[llvm] [AArch64][BTI] Add BTI at EH entries. (PR #155308)

[Eli Friedman via llvm-commits]

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@@ -143,11 +161,33 @@ void AArch64BranchTargets::addBTI(MachineBasicBlock &MBB, bool CouldCall,
        MBBI->getOpcode() == AArch64::PACIBSP))
     return;
 
-  if (HasWinCFI && MBBI->getFlag(MachineInstr::FrameSetup)) {
+  // If there's already a HINT at the insertion point, handle BTI safely.
+  if (MBBI != MBB.end() && MBBI->getOpcode() == AArch64::HINT) {
+    // Only "upgrade" if it is actually a BTI HINT (base 32). Otherwise,
+    // fall through and insert a BTI before this non-BTI hint, so the
+    // first executed instruction is BTI/PAC as required by enforcement.
+    if (MBBI->getOperand(0).isImm()) {
+      int64_t Imm = MBBI->getOperand(0).getImm();
+      if ((Imm & ~int64_t(BTIMask)) == BTIBase) {
+        unsigned Have = unsigned(Imm) & BTIMask;
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efriedma-quic wrote:

Is this code actually reachable?  As far as I can tell, no other pass actually generates BTI instructions, so we should never find one.

https://github.com/llvm/llvm-project/pull/155308