[all-commits] [llvm/llvm-project] 039308: MC: Store MCRelaxableFragment MCInst out-of-line

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 0393084adc9028c6babb95fb48d4ef1897093412
      https://github.com/llvm/llvm-project/commit/0393084adc9028c6babb95fb48d4ef1897093412
  Author: Fangrui Song <i at maskray.me>
  Date:   2025-07-08 (Tue, 08 Jul 2025)

  Changed paths:
    M llvm/include/llvm/MC/MCAsmBackend.h
    M llvm/include/llvm/MC/MCInst.h
    M llvm/include/llvm/MC/MCSection.h
    M llvm/lib/MC/MCAssembler.cpp
    M llvm/lib/MC/MCObjectStreamer.cpp
    M llvm/lib/MC/MCSection.cpp
    M llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUAsmBackend.cpp
    M llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
    M llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.h
    M llvm/lib/Target/CSKY/MCTargetDesc/CSKYAsmBackend.cpp
    M llvm/lib/Target/CSKY/MCTargetDesc/CSKYAsmBackend.h
    M llvm/lib/Target/Hexagon/MCTargetDesc/HexagonAsmBackend.cpp
    M llvm/lib/Target/M68k/MCTargetDesc/M68kAsmBackend.cpp
    M llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
    M llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h
    M llvm/lib/Target/VE/MCTargetDesc/VEAsmBackend.cpp
    M llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
    M llvm/lib/Target/Xtensa/MCTargetDesc/XtensaAsmBackend.cpp

  Log Message:
  -----------
  MC: Store MCRelaxableFragment MCInst out-of-line

Follow-up to #146307

Moved MCInst storage to MCSection, enabling trivial ~MCRelaxableFragment
and eliminating the need for a fragment walk in ~MCSection.

Updated MCRelaxableFragment::getInst to construct an MCInst on demand.
Modified MCAssembler::relaxInstruction's mayNeedRelaxation to accept
opcode and operands instead of an MCInst, avoiding redundant MCInst
creation. Note that MCObjectStreamer::emitInstructionImpl calls
mayNeedRelaxation before determining the target fragment for the MCInst.

Unfortunately, we also have to encode `MCInst::Flags` to support
the EVEX prefix, e.g. `{evex} xorw $foo, %ax`

There is a small decrease in max-rss (stage1-ReleaseLTO-g (link only))
with negligible instructions:u change.
https://llvm-compile-time-tracker.com/compare.php?from=0b533f2d9f0551aaffb13dcac8e0fd0a952185b5&to=f26b57f33bc7ccae749a57dfc841de7ce2acc2ef&stat=max-rss&linkStats=on

Next: Enable MCFragment to store fixed-size data (was MCDataFragment's job)
and optional Opcode/Operands data (was MCRelaxableFragment's job),
and delete MCDataFragment/MCRelaxableFragment.
This will allow re-encoding of Data+Relax+Data+Relax sequences as
Frag+Frag. The saving should outweigh the downside of larger
MCFragment.

Pull Request: https://github.com/llvm/llvm-project/pull/147229



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