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.

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Explain GNU style linker options

Updated in 2025-02

(首先庆祝一下LLVM 2000 commits达成！)

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:

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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.

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gcov与LLVM中的实现

打算以后不定期写一点LLVM的学习(开发)笔记。写作上不想过多花时间(加语文水平所限...)，所以字句不作过多斟酌。

gcov

https://gcc.gnu.org/onlinedocs/gcc/Gcov.html

_Optimally Profiling and Tracing Programs_描述了一个edge-frequency/edge-placement problem。 选择control-flow graph的一些边，加上监控代码，推导所有边的执行次数。 gcov是一种实现。

在gcov的模型中，一个源文件包含若干函数，一个函数包含若干基本块，一个基本块占据若干行，这些信息保存在.gcno文件中。 Instrument程序，在基本块间转移时记录边的执行次数，程序退出时为每个translation unit输出一个.gcda文件。 .gcda文件可以累计多次程序执行的计数。

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