[FastISel] Support unreachable with NoTrapAfterNoReturn (#118296)

Currently FastISel triggers a fallback if there is an unreachable
terminator and the TrapUnreachable option is enabled (the ISD::TRAP
selection does not actually work).

Add handling for NoTrapAfterNoReturn, in which case we don't actually
need to emit a trap. The test is just there to make sure there is no
FastISel fallback (which is why I'm not testing the case without
noreturn). We have other tests that check the actual unreachable codegen
variations.

Author: nikic

Diff for: llvm/lib/CodeGen/SelectionDAG/FastISel.cpp

@@ -1851,11 +1851,19 @@ bool FastISel::selectOperator(const User *I, unsigned Opcode) {
     return false;
   }
 
-  case Instruction::Unreachable:
-    if (TM.Options.TrapUnreachable)
+  case Instruction::Unreachable: {
+    if (TM.Options.TrapUnreachable) {
+      if (TM.Options.NoTrapAfterNoreturn) {
+        const auto *Call =
+            dyn_cast_or_null<CallInst>(cast<Instruction>(I)->getPrevNode());
+        if (Call && Call->doesNotReturn())
+          return true;
+      }
+
       return fastEmit_(MVT::Other, MVT::Other, ISD::TRAP) != 0;
-    else
-      return true;
+    }
+    return true;
+  }
 
   case Instruction::Alloca:
     // FunctionLowering has the static-sized case covered.

Diff for: llvm/test/CodeGen/X86/no-trap-after-noreturn-fastisel.ll

@@ -0,0 +1,13 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -mtriple=x86_64-unknown-linux-gnu -O0 -trap-unreachable -no-trap-after-noreturn -fast-isel-abort=3 < %s | FileCheck %s
+
+declare void @foo()
+
+define void @noreturn_unreachable() nounwind {
+; CHECK-LABEL: noreturn_unreachable:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    pushq %rax
+; CHECK-NEXT:    callq foo@PLT
+  call void @foo() noreturn
+  unreachable
+}