Revert "Revert "[GlobalISel][IRTranslator] Emit trap intrinsic for "unreachable"""

This reverts commit d95cd81141.

Re-land the original patch now that the bug this exposed in selection has been
fixed by 6bc64e24c3

llvm/include/llvm/CodeGen/GlobalISel/IRTranslator.h

@@ -467,9 +467,8 @@ private:
   bool translateSIToFP(const User &U, MachineIRBuilder &MIRBuilder) {
     return translateCast(TargetOpcode::G_SITOFP, U, MIRBuilder);
   }
-  bool translateUnreachable(const User &U, MachineIRBuilder &MIRBuilder) {
-    return true;
-  }
+  bool translateUnreachable(const User &U, MachineIRBuilder &MIRBuilder);
+
   bool translateSExt(const User &U, MachineIRBuilder &MIRBuilder) {
     return translateCast(TargetOpcode::G_SEXT, U, MIRBuilder);
   }

llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp

@@ -2701,6 +2701,28 @@ bool IRTranslator::translateVAArg(const User &U, MachineIRBuilder &MIRBuilder) {
   return true;
 }
 
+bool IRTranslator::translateUnreachable(const User &U, MachineIRBuilder &MIRBuilder) {
+    if (!MF->getTarget().Options.TrapUnreachable)
+    return true;
+
+  auto &UI = cast<UnreachableInst>(U);
+  // We may be able to ignore unreachable behind a noreturn call.
+  if (MF->getTarget().Options.NoTrapAfterNoreturn) {
+    const BasicBlock &BB = *UI.getParent();
+    if (&UI != &BB.front()) {
+      BasicBlock::const_iterator PredI =
+        std::prev(BasicBlock::const_iterator(UI));
+      if (const CallInst *Call = dyn_cast<CallInst>(&*PredI)) {
+        if (Call->doesNotReturn())
+          return true;
+      }
+    }
+  }
+
+  MIRBuilder.buildIntrinsic(Intrinsic::trap, ArrayRef<Register>(), true);
+  return true;
+}
+
 bool IRTranslator::translateInsertElement(const User &U,
                                           MachineIRBuilder &MIRBuilder) {
   // If it is a <1 x Ty> vector, use the scalar as it is

llvm/test/CodeGen/AArch64/GlobalISel/irtranslator-unreachable.ll

@@ -0,0 +1,24 @@
+; NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+; RUN: llc -O0 -mtriple=aarch64-apple-ios -global-isel -stop-after=irtranslator %s -o - | FileCheck %s
+
+declare void @llvm.trap()
+
+define void @unreachable() {
+  ; CHECK-LABEL: name: unreachable
+  ; CHECK: bb.1 (%ir-block.0):
+  ; CHECK-NEXT:   G_INTRINSIC_W_SIDE_EFFECTS intrinsic(@llvm.trap)
+  unreachable
+  ret void
+}
+
+declare void @foo() noreturn
+define void @trap_call_noreturn() {
+  ; CHECK-LABEL: name: trap_call_noreturn
+  ; CHECK: bb.1 (%ir-block.0):
+  ; CHECK-NEXT:   ADJCALLSTACKDOWN 0, 0, implicit-def $sp, implicit $sp
+  ; CHECK-NEXT:   BL @foo, csr_darwin_aarch64_aapcs, implicit-def $lr, implicit $sp
+  ; CHECK-NEXT:   ADJCALLSTACKUP 0, 0, implicit-def $sp, implicit $sp
+  call void @foo()
+  unreachable
+  ret void
+}