[all-commits] [llvm/llvm-project] b22e22: MC: Allocate initial fragment and define section s...

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: b22e22ebfa546cc31e166dd5ca4cc808aa057c9b
      https://github.com/llvm/llvm-project/commit/b22e22ebfa546cc31e166dd5ca4cc808aa057c9b
  Author: Fangrui Song <i at maskray.me>
  Date:   2025-07-26 (Sat, 26 Jul 2025)

  Changed paths:
    M llvm/include/llvm/MC/MCContext.h
    M llvm/include/llvm/MC/MCObjectStreamer.h
    M llvm/include/llvm/MC/MCSection.h
    M llvm/include/llvm/MC/MCStreamer.h
    M llvm/include/llvm/MC/MCXCOFFStreamer.h
    M llvm/lib/MC/ELFObjectWriter.cpp
    M llvm/lib/MC/MCAssembler.cpp
    M llvm/lib/MC/MCContext.cpp
    M llvm/lib/MC/MCELFStreamer.cpp
    M llvm/lib/MC/MCMachOStreamer.cpp
    M llvm/lib/MC/MCObjectStreamer.cpp
    M llvm/lib/MC/MCSection.cpp
    M llvm/lib/MC/MCStreamer.cpp
    M llvm/lib/MC/MCXCOFFStreamer.cpp
    M llvm/test/CodeGen/XCore/section-name.ll
    M llvm/test/MC/ELF/section-sym2.s
    R llvm/test/MC/ELF/undefined-debug.s

  Log Message:
  -----------
  MC: Allocate initial fragment and define section symbol in changeSection

Reland #150574 with a MCStreamer::changeSection change:
In Mach-O, DWARF sections use Begin as a temporary label, requiring a label
definition, unlike section symbols in other file formats.
(Tested by dec978036ef1037753e7de5b78c978e71c49217b)

---

13a79bbfe583e1d8cc85d241b580907260065eb8 (2017) introduced fragment
creation in MCContext for createELFSectionImpl, which was inappropriate.
Fragments should only be created when using MCSteramer, not during
`MCContext::get*Section` calls.
`initMachOMCObjectFileInfo` defines multiple sections, some of which may
not be used by the code generator. This caused symbol names matching
these sections to be incorrectly marked as undefined (see
https://reviews.llvm.org/D55173).

The fragment code was later replicated in other file formats, such as
WebAssembly (see https://reviews.llvm.org/D46561), XCOFF, and GOFF.

This patch fixes the problem by moving initial fragment allocation from
MCContext::createSection to MCStreamer::changeSection.
While MCContext still creates a section symbol, the symbol is not
attached to the initial fragment. In addition,

* Move `emitLabel`/`setFragment` from `switchSection*` and
  overridden changeSection to `MCObjectStreamer::changeSection` for
  consistency.
* De-virtualize `switchSectionNoPrint`.
* test/CodeGen/XCore/section-name.ll now passes. XCore doesn't support
  MCObjectStreamer. I don't think the MCAsmStreamer output behavior
  change matters.

Pull Request: https://github.com/llvm/llvm-project/pull/150574



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