[all-commits] [llvm/llvm-project] dc3a4c: MC: Restructure MCFragment as a fixed part and a v...

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: dc3a4c0fcf4953645bc81ae99db10e48acd4961f
      https://github.com/llvm/llvm-project/commit/dc3a4c0fcf4953645bc81ae99db10e48acd4961f
  Author: Fangrui Song <i at maskray.me>
  Date:   2025-07-15 (Tue, 15 Jul 2025)

  Changed paths:
    M llvm/include/llvm/MC/MCAsmBackend.h
    M llvm/include/llvm/MC/MCAssembler.h
    M llvm/include/llvm/MC/MCCodeView.h
    M llvm/include/llvm/MC/MCContext.h
    M llvm/include/llvm/MC/MCELFStreamer.h
    M llvm/include/llvm/MC/MCObjectStreamer.h
    M llvm/include/llvm/MC/MCSection.h
    M llvm/include/llvm/MC/MCWasmStreamer.h
    M llvm/lib/MC/MCAsmBackend.cpp
    M llvm/lib/MC/MCAssembler.cpp
    M llvm/lib/MC/MCCodeView.cpp
    M llvm/lib/MC/MCContext.cpp
    M llvm/lib/MC/MCELFStreamer.cpp
    M llvm/lib/MC/MCExpr.cpp
    M llvm/lib/MC/MCFragment.cpp
    M llvm/lib/MC/MCMachOStreamer.cpp
    M llvm/lib/MC/MCObjectStreamer.cpp
    M llvm/lib/MC/MCSection.cpp
    M llvm/lib/MC/MCSymbol.cpp
    M llvm/lib/MC/MCWasmStreamer.cpp
    M llvm/lib/MC/MCWin64EH.cpp
    M llvm/lib/MC/MCWinCOFFStreamer.cpp
    M llvm/lib/MC/MCXCOFFStreamer.cpp
    M llvm/lib/MC/MachObjectWriter.cpp
    M llvm/lib/MC/WasmObjectWriter.cpp
    M llvm/lib/MC/WinCOFFObjectWriter.cpp
    M llvm/lib/Target/AArch64/MCTargetDesc/AArch64ELFStreamer.cpp
    M llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
    M llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.h
    M llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp
    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/LoongArch/MCTargetDesc/LoongArchAsmBackend.cpp
    M llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.h
    M llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
    M llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h
    M llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
    M llvm/test/MC/ELF/mc-dump.s

  Log Message:
  -----------
  MC: Restructure MCFragment as a fixed part and a variable tail

Refactor the fragment representation of `push rax; jmp foo; nop; jmp foo`,
previously encoded as
`MCDataFragment(nop); MCRelaxableFragment(jmp foo); MCDataFragment(nop); MCRelaxableFragment(jmp foo)`,

to

```
MCFragment(fixed: push rax, variable: jmp foo)
MCFragment(fixed: nop, variable: jmp foo)
```

Changes:

* Eliminate MCEncodedFragment, moving content and fixup storage to MCFragment.
* The new MCFragment contains a fixed-size content (similar to previous
  MCDataFragment) and an optional variable-size tail.
* The variable-size tail supports FT_Relaxable, FT_LEB, FT_Dwarf, and
  FT_DwarfFrame, with plans to extend to other fragment types.
  dyn_cast/isa should be avoided for the converted fragment subclasses.
* In `setVarFixups`, source fixup offsets are relative to the variable part's start.
  Stored fixup (in `FixupStorage`) offsets are relative to the fixed part's start.
  A lot of code does `getFragmentOffset(Frag) + Fixup.getOffset()`,
  expecting the fixup offset to be relative to the fixed part's start.
* HexagonAsmBackend::fixupNeedsRelaxationAdvanced needs to know the
  associated instruction for a fixup. We have to add a `const MCFragment &` parameter.
* In MCObjectStreamer, extend `absoluteSymbolDiff` to apply to
  FT_Relaxable as otherwise there would be many more FT_DwarfFrame
  fragments in -g compilations.

https://llvm-compile-time-tracker.com/compare.php?from=28e1473e8e523150914e8c7ea50b44fb0d2a8d65&to=778d68ad1d48e7f111ea853dd249912c601bee89&stat=instructions:u

```
stage2-O0-g instructins:u geomeon (-0.07%)
stage1-ReleaseLTO-g (link only) max-rss geomean (-0.39%)
```

```
% /t/clang-old -g -c sqlite3.i -w -mllvm -debug-only=mc-dump &| awk '/^[0-9]+/{s[$2]++;tot++} END{print "Total",tot; n=asorti(s, si); for(i=1;i<=n;i++) print si[i],s[si[i]]}'
Total 59675
Align 2215
Data 29700
Dwarf 12044
DwarfCallFrame 4216
Fill 92
LEB 12
Relaxable 11396
% /t/clang-new -g -c sqlite3.i -w -mllvm -debug-only=mc-dump &| awk '/^[0-9]+/{s[$2]++;tot++} END{print "Total",tot; n=asorti(s, si); for(i=1;i<=n;i++) print si[i],s[si[i]]}'
Total 32287
Align 2215
Data 2312
Dwarf 12044
DwarfCallFrame 4216
Fill 92
LEB 12
Relaxable 11396
```

Pull Request: https://github.com/llvm/llvm-project/pull/148544



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