[MachineOutliner][AArch64] Save + restore LR in noreturn functions

Conservatively always save + restore LR in noreturn functions.

These functions do not end in a RET, and so they aren't guaranteed to have an
instruction which uses LR in any way. So, as a result, you can end up in
unfortunate situations where you can't backtrace out of these functions in a
debugger.

Remove the old noreturn test, and add a new one which is more descriptive.

Remove the restriction that we can't outline from noreturn functions as well
since we now do the right thing.

1 files changed, 11 insertions, 1 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp b/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
index abde3e50021..54f3f7c1013 100644
--- a/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
@@ -5868,11 +5868,21 @@ outliner::OutlinedFunction AArch64InstrInfo::getOutliningCandidateInfo(
     unsigned NumBytesNoStackCalls = 0;
     std::vector<outliner::Candidate> CandidatesWithoutStackFixups;
 
+    // Check if we have to save LR.
     for (outliner::Candidate &C : RepeatedSequenceLocs) {
       C.initLRU(TRI);
 
+      // If we have a noreturn caller, then we're going to be conservative and
+      // say that we have to save LR. If we don't have a ret at the end of the
+      // block, then we can't reason about liveness accurately.
+      //
+      // FIXME: We can probably do better than always disabling this in
+      // noreturn functions by fixing up the liveness info.
+      bool IsNoReturn =
+          C.getMF()->getFunction().hasFnAttribute(Attribute::NoReturn);
+
       // Is LR available? If so, we don't need a save.
-      if (C.LRU.available(AArch64::LR)) {
+      if (C.LRU.available(AArch64::LR) && !IsNoReturn) {
         NumBytesNoStackCalls += 4;
         C.setCallInfo(MachineOutlinerNoLRSave, 4);
         CandidatesWithoutStackFixups.push_back(C);