commit dc3a4c0fcf4953645bc81ae99db10e48acd4961f
author Fangrui Song <i@maskray.me> Tue Jul 15 21:56:55 2025 -0700
committer GitHub <noreply@github.com> Tue Jul 15 21:56:55 2025 -0700
tree 908becce31c10c30762e4636093ec2abbd2d1431
parent 7a9bef0166951a61bc7094514a20471ae45f6090

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