MC: Store fragment content and fixups out-of-line

Moved `Contents` and `Fixups` SmallVector storage to MCSection, enabling
trivial destructors for most fragment subclasses and eliminating the need
for MCFragment::destroy in ~MCSection.

For appending content to the current section, use
getContentsForAppending. During assembler relaxation, prefer
setContents/setFixups, which may involve copying and reduce the benefits
of https://reviews.llvm.org/D145791.

Moving only Contents out-of-line caused a slight performance regression
(Alexis Engelke's 2024 prototype). By also moving Fragments out-of-line,
fragment destructors become trivial, resulting in
neglgible instructions:u increase for "stage2-O0-g" and [large max-rss decrease](https://llvm-compile-time-tracker.com/compare.php?from=84e82746c3ff63ec23a8b85e9efd4f7fccf92590&to=555a28c0b2f8250a9cf86fd267a04b0460283e15&stat=max-rss&linkStats=on)
for the "stage1-ReleaseLTO-g (link only)" benchmark.
(
An older version using fewer inline functions: https://llvm-compile-time-tracker.com/compare.php?from=bb982e733cfcda7e4cfb0583544f68af65211ed1&to=f12d55f97c47717d438951ecddecf8ebd28c296b&linkStats=on
)

Now using plain SmallVector in MCSection for storage, with potential for
future allocator optimizations, such as allocating `Contents` as the
trailing object of MCDataFragment. (GNU Assembler uses gnulib's obstack
for fragment management.)

Co-authored-by: Alexis Engelke <engelke@in.tum.de>

Pull Request: https://github.com/llvm/llvm-project/pull/146307

1 files changed, 7 insertions, 6 deletions

diff --git a/llvm/lib/MC/MCCodeView.cpp b/llvm/lib/MC/MCCodeView.cpp
index e8f0427..5d79143 100644
--- a/llvm/lib/MC/MCCodeView.cpp
+++ b/llvm/lib/MC/MCCodeView.cpp
@@ -510,8 +510,7 @@ void CodeViewContext::encodeInlineLineTable(const MCAssembler &Asm,
 
   MCCVFunctionInfo *SiteInfo = getCVFunctionInfo(Frag.SiteFuncId);
 
-  SmallVectorImpl<char> &Buffer = Frag.getContents();
-  Buffer.clear(); // Clear old contents if we went through relaxation.
+  SmallVector<char, 0> Buffer;
   for (const MCCVLoc &Loc : Locs) {
     // Exit early if our line table would produce an oversized InlineSiteSym
     // record. Account for the ChangeCodeLength annotation emitted after the
@@ -604,15 +603,14 @@ void CodeViewContext::encodeInlineLineTable(const MCAssembler &Asm,
 
   compressAnnotation(BinaryAnnotationsOpCode::ChangeCodeLength, Buffer);
   compressAnnotation(std::min(EndSymLength, LocAfterLength), Buffer);
+  Frag.setContents(Buffer);
 }
 
 void CodeViewContext::encodeDefRange(const MCAssembler &Asm,
                                      MCCVDefRangeFragment &Frag) {
   MCContext &Ctx = Asm.getContext();
-  SmallVectorImpl<char> &Contents = Frag.getContents();
-  Contents.clear();
-  SmallVectorImpl<MCFixup> &Fixups = Frag.getFixups();
-  Fixups.clear();
+  SmallVector<char, 0> Contents;
+  SmallVector<MCFixup, 0> Fixups;
   raw_svector_ostream OS(Contents);
 
   // Compute all the sizes up front.
@@ -692,4 +690,7 @@ void CodeViewContext::encodeDefRange(const MCAssembler &Asm,
       GapStartOffset += GapSize + RangeSize;
     }
   }
+
+  Frag.setContents(Contents);
+  Frag.setFixups(Fixups);
 }