[all-commits] [llvm/llvm-project] de19f7: [MC] Replace fragment ilist with singly-linked lists

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: de19f7b6d46f1c38e10e604154f0fdaaffde9ebd
      https://github.com/llvm/llvm-project/commit/de19f7b6d46f1c38e10e604154f0fdaaffde9ebd
  Author: Fangrui Song <i at maskray.me>
  Date:   2024-06-11 (Tue, 11 Jun 2024)

  Changed paths:
    M llvm/include/llvm/MC/MCFragment.h
    M llvm/include/llvm/MC/MCObjectStreamer.h
    M llvm/include/llvm/MC/MCSection.h
    M llvm/lib/MC/MCAssembler.cpp
    M llvm/lib/MC/MCExpr.cpp
    M llvm/lib/MC/MCFragment.cpp
    M llvm/lib/MC/MCObjectStreamer.cpp
    M llvm/lib/MC/MCSection.cpp
    M llvm/lib/MC/WasmObjectWriter.cpp
    M llvm/lib/Target/Hexagon/MCTargetDesc/HexagonAsmBackend.cpp
    M llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCExpr.cpp
    M llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp

  Log Message:
  -----------
  [MC] Replace fragment ilist with singly-linked lists

Fragments are allocated with `operator new` and stored in an ilist with
Prev/Next/Parent pointers. A more efficient representation would be an
array of fragments without the overhead of Prev/Next pointers.

As the first step, replace ilist with singly-linked lists.

* `getPrevNode` uses have been eliminated by previous changes.
* The last use of the `Prev` pointer remains: for each subsection, there is an insertion point and
  the current insertion point is stored at `CurInsertionPoint`.
* `HexagonAsmBackend::finishLayout` needs a backward iterator. Save all
  fragments within `Frags`. Hexagon programs are usually small, and the
  performance does not matter that much.

To eliminate `Prev`, change the subsection representation to
singly-linked lists for subsections and a pointer to the active
singly-linked list. The fragments from all subsections will be chained
together at layout time.

Since fragment lists are disconnected before layout time, we can remove
`MCFragment::SubsectionNumber` (https://reviews.llvm.org/D69411). The
current implementation of `AttemptToFoldSymbolOffsetDifference` requires
future improvement for robustness.

Pull Request: https://github.com/llvm/llvm-project/pull/95077



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