LLVM 7.0.0 Release Notes

Introduction

This document contains the release notes for the LLVM Compiler Infrastructure, release 7.0.0. 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.

Non-comprehensive list of changes in this release

• The Windows installer no longer includes a Visual Studio integration. Instead, a new LLVM Compiler Toolchain Visual Studio extension is available on the Visual Studio Marketplace. The new integration supports Visual Studio 2017.
• Libraries have been renamed from 7.0 to 7. This change also impacts downstream libraries like lldb.
• The LoopInstSimplify pass (-loop-instsimplify) has been removed.
• Symbols starting with ? are no longer mangled by LLVM when using the Windows x or w IR mangling schemes.
• A new tool named llvm-exegesis has been added. llvm-exegesis automatically measures instruction scheduling properties (latency/uops) and provides a principled way to edit scheduling models.
• A new tool named llvm-mca has been added. llvm-mca is a static performance analysis tool that uses information available in LLVM to statically predict the performance of machine code for a specific CPU.
• Optimization of floating-point casts is improved. This may cause surprising results for code that is relying on the undefined behavior of overflowing casts. The optimization can be disabled by specifying a function attribute: "strict-float-cast-overflow"="false". This attribute may be created by the clang option -fno-strict-float-cast-overflow. Code sanitizers can be used to detect affected patterns. The clang option for detecting this problem alone is -fsanitize=float-cast-overflow:

int main() {
  float x = 4294967296.0f;
  x = (float)((int)x);
  printf("junk in the ftrunc: %f\n", x);
  return 0;
}

clang -O1 ftrunc.c -fsanitize=float-cast-overflow ; ./a.out
ftrunc.c:5:15: runtime error: 4.29497e+09 is outside the range of representable values of type 'int'
junk in the ftrunc: 0.000000

• LLVM_ON_WIN32 is no longer set by llvm/Config/config.h and llvm/Config/llvm-config.h. If you used this macro, use the compiler-set _WIN32 instead which is set exactly when LLVM_ON_WIN32 used to be set.
• The DEBUG macro has been renamed to LLVM_DEBUG, the interface remains the same. If you used this macro you need to migrate to the new one. You should also clang-format your code to make it easier to integrate future changes locally. This can be done with the following bash commands:

git grep -l 'DEBUG' | xargs perl -pi -e 's/\bDEBUG\s?\(/LLVM_DEBUG(/g'
git diff -U0 master | ../clang/tools/clang-format/clang-format-diff.py -i -p1 -style LLVM

• Early support for UBsan, X-Ray instrumentation and libFuzzer (x86 and x86_64) for OpenBSD. Support for MSan (x86_64), X-Ray instrumentation and libFuzzer (x86 and x86_64) for FreeBSD.
• SmallVector<T, 0> shrank from sizeof(void*) * 4 + sizeof(T) to sizeof(void*) + sizeof(unsigned) * 2, smaller than std::vector<T> on 64-bit platforms. The maximum capacity is now restricted to UINT32_MAX. Since SmallVector doesn’t have the exception-safety pessimizations some implementations saddle std::vector with and is better at using realloc, it’s now a better choice even on the heap (although when TinyPtrVector works, that’s even smaller).
• Preliminary/experimental support for DWARF v5 debugging information, including the new .debug_names accelerator table. DWARF emitted at -O0 should be fully DWARF v5 compliant. Type units and split DWARF are known not to be compliant, and higher optimization levels will still emit some information in v4 format.
• Added support for the .rva assembler directive for COFF targets.
• The llvm-rc tool (Windows Resource Compiler) has been improved a bit. There are still known missing features, but it is generally usable in many cases. (The tool still doesn’t preprocess input files automatically, but it can now handle leftover C declarations in preprocessor output, if given output from a preprocessor run externally.)
• CodeView debug info can now be emitted for MinGW configurations, if requested.
• The opt tool now supports the -load-pass-plugin option for loading pass plugins for the new PassManager.
• Support for profiling JITed code with perf.

Changes to the LLVM IR

• The signatures for the builtins @llvm.memcpy, @llvm.memmove, and @llvm.memset have changed. Alignment is no longer an argument, and are instead conveyed as parameter attributes.
• invariant.group.barrier has been renamed to launder.invariant.group.
• invariant.group metadata can now refer only to empty metadata nodes.

Changes to the AArch64 Target

• The .inst assembler directive is now usable on both COFF and Mach-O targets, in addition to ELF.
• Support for most remaining COFF relocations has been added.
• Support for TLS on Windows has been added.
• Assembler and disassembler support for the ARM Scalable Vector Extension has been added.

Changes to the ARM Target

• The .inst assembler directive is now usable on both COFF and Mach-O targets, in addition to ELF. For Thumb, it can now also automatically deduce the instruction size, without having to specify it with e.g. .inst.w as before.

Changes to the Hexagon Target

• Hexagon now supports auto-vectorization for HVX. It is disabled by default and can be turned on with -fvectorize. For auto-vectorization to take effect, code generation for HVX needs to be enabled with -mhvx. The complete set of options should include -fvectorize, -mhvx, and -mhvx-length={64b|128b}.
• The support for Hexagon ISA V4 is deprecated and will be removed in the next release.

Changes to the MIPS Target

During this release the MIPS target has:

• Added support for Virtualization, Global INValidate ASE, and CRC ASE instructions.
• Introduced definitions of [d]rem, [d]remu, and microMIPSR6 ll/sc instructions.
• Shrink-wrapping is now supported and enabled by default (except for -O0).
• Extended size reduction pass by the LWP and SWP instructions.
• Gained initial support of GlobalISel instruction selection framework.
• Updated the P5600 scheduler model not to use instruction itineraries.
• Added disassembly support for comparison and fused (negative) multiply add/sub instructions.
• Improved the selection of multiple instructions.
• Load/store lb, sb, ld, sd, lld, … instructions now support 32/64-bit offsets.
• Added support for y, M, and L inline assembler operand codes.
• Extended list of relocations supported by the .reloc directive
• Fixed using a wrong register class for creating an emergency spill slot for mips3 / n64 ABI.
• MIPS relocation types were generated for microMIPS code.
• Corrected definitions of multiple instructions (lwp, swp, ctc2, cfc2, sync, synci, cvt.d.w, …).
• Fixed atomic operations at -O0 level.
• Fixed local dynamic TLS with Sym64

Changes to the PowerPC Target

During this release the PowerPC target has:

• Replaced the list scheduler for post register allocation with the machine scheduler.
• Added support for coldcc calling convention.
• Added support for symbol@high and symbol@higha symbol modifiers.
• Added support for quad-precision floating point type (__float128) under the llvm option -enable-ppc-quad-precision.
• Added dump function to LatencyPriorityQueue.
• Completed the Power9 scheduler model.
• Optimized TLS code generation.
• Improved MachineLICM for hoisting constant stores.
• Improved code generation to reduce register use by using more register + immediate instructions.
• Improved code generation to better exploit rotate-and-mask instructions.
• Fixed the bug in dynamic loader for JIT which crashed NNVM.
• Numerous bug fixes and code cleanups.

Changes to the SystemZ Target

During this release the SystemZ target has:

• Added support for vector registers in inline asm statements.
• Added support for stackmaps, patchpoints, and the anyregcc calling convention.
• Changed the default function alignment to 16 bytes.
• Improved codegen for condition code handling.
• Improved instruction scheduling and microarchitecture tuning for z13/z14.
• Fixed support for generating GCOV coverage data.
• Fixed some codegen bugs.

Changes to the X86 Target

• The calling convention for the f80 data type on MinGW targets has been fixed. Normally, the calling convention for this type is handled within clang, but if an intrinsic is used, which LLVM expands into a libcall, the proper calling convention needs to be supported in LLVM as well. (Note, on Windows, this data type is only used for long doubles in MinGW environments - in MSVC environments, long doubles are the same size as normal doubles.)

Changes to the OCaml bindings

• Removed add_bb_vectorize.

Changes to the C API

• Removed LLVMAddBBVectorizePass. The implementation was removed and the C interface was made a deprecated no-op in LLVM 5. Use LLVMAddSLPVectorizePass instead to get the supported SLP vectorizer.
• Expanded the OrcJIT APIs so they can register event listeners like debuggers and profilers.

Changes to the DAG infrastructure

• ADDC/ADDE/SUBC/SUBE are now deprecated and will default to expand. Backends that wish to continue to use these opcodes should explicitely request to do so using setOperationAction in their TargetLowering. New backends should use UADDO/ADDCARRY/USUBO/SUBCARRY instead of the deprecated opcodes.
• The SETCCE opcode has now been removed in favor of SETCCCARRY.
• TableGen now supports multi-alternative pattern fragments via the PatFrags class. PatFrag is now derived from PatFrags, which may require minor changes to backends that directly access PatFrag members.

External Open Source Projects Using LLVM 7

Zig Programming Language

Zig is an open-source programming language designed for robustness, optimality, and clarity. Zig is an alternative to C, providing high level features such as generics, compile time function execution, partial evaluation, and LLVM-based coroutines, while exposing low level LLVM IR features such as aliases and intrinsics. Zig uses Clang to provide automatic import of .h symbols - even inline functions and macros. Zig uses LLD combined with lazily building compiler-rt to provide out-of-the-box cross-compiling for all supported targets.

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.