[llvm] MCFragment: Use trailing data for fixed-size part (PR #150846)

[Alexis Engelke via llvm-commits]

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@@ -46,23 +46,84 @@ MCAssembler *MCObjectStreamer::getAssemblerPtr() {
   return nullptr;
 }
 
+constexpr size_t FragChunkSize = 16384;
+// Ensure the new fragment can at least at least a few bytes.
+constexpr size_t NewFragHeadroom = 8;
+
+static_assert(NewFragHeadroom >= alignof(MCFragment));
+static_assert(FragChunkSize >= sizeof(MCFragment) + NewFragHeadroom);
+
+MCFragment *MCObjectStreamer::allocFragSpace(size_t Headroom) {
+  auto Size = std::max(FragChunkSize, sizeof(MCFragment) + Headroom);
+  FragSpace = Size - sizeof(MCFragment);
+  auto Chunk = std::unique_ptr<char[]>(new char[Size]);
+  auto *F = reinterpret_cast<MCFragment *>(Chunk.get());
+  FragStorage.push_back(std::move(Chunk));
+  return F;
+}
+
 void MCObjectStreamer::newFragment() {
-  addFragment(getContext().allocFragment<MCFragment>());
+  MCFragment *F;
+  if (LLVM_LIKELY(sizeof(MCFragment) + NewFragHeadroom <= FragSpace)) {
+    auto End = size_t(getCurFragEnd());
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aengelke wrote:

`reinterpret_cast<uintptr_t>`?

https://github.com/llvm/llvm-project/pull/150846