WebAssembly lld port

The WebAssembly version of lld takes WebAssembly binaries as inputs and produces a WebAssembly binary as its output. For the most part it tries to mimic the behaviour of traditional ELF linkers and specifically the ELF lld port. Where possible the command line flags and the semantics should be the same.

Object file format

The WebAssembly object file format used by LLVM and LLD is specified as part of the WebAssembly tool conventions on linking.

This is the object format that the llvm will produce when run with the wasm32-unknown-unknown target.

Usage

The WebAssembly version of lld is installed as wasm-ld. It shared many common linker flags with ld.lld but also includes several WebAssembly-specific options:

--no-entry

Don’t search for the entry point symbol (by default _start).

--export-table

Export the function table to the environment.

--import-table

Import the function table from the environment.

--export-all

Export all symbols (normally combined with –no-gc-sections)

Note that this will not export linker-generated mutable globals unless the resulting binaryen already includes the ‘mutable-globals’ features since that would otherwise create and invalid binaryen.

--export-dynamic

When building an executable, export any non-hidden symbols. By default only the entry point and any symbols marked as exports (either via the command line or via the export-name source attribute) are exported.

--global-base=<value>

Address at which to place global data.

--no-merge-data-segments

Disable merging of data segments.

--stack-first

Place stack at start of linear memory rather than after data.

--compress-relocations

Relocation targets in the code section are 5-bytes wide in order to potentially accommodate the largest LEB128 value. This option will cause the linker to shrink the code section to remove any padding from the final output. However because it affects code offset, this option is not compatible with outputting debug information.

--allow-undefined

Allow undefined symbols in linked binary. This is the legacy flag which corresponds to --unresolve-symbols=ignore + --import-undefined.

--allow-undefined-file=<filename>

Like --allow-undefined, but the filename specified a flat list of symbols, one per line, which are allowed to be undefined.

--unresolved-symbols=<method>

This is a more full featured version of --allow-undefined. The semanatics of the different methods are as follows:

report-all:

Report all unresolved symbols. This is the default. Normally the linker will generate an error message for each reported unresolved symbol but the option --warn-unresolved-symbols can change this to a warning.

ignore-all:

Resolve all undefined symbols to zero. For data and function addresses this is trivial. For direct function calls, the linker will generate a trapping stub function in place of the undefined function.

import-dynamic:

Undefined symbols generate WebAssembly imports, including undefined data symbols. This is somewhat similar to the –import-undefined option but works all symbol types. This options puts limitations on the type of relocations that are allowed for imported data symbols. Relocations that require absolute data addresses (i.e. All R_WASM_MEMORY_ADDR_I32) will generate an error if they cannot be resolved statically. For clang/llvm this means inputs should be compiled with -fPIC (i.e. pic or dynamic-no-pic relocation models). This options is useful for linking binaries that are themselves static (non-relocatable) but whose undefined symbols are resolved by a dynamic linker. Since the dynamic linking API is experimental, this option currently requires –experimental-pic to also be specified.

--import-memory

Import memory from the environment.

--import-undefined

Generate WebAssembly imports for undefined symbols, where possible. For example, for function symbols this is always possible, but in general this is not possible for undefined data symbols. Undefined data symbols will still be reported as normal (in accordance with --unresolved-symbols).

--initial-heap=<value>

Initial size of the heap. Default: zero.

--initial-memory=<value>

Initial size of the linear memory. Default: the sum of stack, static data and heap sizes.

--max-memory=<value>

Maximum size of the linear memory. Default: unlimited.

By default the function table is neither imported nor exported, but defined for internal use only.

Behaviour

In general, where possible, the WebAssembly linker attempts to emulate the behaviour of a traditional ELF linker, and in particular the ELF port of lld. For more specific details on how this is achieved see the tool conventions on linking.

Function Signatures

One way in which the WebAssembly linker differs from traditional native linkers is that function signature checking is strict in WebAssembly. It is a validation error for a module to contain a call site that doesn’t agree with the target signature. Even though this is undefined behaviour in C/C++, it is not uncommon to find this in real-world C/C++ programs. For example, a call site in one compilation unit which calls a function defined in another compilation unit but with too many arguments.

In order not to generate such invalid modules, lld has two modes of handling such mismatches: it can simply error-out or it can create stub functions that will trap at runtime (functions that contain only an unreachable instruction) and use these stub functions at the otherwise invalid call sites.

The default behaviour is to generate these stub function and to produce a warning. The --fatal-warnings flag can be used to disable this behaviour and error out if mismatched are found.

Exports

When building a shared library any symbols marked as visibility=default will be exported.

When building an executable, only the entry point (_start) and symbols with the WASM_SYMBOL_EXPORTED flag are exported by default. In LLVM the WASM_SYMBOL_EXPORTED flag is set by the wasm-export-name attribute which in turn can be set using __attribute__((export_name)) clang attribute.

In addition, symbols can be exported via the linker command line using --export (which will error if the symbol is not found) or --export-if-defined (which will not).

Finally, just like with native ELF linker the --export-dynamic flag can be used to export symbols in the executable which are marked as visibility=default.

Imports

By default no undefined symbols are allowed in the final binary. The flag --allow-undefined results in a WebAssembly import being defined for each undefined symbol. It is then up to the runtime to provide such symbols. --allow-undefined-file is the same but allows a list of symbols to be specified.

Alternatively symbols can be marked in the source code as with the import_name and/or import_module clang attributes which signals that they are expected to be undefined at static link time.

Stub Libraries

Another way to specify imports and exports is via a “stub library”. This feature is inspired by the ELF stub objects which are supported by the Solaris linker. Stub libraries are text files that can be passed as normal linker inputs, similar to how linker scripts can be passed to the ELF linker. The stub library is a stand-in for a set of symbols that will be available at runtime, but doesn’t contain any actual code or data. Instead it contains just a list of symbols, one per line. Each symbol can specify zero or more dependencies. These dependencies are symbols that must be defined, and exported, by the output module if the symbol is question is imported/required by the output module.

For example, imagine the runtime provides an external symbol foo that depends on the malloc and free. This can be expressed simply as:

#STUB
foo: malloc,free

Here we are saying that foo is allowed to be imported (undefined) but that if it is imported, then the output module must also export malloc and free to the runtime. If foo is imported (undefined), but the output module does not define malloc and free then the link will fail.

Stub libraries must begin with #STUB on a line by itself.

Garbage Collection

Since WebAssembly is designed with size in mind the linker defaults to --gc-sections which means that all unused functions and data segments will be stripped from the binary.

The symbols which are preserved by default are:

  • The entry point (by default _start).

  • Any symbol which is to be exported.

  • Any symbol transitively referenced by the above.

Weak Undefined Functions

On native platforms, calls to weak undefined functions end up as calls to the null function pointer. With WebAssembly, direct calls must reference a defined function (with the correct signature). In order to handle this case the linker will generate function a stub containing only the unreachable instruction and use this for any direct references to an undefined weak function.

For example a runtime call to a weak undefined function foo will up trapping on unreachable inside and linker-generated function called undefined:foo.

Missing features