UNDER CONSTRUCTION (COFF, Mach-O)
Symbol processing is a major step in a linker. In most binary formats, the linker maintains a global symbol table and performs symbols resolution for each input file (object file, shared object, archive, LLVM bitcode file). Some command line options can define/undefine symbols as well. The symbol resolution can affect archive processing and many subsequent steps (LTO, relocation processing, as-needed shared objects, etc).
This article describes the dependency related linker options -z defs, --no-allow-shlib-undefined, and --warn-backrefs. Deploying them in a build system can improve build health.
-z defsThis 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.)
Updated in 2022-05.
This article is a continuation to ELF interposition and -Bsymbolic. It focuses on the GCC/Clang option -fno-semantic-interposition and why it can (sometimes incredibly) optimize -fPIC programs.
GCC and Clang support __attribute__((weak)) which marks a symbol weak. The same effect can be achieved with a preprocessor directive #pragma weak symbol.
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.In research papers, a segment tree refers to a tree data structure allowing retrieving a list of segments which contain the given point. In competitive programming, the name "segment tree" usually refers to a data structure maintaining an array. According to http://web.ntnu.edu.tw/~algo/Sequence2.html, the data structure originated from Baltic OI 2001: Mars Maps.
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.
A program may have a lot of unused code and data. There can be many reasons:
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.