LLD 4.0.0 Release Notes

Introduction

LLD is a linker which supports ELF (Unix), COFF (Windows) and Mach-O (macOS). It is generally faster than the GNU BFD/gold linkers or the MSVC linker.

LLD is designed to be a drop-in replacement for the system linkers, so that users don’t need to change their build systems other than swapping the linker command.

This document contains the release notes for LLD 4.0.0. Here we describe the status of LLD, including major improvements from the previous release. All LLD releases may be downloaded from the LLVM releases web site.

What’s New in LLD 4.0?

ELF Improvements

LLD provides much better compatibility with the GNU linker than before. Now it is able to link the entire FreeBSD base system including the kernel out of the box. We are working closely with the FreeBSD project to make it usable as the system linker in a future release of the operating system.

Multi-threading performance has been improved, and multi-threading is now enabled by default. Combined with other optimizations, LLD 4.0 is about 1.5 times faster than LLD 3.9 when linking large programs in our test environment.

Other notable changes are listed below:

• Error messages contain more information than before. If debug info is available, the linker prints out not only the object file name but the source location of unresolved symbols.
• Error messages are printed in red just like Clang by default. You can disable it by passing -no-color-diagnostics.
• LLD’s version string is now embedded in a .comment section in the result output file. You can dump it with this command: objdump -j -s .comment <file>.
• The -Map option is supported. With that, you can print out section and symbol information to a specified file. This feature is useful for analyzing link results.
• The file format for the -reproduce option has changed from cpio to tar.
• When creating a copy relocation for a symbol, LLD now scans the DSO’s header to see if the symbol is in a read-only segment. If so, space for the copy relocation is reserved in .bss.rel.ro instead of .bss. This fixes a security issue that read-only data in a DSO becomes writable if it is copied by a copy relocation. This issue was disclosed originally on the binutils mailing list.
• Compressed input sections are supported.
• --oformat binary, --section-start, -Tbss, -Tdata, -Ttext, -b binary, -build-id=uuid, -no-rosegment, -nopie, -nostdlib, -omagic, -retain-symbols-file, -sort-section, -z max-page-size and -z wxneeded are supported.
• A lot of linker script directives have been added.
• Default image base address for x86-64 has changed from 0x10000 to 0x200000 to make it huge-page friendly.
• ARM port now supports GNU ifunc, the ARM C++ exceptions ABI, TLS relocations and static linking. Problems with dlopen() on systems using eglibc fixed.
• MIPS port now supports input files in new R6 revision of MIPS ABIs or N32 ABI. Generated file now contains .MIPS.abiflags section and complete set of ELF headers flags.
• Relocations produced by the -mxgot compiler flag is supported for MIPS. Now it is possible to generate “large” GOT that exceeds the 64K limit.

COFF Improvements

• Performance on Windows has been improved by parallelizing parts of the linker and optimizing file system operations. As a result of these improvements, LLD 4.0 has been measured to be about 2.5 times faster than LLD 3.9 when linking a large Chromium DLL.