LLD, the LLVM linker, is a mature and fast linker supporting multiple binary formats (ELF, Mach-O, PE/COFF, WebAssembly). Designed as a standalone program, the code base relies heavily on global state, making it less than ideal for library integration. As outlined in RFC: Revisiting LLD-as-a-library design, two main hurdles exist:

• Fatal errors: they exit the process without returning control to the caller. This was actually addressed for most scenarios in 2020 by utilizing llvm::sys::Process::Exit(val, /*NoCleanup=*/true) and CrashRecoveryContext (longjmp under the hood).
• Global variable conflicts: shared global variables do not allow two concurrent invocation.

I understand that calling a linker API could be convenient, especially when you want to avoid shipping another executable (which can be large when you link against LLVM statically). However, I believe that invoking LLD as a separate process remains the recommended approach. There are several advantages:

• Build system control: Build systems gain greater control over scheduling and resource allocation for LLD. In an edit-compile-link cycle, the link could need more resources and threading is more useful.
• Better parallelism management
• Global state isolation: LLVM's global state (primarily cl::opt and ManagedStatic) is isolated.

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LLVM's C++ API doesn't offer a stability guarantee. This means function signatures can change or be removed between versions, forcing projects to adapt.

On the other hand, LLVM has an extensive API surface. When a library like llvm/lib/Y relies functionality from another library, the API is often exported in header files under llvm/include/llvm/X/, even if it is not intended to be user-facing.

To be compatible with multiple LLVM versions, many projects rely on #if directives based on the LLVM_VERSION_MAJOR macro. This post explores the specific techniques used by ccls to ensure compatibility with LLVM versions 7 to 19. For the latest release (ccls 0.20241108), support for LLVM versions 7 to 9 has been discontinued.

Given the tight coupling between LLVM and Clang, the LLVM_VERSION_MAJOR macro can be used for both version detection. There's no need to check CLANG_VERSION_MAJOR.

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After migrating from Vim to Emacs as my primary C++ editor in 2015, I switched from Vim to Neovim for miscellaneous non-C++ tasks as it is more convenient in a terminal. Customizing the editor with a language you are comfortable with is important. I found myself increasingly drawn to Neovim's terminal-based simplicity for various tasks. Recently, I've refined my Neovim setup to the point where I can confidently migrate my entire C++ workflow away from Emacs.

This post explores the key improvements I've made to achieve this transition. My focus is on code navigation.

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I've spent countless hours writing and reading C++ code. For many years, Emacs has been my primary editor, and I leverage ccls' (my C++ language server) rainbow semantic highlighting feature.

The feature relies on two custom notification messages $ccls/publishSemanticHighlight and $ccls/publishSkippedRanges. $ccls/publishSemanticHighlight provides a list of symbols, each with kind information (function, type, or variable) of itself and its semantic parent (e.g. a member function's parent is a class), storage duration, and a list of ranges.

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struct CclsSemanticHighlightSymbol {
  int id = 0;
  SymbolKind parentKind;
  SymbolKind kind;
  uint8_t storage;
  std::vector<std::pair<int, int>> ranges;

  std::vector<lsRange> lsRanges; // Only used by vscode-ccls
};

struct CclsSemanticHighlight {
  DocumentUri uri;
  std::vector<CclsSemanticHighlightSymbol> symbols;
};

An editor can use consistent colors to highlight different occurrences of a symbol. Different colors can be assigned to different symbols.

Tobias Pisani created emacs-cquery (the predecessor to emacs-ccls) in Nov 2017. Despite not being a fan of Emacs Lisp, I added the rainbow semantic highlighting feature for my own use in early 2018. My setup also relied heavily on these two settings:

• Bolding and underlining variables of static duration storage
• Italicizing member functions and variables

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(setq ccls-sem-highlight-method 'font-lock)
(ccls-use-default-rainbow-sem-highlight)

Key symbol properties (member, static) were visually prominent in my Emacs environment.

My Emacs hacking days are a distant memory – beyond basic configuration tweaks, I haven't touched elisp code since 2018. As my Elisp skills faded, I increasingly turned to Neovim for various editing tasks. Naturally, I wanted to migrate my C++ development workflow to Neovim as well. However, a major hurdle emerged: Neovim lacked the beloved rainbow highlighting I enjoyed in Emacs.

Thankfully, Neovim supports "semantic tokens" from LSP 3.16, a standardized approach adopted by many editors.

I've made changes to ccls (available on a branch; PR) to support semantic tokens. This involves adapting the $ccls/publishSemanticHighlight code to additionally support textDocument/semanticTokens/full and textDocument/semanticTokens/range.

I utilize a few token modifiers (static, classScope, functionScope, namespaceScope) for highlighting:

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vim.cmd([[
hi @lsp.mod.classScope.cpp gui=italic
hi @lsp.mod.static.cpp gui=bold
hi @lsp.typemod.variable.namespaceScope.cpp gui=bold,underline
]])

While this approach is a significant improvement over relying solely on nvim-treesitter, I'm still eager to implement rainbow semantic tokens. Although LSP semantic tokens don't directly distinguish symbols, we can create custom modifiers to achieve similar results.

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tokenModifiers: {
  "declaration", "definition", "static", ...

  "id0", "id1", ... "id9",
}

In the user-provided initialization options, I set highlight.rainbow to 10.

ccls assigns the same modifier ID to tokens belonging to the same symbol, aiming for unique IDs for different symbols. While we only have a few predefined IDs (each linked to a specific color), there's a slight possibility of collisions. However, this is uncommon and generally acceptable.

For a token with type variable, Neovim's built-in LSP plugin assigns a highlight group @lsp.typemod.variable.id$i.cpp where $i is an integer between 0 and 9. This allows us to customize a unique foreground color for each modifier ID.

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local func_colors = {
  '#e5b124', '#927754', '#eb992c', '#e2bf8f', '#d67c17',
  '#88651e', '#e4b953', '#a36526', '#b28927', '#d69855',
}
local type_colors = {
  '#e1afc3', '#d533bb', '#9b677f', '#e350b6', '#a04360',
  '#dd82bc', '#de3864', '#ad3f87', '#dd7a90', '#e0438a',
}
local param_colors = {
  '#e5b124', '#927754', '#eb992c', '#e2bf8f', '#d67c17',
  '#88651e', '#e4b953', '#a36526', '#b28927', '#d69855',
}
local var_colors = {
  '#429921', '#58c1a4', '#5ec648', '#36815b', '#83c65d',
  '#419b2f', '#43cc71', '#7eb769', '#58bf89', '#3e9f4a',
}
local all_colors = {
  class = type_colors,
  constructor = func_colors,
  enum = type_colors,
  enumMember = var_colors,
  field = var_colors,
  ['function'] = func_colors,
  method = func_colors,
  parameter = param_colors,
  struct = type_colors,
  typeAlias = type_colors,
  typeParameter = type_colors,
  variable = var_colors
}
for type, colors in pairs(all_colors) do
  for i = 1,#colors do
    for _, lang in pairs({'c', 'cpp'}) do
      vim.api.nvim_set_hl(0, string.format('@lsp.typemod.%s.id%s.%s', type, i-1, lang), {fg=colors[i]})
    end
  end
end

vim.cmd([[
hi @lsp.mod.classScope.cpp gui=italic
hi @lsp.mod.static.cpp gui=bold
hi @lsp.typemod.variable.namespaceScope.cpp gui=bold,underline
]])

Now, let's analyze the C++ code above using this configuration.

While the results are visually pleasing, I need help implementing code lens functionality.

Inactive code highlighting

Inactive code regions (skipped ranges in Clang) are typically displayed in grey. While this can be helpful for identifying unused code, it can sometimes hinder understanding the details. I simply disabled the inactive code feature.

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#ifdef X
... // colorful
#else
... // normal instead of grey
#endif

Refresh

When opening a large project, the initial indexing or cache loading process can be time-consuming, often leading to empty lists of semantic tokens for the initially opened files. While ccls prioritizes indexing these files, it's unclear how to notify the client to refresh the files. The existing workspace/semanticTokens/refresh request, unfortunately, doesn't accept text document parameters.

In contrast, with $ccls/publishSemanticHighlight, ccls proactively sends the notification after an index update (see main_OnIndexed).

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void main_OnIndexed(DB *db, WorkingFiles *wfiles, IndexUpdate *update) {
  ...

  db->applyIndexUpdate(update);

  // Update indexed content, skipped ranges, and semantic highlighting.
  if (update->files_def_update) {
    auto &def_u = *update->files_def_update;
    if (WorkingFile *wfile = wfiles->getFile(def_u.first.path)) {
      wfile->setIndexContent(g_config->index.onChange ? wfile->buffer_content
                                                      : def_u.second);
      QueryFile &file = db->files[update->file_id];
      // Publish notifications to the file.
      emitSkippedRanges(wfile, file);
      emitSemanticHighlight(db, wfile, file);
      // But how do we send a workspace/semanticTokens/refresh request?????
    }
  }
}

While the semantic token request supports partial results in the specification, Neovim lacks this implementation. Even if it were, I believe a notification message with a text document parameter would be a more efficient and direct approach.

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export interface SemanticTokensParams extends WorkDoneProgressParams,
	PartialResultParams {
	/**
	 * The text document.
	 */
	textDocument: TextDocumentIdentifier;
}

Other clients

emacs-ccls

Once this feature branch is merged, Emacs users can simply remove the following lines:

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(setq ccls-sem-highlight-method 'font-lock)
(ccls-use-default-rainbow-sem-highlight)

How to change lsp-semantic-token-modifier-faces to support rainbow semantic tokens in lsp-mode and emacs-ccls?

The general approach is similar to the following, but we need a feature from lsp-mode (https://github.com/emacs-lsp/lsp-mode/issues/4590).

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(setq lsp-semantic-tokens-enable t)
(defface lsp-face-semhl-namespace-scope
         '((t :weight bold)) "highlight for namespace scope symbols" :group 'lsp-semantic-tokens)
(cl-loop for color in '("#429921" "#58c1a4" "#5ec648" "#36815b" "#83c65d"
                        "#417b2f" "#43cc71" "#7eb769" "#58bf89" "#3e9f4a")
       for i = 0 then (1+ i)
       do (custom-declare-face (intern (format "lsp-face-semhl-id%d" i))
                               `((t :foreground ,color))
                               "" :group 'lsp-semantic-tokens))
(setq lsp-semantic-token-modifier-faces
      `(("declaration" . lsp-face-semhl-interface)
        ("definition" . lsp-face-semhl-definition)
        ("implementation" . lsp-face-semhl-implementation)
        ("readonly" . lsp-face-semhl-constant)
        ("static" . lsp-face-semhl-static)
        ("deprecated" . lsp-face-semhl-deprecated)
        ("abstract" . lsp-face-semhl-keyword)
        ("async" . lsp-face-semhl-macro)
        ("modification" . lsp-face-semhl-operator)
        ("documentation" . lsp-face-semhl-comment)
        ("defaultLibrary" . lsp-face-semhl-default-library)
        ("classScope" . lsp-face-semhl-member)
        ("namespaceScope" . lsp-face-semhl-namespace-scope)
        ,@(cl-loop for i from 0 to 10
                   collect (cons (format "id%d" i)
                                 (intern (format "lsp-face-semhl-id%d" i))))
        ))

vscode-ccls

We require assistance to eliminate the $ccls/publishSemanticHighlight feature and adopt built-in semantic tokens support. Due to the lack of active maintenance for vscode-ccls, I'm unable to maintain this plugin for an editor I don't frequently use.

Misc

I use a trick to switch ccls builds without changing editor configurations.

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#!/bin/zsh
#export CCLS_TRACEME=s
export LD_PRELOAD=/usr/lib/libmimalloc.so

type=
[[ -f /tmp/ccls-build ]] && type=$(</tmp/ccls-build)

case $type in
  strace)
    exec strace -s999 -e read,write -o /tmp/strace.log -f ~/ccls/out/debug/ccls --log-file=/tmp/cc.log -v=1 "$@";;
  debug)
    exec ~/ccls/out/debug/ccls --log-file=/tmp/cc.log -v=2 "$@";;
  release)
    exec ~/ccls/out/release/ccls --log-file=/tmp/cc.log -v=1 "$@";;
  *)
    exec /usr/bin/ccls --log-file=/tmp/cc.log -v=1 "$@";;
esac

Usage:

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echo debug > /tmp/ccls-build
nvim  # out/debug/ccls is now used

LLVM 19.1 will soon be released. This post provides a summary of my contributions in this release cycle to record my learning progress.

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LLVM 19 will be released. As usual, I maintain lld/ELF and have added some notes to https://github.com/llvm/llvm-project/blob/release/19.x/lld/docs/ReleaseNotes.rst. I've meticulously reviewed nearly all the patches that are not authored by me. I'll delve into some of the key changes.

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In object files, certain code patterns embed data within instructions or transitions occur between instruction sets. This can create hurdles for disassemblers, which might misinterpret data as code, resulting in inaccurate output. Furthermore, code written for one instruction set could be incorrectly disassembled as another. To address these issues, some architectures (Arm, C-SKY, NDS32, RISC-V, etc) define mapping symbols to explicitly denote state transition. Let's explore this concept using an AArch32 code example:

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The output of ld.lld -v includes a message "compatible with GNU linkers" to address detection mechanism used by GNU Libtool. This problem is described by Software compatibility and our own "User-Agent" problem.

The latest m4/libtool.m4 continues to rely on a GNU check.

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Within the LLVM project, MC is a library responsible for handling assembly, disassembly, and object file formats. Intro to the LLVM MC Project, which was written back in 2010, remains a good source to understand the high-level structures.

In the latest release cycle, substantial effort has been dedicated to refining MC's internal representation for improved performance and readability. These changes have decreased compile time significantly. This blog post will delve into the details, providing insights into the specific changes.

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GNU ld's output section layout is determined by a linker script, which can be either internal (default) or external (specified with -T or -dT). Within the linker script, SECTIONS commands define how input sections are mapped into output sections.

Input sections not explicitly placed by SECTIONS commands are termed "orphan sections".

Orphan sections are sections present in the input files which are not explicitly placed into the output file by the linker script. The linker will still copy these sections into the output file by either finding, or creating a suitable output section in which to place the orphaned input section.

GNU ld's default behavior is to create output sections to hold these orphan sections and insert these output sections into appropriate places.

Orphan section placement is crucial because GNU ld's built-in linker scripts, while understanding common sections like .text/.rodata/.data, are unaware of custom sections. These custom sections should still be included in the final output file.

• Grouping: Orphan input sections are grouped into orphan output sections that share the same name.
• Placement: These grouped orphan output sections are then inserted into the output sections defined in the linker script. They are placed near similar sections to minimize the number of PT_LOAD segments needed.

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