[lld] [LLD][ELF][AArch64] Add BTI Aware long branch thunks (PR #108989)

[Fangrui Song via llvm-commits]

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@@ -578,6 +622,48 @@ void AArch64ADRPThunk::addSymbols(ThunkSection &isec) {
   addSymbol("$x", STT_NOTYPE, 0, isec);
 }
 
+void AArch64BTILandingPadThunk::addSymbols(ThunkSection &isec) {
+  addSymbol(saver().save("__AArch64BTIThunk_" + destination.getName()),
+            STT_FUNC, 0, isec);
+  addSymbol("$x", STT_NOTYPE, 0, isec);
+}
+
+InputSection *AArch64BTILandingPadThunk::getTargetInputSection() const {
+  auto &dr = cast<Defined>(destination);
+  return dyn_cast<InputSection>(dr.section);
+}
+
+void AArch64BTILandingPadThunk::writeTo(uint8_t *buf) {
+  if (!getMayUseShortThunk()) {
+    writeLong(buf);
+    return;
+  }
+  write32(buf, 0xd503245f); // BTI c
+  // Control falls through to target in following section.
+}
+
+bool AArch64BTILandingPadThunk::getMayUseShortThunk() {
+  if (!mayUseShortThunk)
+    return false;
+  // If the target is the following instruction then
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MaskRay wrote:

The wrap column is smaller than 80.

https://github.com/llvm/llvm-project/pull/108989