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

1 files changed, 3 insertions, 4 deletions

diff --git a/llvm/lib/MC/MCMachOStreamer.cpp b/llvm/lib/MC/MCMachOStreamer.cpp
index a54dfb9..43598ef 100644
--- a/llvm/lib/MC/MCMachOStreamer.cpp
+++ b/llvm/lib/MC/MCMachOStreamer.cpp
@@ -161,7 +161,7 @@ void MCMachOStreamer::emitLabel(MCSymbol *Symbol, SMLoc Loc) {
   // We have to create a new fragment if this is an atom defining symbol,
   // fragments cannot span atoms.
   if (cast<MCSymbolMachO>(Symbol)->isSymbolLinkerVisible())
-    insert(getContext().allocFragment<MCDataFragment>());
+    insert(getContext().allocFragment<MCFragment>());
 
   MCObjectStreamer::emitLabel(Symbol, Loc);
 
@@ -483,8 +483,7 @@ void MCMachOStreamer::finalizeCGProfile() {
   // For each entry, reserve space for 2 32-bit indices and a 64-bit count.
   size_t SectionBytes =
       W.getCGProfile().size() * (2 * sizeof(uint32_t) + sizeof(uint64_t));
-  cast<MCDataFragment>(*CGProfileSection->begin())
-      .appendContents(SectionBytes, 0);
+  (*CGProfileSection->begin()).appendContents(SectionBytes, 0);
 }
 
 MCStreamer *llvm::createMachOStreamer(MCContext &Context,
@@ -513,7 +512,7 @@ void MCMachOStreamer::createAddrSigSection() {
   MCSection *AddrSigSection =
       Asm.getContext().getObjectFileInfo()->getAddrSigSection();
   changeSection(AddrSigSection);
-  auto *Frag = cast<MCDataFragment>(AddrSigSection->curFragList()->Head);
+  auto *Frag = cast<MCFragment>(AddrSigSection->curFragList()->Head);
   // We will generate a series of pointer-sized symbol relocations at offset
   // 0x0. Set the section size to be large enough to contain a single pointer
   // (instead of emitting a zero-sized section) so these relocations are