[all-commits] [llvm/llvm-project] 40933f: [lld][macho] Support order cstrings with -order_fi...

[SharonXSharon via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 40933fd410dd32870a325a9b6d3c741441d5e213
      https://github.com/llvm/llvm-project/commit/40933fd410dd32870a325a9b6d3c741441d5e213
  Author: SharonXSharon <xiaoranxu.nju at gmail.com>
  Date:   2025-06-05 (Thu, 05 Jun 2025)

  Changed paths:
    M lld/MachO/SectionPriorities.cpp
    M lld/MachO/SectionPriorities.h
    M lld/MachO/SyntheticSections.cpp
    A lld/test/MachO/ordre-file-cstring.s

  Log Message:
  -----------
  [lld][macho] Support order cstrings with -order_file (#140307)

Expand the `-order_file` also accept cstrings to order.
The purpose is to order hot cstrings for performance (implemented in
this diff), and then later on we can also order cold cstrings for
compression size win.

Due to the speciality of cstrings, there's no way to pass in symbol
names in the order file as the existing -order_file, so we expect `<hash
of cstring literal content>` to represent/identify each cstring.

```
// An order file has one entry per line, in the following format:
  //
  //   <cpu>:<object file>:[<symbol name> | CStringEntryPrefix <cstring hash>]
  //
  // <cpu> and <object file> are optional.
  // If not specified, then that entry tries to match either,
  //
  // 1) any symbol of the <symbol name>;
  // Parsing this format is not quite straightforward because the symbol name
  // itself can contain colons, so when encountering a colon, we consider the
  // preceding characters to decide if it can be a valid CPU type or file path.
  // If a symbol is matched by multiple entries, then it takes the
  // lowest-ordered entry (the one nearest to the front of the list.)
  //
  // or 2) any cstring literal with the given hash, if the entry has the
  // CStringEntryPrefix prefix defined below in the file. <cstring hash> is the
  // hash of cstring literal content.
  //
  // Cstring literals are not symbolized, we can't identify them by name
  // However, cstrings are deduplicated, hence unique, so we use the hash of
  // the content of cstring literals to identify them and assign priority to it.
  // We use the same hash as used in StringPiece, i.e. 31 bit:
  // xxh3_64bits(string) & 0x7fffffff
  //
```

The ordering of cstring has to happen during/before the finalizing of
the cstring section content in the `finalizeContents()` function, which
happens before the writer is run

---------

Co-authored-by: Sharon Xu <sharonxu at fb.com>



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