
!1 = distinct !{!1}
!2 = distinct !{!2, !1}
!3 = !{!2, !1}

MDLocation (aka DebugLoc aka DILocation)

There's a new first-class metadata construct called MDLocation (to be followed in subsequent releases by others). It's used for the locations referenced by !dbg metadata attachments.

For example, if an old !dbg attachment looked like this:

define i32 @foo(i32 %a, i32 %b) {
entry:
  %add = add i32 %a, %b, !dbg !0
  ret %add, !dbg !1
}

!0 = metadata !{i32 10, i32 3, metadata !2, metadata !1)
!1 = metadata !{i32 20, i32 7, metadata !3)
!2 = metadata !{...}
!3 = metadata !{...}

the new attachment looks like this:

define i32 @foo(i32 %a, i32 %b) {
entry:
  %add = add i32 %a, %b, !dbg !0
  ret %add, !dbg !1
}

!0 = !MDLocation(line: 10, column: 3, scope: !2, inlinedAt: !1)
!1 = !MDLocation(line: 20, column: 7, scope: !3)
!2 = !{...}
!3 = !{...}

The fields are named, can be reordered, and have sane defaults if left out (although scope: is required).

Alias syntax change

The syntax for aliases is now closer to what is used for global variables

@a = weak global ...
@b = weak alias ...

The order of the alias keyword and the linkage was swapped before.

The old JIT has been removed

All users should transition to MCJIT.

object::Binary doesn't owns the file buffer

It is now just a wrapper, which simplifies using object::Binary with other users of the underlying file.

IR in object files is now supported

Regular object files can contain IR in a section named .llvmbc.

The gold plugin has been rewritten

It is now implemented directly on top of lib/Linker instead of lib/LTO. The API of lib/LTO is sufficiently different from gold's view of the linking process that some cases could not be conveniently implemented.

The new implementation is also lazier and has a save-temps option.

Change in the representation of lazy loaded funcs

Lazy loaded functions are now represented is a way that isDeclaration returns the correct answer even before reading the body.

The opt option -std-compile-opts was removed

It was effectively an alias of -O3.

Python 2.7 is now required

This was done to simplify compatibility with python 3.

The leak detector has been removed

In practice tools like asan and valgrind were finding way more bugs than the old leak detector, so it was removed.

New comdat syntax

The syntax of comdats was changed to

$c = comdat any
@g = global i32 0, comdat($c)
@c = global i32 0, comdat

The version without the parentheses is a syntatic sugar for a comdat with the same name as the global.

Diagnotic infrastructure used by lib/Linker and lib/Bitcode

These libraries now use the diagnostic handler to print errors and warnings. This provides better error messages and simpler error handling.

The PreserveSource linker mode was removed

It was fairly broken and was removed.

Changes to the ARM Backend

During this release ...

Changes to the MIPS Target

During this release the MIPS target has reached a few major milestones. The compiler has gained support for MIPS-II and MIPS-III; become ABI-compatible with GCC for big and little endian O32, N32, and N64; and is now able to compile the Linux kernel for 32-bit targets. Additionally, LLD now supports microMIPS for the O32 ABI on little endian targets, and code generation for microMIPS is almost completely passing the test-suite.

ABI

A large number of bugs have been fixed for big-endian MIPS targets using the N32 and N64 ABI's as well as a small number of bugs affecting other ABI's. Please note that some of these bugs will still affect LLVM-IR generated by LLVM 3.5 since correct code generation depends on appropriate usage of the inreg, signext, and zeroext attributes on all function arguments and returns.

There are far too many corrections to provide a complete list but here are a few notable ones:

  • Big-endian N32 and N64 now interlinks successfully with GCC compiled code. Previously this didn't work for the majority of cases.
  • The registers used to return a structure containing a single 128-bit floating point member on the N32/N64 ABI's have been changed from those specified by the ABI documentation to match those used by GCC. The documentation specifies that $f0 and $f2 should be used but GCC has used $f0 and $f1 for many years.
  • Returning a zero-byte struct no longer causes arguments to be read from the wrong registers when using the O32 ABI.
  • The exception personality has been changed for 64-bit MIPS targets to eliminate warnings about relocations in a read-only section.
  • Incorrect usage of odd-numbered single-precision floating point registers has been fixed when the fastcc calling convention is used with 64-bit FPU's and -mno-odd-spreg.

LLVMLinux

It is now possible to compile the Linux kernel. This currently requires a small number of kernel patches. See the LLVMLinux project for details.

  • Added -mabicalls and -mno-abicalls. The implementation may not be complete but works sufficiently well for the Linux kernel.
  • Fixed multiple compatibility issues between LLVM's inline assembly support and GCC's.
  • Added support for a number of directives used by Linux to the Integrated Assembler.

Miscellaneous

  • Attempting to disassemble l[wd]c[23], s[wd]c[23], cache, and pref no longer triggers an assertion.
  • Added -muclibc and -mglibc to support toolchains that provide both uClibC and GLibC.
  • __SIZEOF_INT128__ is no longer defined for 64-bit targets since 128-bit integers do not work at this time for this target.
  • Using $t4-$t7 with the N32 and N64 ABI is deprecated when -fintegrated-as is in use and will be removed in LLVM 3.7. These names have never been supported by the GNU Assembler for these ABI's.

Changes to the PowerPC Target

There are numerous improvements to the PowerPC target in this release:

  • LLVM now generates the Vector-Scalar eXtension (VSX) instructions from version 2.06 of the Power ISA, for both big- and little-endian targets.
  • LLVM now has a POWER8 instruction scheduling description.
  • Address Sanitizer (ASAN) support is now fully functional.
  • Performance of simple atomic accesses has been greatly improved.
  • Atomic fences now use light-weight syncs where possible, again providing significant performance benefit.
  • The PowerPC target now supports PIC levels (-fPIC vs. -fpic).
  • PPC32 SVR4 now supports small-model PIC.
  • There have been many smaller bug fixes and performance improvements.

Changes to the OCaml bindings

  • The bindings now require OCaml >=4.00.0, ocamlfind, ctypes >=0.3.0 <0.4 and OUnit 2 if tests are enabled.
  • The bindings can now be built using cmake as well as autoconf.
  • LLVM 3.5 has, unfortunately, shipped a broken Llvm_executionengine implementation. In LLVM 3.6, the bindings now fully support MCJIT, however the interface is reworked from scratch using ctypes and is not backwards compatible.
  • Llvm_linker.Mode was removed following the changes in LLVM. This breaks the interface of Llvm_linker.
  • All combinations of ocamlc/ocamlc -custom/ocamlopt and shared/static builds of LLVM are now supported.
  • Absolute paths are not embedded into the OCaml libraries anymore. Either OCaml >=4.02.2 must be used, which includes an rpath-like $ORIGIN mechanism, or META file must be updated for out-of-tree installations; see r221139.
  • As usual, many more functions have been exposed to OCaml.

External Open Source Projects Using LLVM 3.6

An exciting aspect of LLVM is that it is used as an enabling technology for a lot of other language and tools projects. This section lists some of the projects that have already been updated to work with LLVM 3.6.

Portable Computing Language (pocl)

In addition to producing an easily portable open source OpenCL implementation, another major goal of pocl is improving performance portability of OpenCL programs with compiler optimizations, reducing the need for target-dependent manual optimizations. An important part of pocl is a set of LLVM passes used to statically parallelize multiple work-items with the kernel compiler, even in the presence of work-group barriers. This enables static parallelization of the fine-grained static concurrency in the work groups in multiple ways.

TTA-based Co-design Environment (TCE)

TCE is a toolset for designing customized exposed datapath processors based on the Transport triggered architecture (TTA).

The toolset provides a complete co-design flow from C/C++ programs down to synthesizable VHDL/Verilog and parallel program binaries. Processor customization points include the register files, function units, supported operations, and the interconnection network.

TCE uses Clang and LLVM for C/C++/OpenCL C language support, target independent optimizations and also for parts of code generation. It generates new LLVM-based code generators "on the fly" for the designed processors and loads them in to the compiler backend as runtime libraries to avoid per-target recompilation of larger parts of the compiler chain.

Additional Information

A wide variety of additional information is available on the LLVM web page, in particular in the documentation section. The web page also contains versions of the API documentation which is up-to-date with the Subversion version of the source code. You can access versions of these documents specific to this release by going into the llvm/docs/ directory in the LLVM tree.

If you have any questions or comments about LLVM, please feel free to contact us via the mailing lists.