[ARM][AArch64] Bail out if CandidatesWithoutStackFixups is empty (#95410)

The following code assumes that RepeatedSequenceLocs is non-empty. Bail
out if there are less than 2 candidates left, as no outlining is
possible in that case. The same check is already present in all the
other places where elements from RepeatedSequenceLocs may be dropped.

This fixes the issue reported at:
#93965 (comment)

Author: nikic

llvm/lib/Target/AArch64/AArch64InstrInfo.cpp

@@ -8556,6 +8556,8 @@ AArch64InstrInfo::getOutliningCandidateInfo(
         NumBytesNoStackCalls <= RepeatedSequenceLocs.size() * 12) {
       RepeatedSequenceLocs = CandidatesWithoutStackFixups;
       FrameID = MachineOutlinerNoLRSave;
+      if (RepeatedSequenceLocs.size() < 2)
+        return std::nullopt;
     } else {
       SetCandidateCallInfo(MachineOutlinerDefault, 12);
 

llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp

@@ -6060,6 +6060,8 @@ ARMBaseInstrInfo::getOutliningCandidateInfo(
         RepeatedSequenceLocs.size() * Costs.CallDefault) {
       RepeatedSequenceLocs = CandidatesWithoutStackFixups;
       FrameID = MachineOutlinerNoLRSave;
+      if (RepeatedSequenceLocs.size() < 2)
+        return std::nullopt;
     } else
       SetCandidateCallInfo(MachineOutlinerDefault, Costs.CallDefault);
   }

llvm/test/CodeGen/ARM/machine-outliner-no-candidates-without-stack-fixup.ll

@@ -0,0 +1,30 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -mtriple=thumbv8.1m.main-unknown-unknown < %s | FileCheck %s
+
+; Make sure this does not assert during machine outlining.
+
+declare void @a(...)
+
+define void @b(i32 %a, i32 %b, i32 %c, ptr %d, ptr %e) minsize {
+; CHECK-LABEL: b:
+; CHECK:       @ %bb.0:
+; CHECK-NEXT:    ldr r3, [sp]
+; CHECK-NEXT:    mov r2, r1
+; CHECK-NEXT:    mov r1, r0
+; CHECK-NEXT:    movs r0, #2
+; CHECK-NEXT:    b a
+  tail call void @a(i32 2, i32 %a, i32 %b, ptr %e)
+  ret void
+}
+
+define void @c(i32 %a, i32 %b, i32 %c, ptr %d, ptr %e) minsize {
+; CHECK-LABEL: c:
+; CHECK:       @ %bb.0:
+; CHECK-NEXT:    ldr r3, [sp]
+; CHECK-NEXT:    mov r2, r1
+; CHECK-NEXT:    mov r1, r0
+; CHECK-NEXT:    movs r0, #4
+; CHECK-NEXT:    b a
+  tail call void @a(i32 4, i32 %a, i32 %b, ptr %e)
+  ret void
+}