2017年10月第一次给LLVM提交patch，到2020年底已经贡献三年多了。今年~1000 commits，灌水很多，挑出一些值得一提的。
LLD and GNU linker incompatibilities
Updated in 2024-06.
Subtitle: Is ld.lld a drop-in replacement for GNU ld?
The motivation for this article was someone challenging the "drop-in replacement" claim on LLD's website (the discussion was about Linux-like ELF toolchain):
LLD is a linker from the LLVM project that is a drop-in replacement for system linkers and runs much faster than them. It also provides features that are useful for toolchain developers.
99.9% pieces of software work with ld.lld without a change. Some linker script applications may need an adaption (such adaption is oftentimes due to brittle assumptions: asking too much from GNU ld's behavior which should be fixed anyway). So I defended for this claim.
C++ exception handling ABI
Updated in 2024-11.
I wrote an article a few weeks ago to introduce stack unwinding in detail. Today I will introduce C++ exception handling, an application of stack unwinding. Exception handling has a variety of ABI (interoperability of C++ implementations), the most widely used of which is Itanium C++ ABI: Exception Handling
Dominator tree
Lengauer-Tarjan algorithm
All about symbol versioning
Updated in 2026-01.
Many people just want to know how to define or reference versioned symbols properly. You may jump to Recommended usage below.
In 1995, Solaris' link editor and ld.so introduced the symbol versioning mechanism. Ulrich Drepper and Eric Youngdale borrowed Solaris' symbol versioning in 1997 and designed the GNU style symbol versioning for glibc.
gnu-gabi specification: https://sourceware.org/gnu-gabi/program-loading-and-dynamic-linking.txt#:~:text=versioning
Explain GNU style linker options
Updated in 2025-02
(首先慶祝一下LLVM 2000 commits達成！)
Compiler driver options
Before describing the linker options, let's introduce the concept of
driver options. The user-facing options of gcc and
clang are called driver options. Some driver options affect
the options passed to the linker. Many such options have the same name
as the linker's, and they often have additional functions in addition to
the options of the same name passed to the linker, such as:
Stack unwinding
Update in 2025-11.
The main usage of stack unwinding is:
- To obtain a stack trace for debuggers, crash reporters, profilers, garbage collectors, etc.
- To implement C++ exceptions (Itanium C++ ABI) using personality routines and language-specific data area. See C++ exception handling ABI
Some people use "stack walking" to refer to obtaining a stack trace, while "stack unwinding" refers to an enhanced operation that also recovers callee-saved registers. In this post, we don't make the distinction.
模塊內函數調用和libc符號重命名
Translation unit外的函數調用
Command line processing in LLVM
Updated in 2023-05.
There are two libraries for processing command line options in LLVM.
llvm/Support/ComandLine.h
See https://llvm.org/docs/CommandLine.html for documentation.
Global variables (mostly llvm::cl::opt<type>, some
llvm::cl::list<type>) are most common to represent
command-line options. The llvm::cl::opt constructor
registers this command line option in a global registry. The program
calls llvm::cl::ParseCommandLineOptions(argc, argv, ...) in
main to parse the command line options. opt
supports various integer types, bool,
std::string, etc. Defining some specialization can support
support custom class/enum types.
gcov與LLVM中的實現
打算以後不定期寫一點LLVM的學習(開發)筆記。寫作上不想過多花時間(加語文水平所限...)，所以字句不作過多斟酌。
gcov
https://gcc.gnu.org/onlinedocs/gcc/Gcov.html
_Optimally Profiling and Tracing Programs_描述了一個edge-frequency/edge-placement problem。 選擇control-flow graph的一些邊，加上監控代碼，推導所有邊的執行次數。 gcov是一種實現。
在gcov的模型中，一個源文件包含若干函數，一個函數包含若干基本塊，一個基本塊佔據若干行，這些信息保存在.gcno文件中。
Instrument程序，在基本塊間轉移時記錄邊的執行次數，程序退出時爲每個translation
unit輸出一個.gcda文件。
.gcda文件可以累計多次程序執行的計數。