[all-commits] [llvm/llvm-project] ba3c1f: [WebAssembly] Add segment RETAIN flag to support p...

[Yuta Saito via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: ba3c1f9ce30cf4f8aee5f1961df74d65e11d53bc
      https://github.com/llvm/llvm-project/commit/ba3c1f9ce30cf4f8aee5f1961df74d65e11d53bc
  Author: Yuta Saito <kateinoigakukun at gmail.com>
  Date:   2024-02-21 (Wed, 21 Feb 2024)

  Changed paths:
    A lld/test/wasm/no-strip-segment.s
    M lld/wasm/InputChunks.h
    M lld/wasm/MarkLive.cpp
    M llvm/include/llvm/BinaryFormat/Wasm.h
    M llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
    M llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
    M llvm/lib/MC/MCParser/WasmAsmParser.cpp
    M llvm/lib/MC/MCSectionWasm.cpp
    M llvm/lib/ObjectYAML/WasmYAML.cpp
    A llvm/test/CodeGen/WebAssembly/no-strip.ll
    M llvm/test/MC/WebAssembly/no-dead-strip.ll

  Log Message:
  -----------
  [WebAssembly] Add segment RETAIN flag to support private retained data (#81539)

In WebAssembly, we have `WASM_SYMBOL_NO_STRIP` symbol flag to mark the
referenced content as retained. However, the flag is not enough to
express retained data that is not referenced by any symbol. This patch
adds a new segment flag`WASM_SEG_FLAG_RETAIN` to support "private"
linkage data that is retained by llvm.used.

This kind of data that is not referenced but must be retained is usually
used with encapsulation symbols (__start/__stop). Swift runtime uses
this technique and depends on the fact "all metadata sections in live
objects are retained", which was not guaranteed with `--gc-sections`
before this patch.

This is a revised version of https://reviews.llvm.org/D126950 (has been
reverted) based on @MaskRay's comments



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