lld 9.0.0 Release Notes — lld 9 documentation

# lld 9.0.0 Release Notes

- Introduction

- Non-comprehensive list of changes in this release

  - ELF Improvements

  - COFF Improvements

  - WebAssembly Improvements

  - MinGW Improvements

## Introduction

lld is a high-performance linker that supports ELF (Unix), COFF (Windows), Mach-O (macOS), MinGW and WebAssembly. lld is command-line-compatible with GNU linkers and Microsoft link.exe and is significantly faster than the system default linkers.

lld 9 has lots of feature improvements and bug fixes.

## Non-comprehensive list of changes in this release

### ELF Improvements

- ld.lld now has typo suggestions for flags: `$ ld.lld --call-shared` now prints `unknown argument '--call-shared', did you mean '--call_shared'`. (r361518)

- `--allow-shlib-undefined` and `--no-allow-shlib-undefined` options are added. `--no-allow-shlib-undefined` is the default for executables. (r352826)

- `-nmagic` and `-omagic` options are fully supported. (r360593)

- Segment layout has changed. PT_GNU_RELRO, which was previously placed in the middle of readable/writable PT_LOAD segments, is now placed at the beginning of them. This change permits lld-produced ELF files to be read correctly by GNU strip older than 2.31, which has a bug to discard a PT_GNU_RELRO in the former layout.

- `-z common-page-size` is supported. (r360593)

- Diagnostics messages have improved. A new flag `--vs-diagnostics` alters the format of diagnostic output to enable source hyperlinks in Microsoft Visual Studio IDE.

- Linker script compatibility with GNU BFD linker has generally improved.

- The clang `--dependent-library` form of autolinking is supported.

  This feature is added to implement the Windows-style autolinking for Unix. On Unix, in order to use a library, you usually have to include a header file provided by the library and then explicitly link the library with the linker `-l` option. On Windows, header files usually contain pragmas that list needed libraries. Compilers copy that information to object files, so that linkers can automatically link needed libraries. `--dependent-library` is added for implementing that Windows semantics on Unix. (r360984)

- AArch64 BTI and PAC are supported. (r362793)

- lld now supports replacing `JAL` with `JALX` instructions in case of MIPS-microMIPS cross-mode jumps. (r354311)

- lld now creates LA25 thunks for MIPS R6 code. (r354312)

- Put MIPS-specific .reginfo, .MIPS.options, and .MIPS.abiflags sections into corresponding PT_MIPS_REGINFO, PT_MIPS_OPTIONS, and PT_MIPS_ABIFLAGS segments.

- The quality of RISC-V and PowerPC ports have greatly improved. Many applications can now be linked by lld. PowerPC64 is now almost production ready.

- The Linux kernel for arm32_7, arm64, ppc64le and x86_64 can now be linked by lld.

- x86-64 TLSDESC is supported. (r361911, r362078)

- DF_STATIC_TLS flag is set for i386 and x86-64 when needed. (r353293, r353378)

- The experimental partitioning feature is added to allow a program to be split into multiple pieces.

  The feature allows you to semi-automatically split a single program into multiple ELF files called “partitions”. Since all partitions share the same memory address space and don’t use PLT/GOT, split programs run as fast as regular programs.

  With the mechanism, you can start a program only with a “main” partition and load remaining partitions on-demand. For example, you can split a web browser into a main partition and a PDF reader sub-partition and load the PDF reader partition only when a user tries to open a PDF file.

  See the documentation for more information.

- If “-” is given as an output filename, lld writes the final result to the standard output. Previously, it created a file “-” in the current directory. (r351852)

- `-z ifunc-noplt` option is added to reduce IFunc function call overhead in a freestanding environment such as the OS kernel.

  Functions resolved by the IFunc mechanism are usually dispatched via PLT and thus slower than regular functions because of the cost of indirection. With `-z ifunc-noplt`, you can eliminate it by doing text relocations at load-time. You need a special loader to utilize this feature. This feature is added for the FreeBSD kernel but can be used by any operating systems. (r360685)

- `--undefined-glob` option is added. The new option is an extension to `--undefined` to take a glob pattern instead of a single symbol name. (r363396)

### COFF Improvements

- Like the ELF driver, lld-link now has typo suggestions for flags. (r361518)

- lld-link now correctly reports duplicate symbol errors for object files that were compiled with `/Gy`. (r352590)

- lld-link now correctly reports duplicate symbol errors when several resource (.res) input files define resources with the same type, name and language. This can be demoted to a warning using `/force:multipleres`. (r359829)

- lld-link now rejects more than one resource object input files, matching link.exe. Previously, lld-link would silently ignore all but one. If you hit this: Don’t pass resource object files to the linker, instead pass res files to the linker directly. Don’t put resource files in static libraries, pass them on the command line. (r359749)

- Having more than two `/natvis:` now works correctly; it used to not work for larger binaries before. (r327895)

- Undefined symbols are now printed only in demangled form. Pass `/demangle:no` to see raw symbol names instead. (r355878)

- Several speed and memory usage improvements.

- lld-link now supports resource object files created by GNU windres and MS cvtres, not only llvm-cvtres.

- The generated thunks for delayimports now share the majority of code among thunks, significantly reducing the overhead of using delayimport. (r365823)

- `IMAGE_REL_ARM{,64}_REL32` relocations are supported. (r352325)

- Range extension thunks for AArch64 are now supported, so lld can create large executables for Windows/ARM64. (r352929)

- The following flags have been added: `/functionpadmin` (r354716), `/swaprun:` (r359192), `/threads:no` (r355029), `/filealign` (r361634)

### WebAssembly Improvements

- Imports from custom module names are supported. (r352828)

- Symbols that are in llvm.used are now exported by default. (r353364)

- Initial support for PIC and dynamic linking has landed. (r357022)

- wasm-ld now add `__start_`/`__stop_` symbols for data sections. (r361236)

- wasm-ld now doesn’t report an error on archives without a symbol index. (r364338)

- The following flags have been added: `--emit-relocs` (r361635), `--wrap` (r361639), `--trace` and `--trace-symbol` (r353264).

### MinGW Improvements

- lld now correctly links crtend.o as the last object file, handling terminators for the sections such as .eh_frame properly, fixing DWARF exception handling with libgcc and gcc’s crtend.o.

- lld now also handles DWARF unwind info generated by GCC, when linking with libgcc.

- PDB output can be requested without manually specifying the PDB file name, with the new option `-pdb=` with an empty value to the option. (The old existing syntax `-pdb <filename>` was more cumbersome to use with an empty parameter value.)

- `--no-insert-timestamp` option is added as an alias to `/timestamp:0`. (r353145)

- Many more GNU ld options are now supported, which e.g. allows the lld MinGW frontend to be called by GCC.

- The following options are added: `--exclude-all-symbols`, `--appcontainer`, `--undefined`