Reapply "MCFragment: Use trailing data for fixed-size part" (#150846)

The fixed-size content of the MCFragment object is now stored as
trailing data, replacing ContentStart/ContentEnd with ContentSize. The
available space for trailing data is tracked using `FragSpace`. If the
available space is insufficient, a new block is allocated within the
bump allocator `MCObjectStreamer::FragStorage`.

FragList::Tail cannot be reused when switching sections or subsections,
as it is not associated with the fragment space tracked by `FragSpace`.
Instead, allocate a new fragment, which becomes less expensive after #150574.

Data can only be appended to the tail fragment of a subsection, not to
fragments in the middle. Post-assembler-layout adjustments (such as
.llvm_addrsig and .llvm.call-graph-profile) have been updated to use the
variable-size part instead.

---

This reverts commit a2fef664c29a53bfa8a66927fcf8b2e5c9da4876,
which reverted the innocent f1aa6050bd90f8ec4273da55d362e23905ad3a81 .
Commit df71243fa885cd3db701dc35a0c8d157adaf93b3, the MCOrgFragment fix,
has fixed the root cause of https://github.com/ClangBuiltLinux/linux/issues/2116

1 files changed, 3 insertions, 0 deletions

diff --git a/llvm/lib/MC/MCWin64EH.cpp b/llvm/lib/MC/MCWin64EH.cpp
index 72a8dd7..a87648a 100644
--- a/llvm/lib/MC/MCWin64EH.cpp
+++ b/llvm/lib/MC/MCWin64EH.cpp
@@ -318,6 +318,9 @@ static void EmitUnwindInfo(MCStreamer &streamer, WinEH::FrameInfo *info) {
 
   // Emit the epilog instructions.
   if (EnableUnwindV2) {
+    // Ensure the fixups and appended content apply to the same fragment.
+    OS->ensureHeadroom(info->EpilogMap.size() * 2);
+
     bool IsLast = true;
     for (const auto &Epilog : llvm::reverse(info->EpilogMap)) {
       if (IsLast) {