ELF interposition and -Bsymbolic

This article describes ELF interposition, the linker option -Bsymbolic, and its friends. In the end, it will discuss an ambitious plan which I dubbed "the Last Alliance of ELF and Men".

Motivated by a great post by Daniel Colascione ("Python is 1.3x faster when compiled in a way that re-examines shitty technical decisions from the 1990s.") and a recent rant from Linus Torvalds on shared objects' performance issues, I have summarized the current unfortunate ELF state and filed some GCC/binutils feature requests. I believe the performance of our shared object oriented world will be no slower than one with mostly statically linked executables.

(I wrote -fno-semantic-interposition first but then realized reorganization would improve readability, so moved some parts and added some stuff to this new article.)

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Weak symbol

Updated in 2023-10.

C/C++

GCC and Clang support __attribute__((weak)) which marks a symbol weak. The same effect can be achieved with a preprocessor directive #pragma weak symbol.

Object file format

In ELF, there are three main symbol bindings. The ELF specification says:

  • STB_LOCAL: Local symbols are not visible outside the object file containing their definition. Local symbols of the same name may exist in multiple files without interfering with each other.
  • STB_GLOBAL: Global symbols are visible to all object files being combined. One file's definition of a global symbol will satisfy another file's undefined reference to the same global symbol.
  • STB_WEAK: Weak symbols resemble global symbols, but their definitions have lower precedence.

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The dark side of RISC-V linker relaxation

Updated in 2025-07.

This article introduces RISC-V linker relaxation and describes the downside.

Linker optimization/relaxation

Because the linker has a global view and layout information, it can perform some peephole optimizations which are difficult/impossible to do on the compiler side. Generic link-time code sequence transformation is risky, because semantic information is lost and what the linker sees are byte streams. However, if every instruction in the candidate code sequence is associated with one ore more relocations, the ABI and the implementation can assign (additional) semantics to the relocation types and make such transformation safe. This technique is usually called linker optimization or linker relaxation. It seems that the term "linker optimization" is often used when the number of bytes does not change while "linker relaxation" is used when the number of bytes decreases.

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All about thread-local storage

Updated in 2025-02.

Thread-local storage (TLS) provides a mechanism allocating distinct objects for different threads. It is the usual implementation for GCC extension __thread, C11 _Thread_local, and C++11 thread_local, which allow the use of the declared name to refer to the entity associated with the current thread. This article will describe thread-local storage on ELF platforms in detail, and touch on other related topics, such as: thread-specific data keys and Windows/macOS TLS.

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Metadata sections, COMDAT and SHF_LINK_ORDER

Updated in 2025-09.

Modern compilers create metadata sections to support instrumentation, debugging, profiling, and other features. These sections should be retained when their associated code is kept and discarded when their code is removed-but achieving this requires careful linker considerations. This post explores the evolution of solutions, from early workarounds to modern mechanisms like section groups and SHF_LINK_ORDER, examining their trade-offs and real-world applications.

Metadata sections: properties and categories

Metadata sections usually have the following property:

  • Relocations within a metadata section refer to its associated text section or any other auxiliary metadata sections. In many applications, a metadata section doesn't need additional auxiliary sections.
  • Text sections don't have relocations that point to their metadata section. In some cases, a text section might reference its metadata section, but this means that the text section can't be inlined into another.

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Everything I know about GNU toolchain

As mainly an LLVM person, I occasionally contribute to GNU toolchain projects. This is sometimes for fun, sometimes for investigating why an (usually ancient) feature works in a particular way, sometimes for pushing forward a toolchain feature with the mind of both communities, or sometimes just for getting sense of how things work with mailing list+GNU make.

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