[WPD]Regard unreachable function as a possible devirtualizable target

llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp

@@ -167,6 +167,18 @@ static cl::list<std::string>
                       cl::desc("Prevent function(s) from being devirtualized"),
                       cl::Hidden, cl::CommaSeparated);
 
+/// A function is unreachable if its entry block ends with 'unreachable' IR
+/// instruction. In some cases, the program intends to run such functions and
+/// terminate, for instance, a unit test may run a death test. A non-test
+/// program might (or allowed to) invoke such functions to report failures
+/// (whether/when it's a good practice or not is a different topic). Regard
+/// unreachable function as possible devirtualize targets to keep the program
+/// behavior.
+static cl::opt<bool> WholeProgramDevirtKeepUnreachableFunction(
+    "wholeprogramdevirt-keep-unreachable-function",
+    cl::desc("Regard unreachable functions as possible devirtualize targets."),
+    cl::Hidden, cl::init(true));
+
 /// If explicitly specified, the devirt module pass will stop transformation
 /// once the total number of devirtualizations reach the cutoff value. Setting
 /// this option to 0 explicitly will do 0 devirtualization.
@@ -386,6 +398,9 @@ template <> struct DenseMapInfo<VTableSlotSummary> {
 //   2) All function summaries indicate it's unreachable
 //   3) There is no non-function with the same GUID (which is rare)
 static bool mustBeUnreachableFunction(ValueInfo TheFnVI) {
+  if (WholeProgramDevirtKeepUnreachableFunction)
+    return false;
+
   if ((!TheFnVI) || TheFnVI.getSummaryList().empty()) {
     // Returns false if ValueInfo is absent, or the summary list is empty
     // (e.g., function declarations).
@@ -2241,6 +2256,8 @@ DevirtModule::lookUpFunctionValueInfo(Function *TheFn,
 
 bool DevirtModule::mustBeUnreachableFunction(
     Function *const F, ModuleSummaryIndex *ExportSummary) {
+  if (WholeProgramDevirtKeepUnreachableFunction)
+    return false;
   // First, learn unreachability by analyzing function IR.
   if (!F->isDeclaration()) {
     // A function must be unreachable if its entry block ends with an

llvm/test/Transforms/WholeProgramDevirt/devirt_single_after_filtering_unreachable_function.ll

@@ -1,11 +1,15 @@
+; Test that static devirtualization doesn't happen because there are two
+; devirtualizable targets. Unreachable functions are kept in the devirtualizable
+; target set by default.
+; RUN: opt -S -passes=wholeprogramdevirt -whole-program-visibility -pass-remarks=wholeprogramdevirt %s 2>&1 | FileCheck %s  --implicit-check-not="single-impl"
+
 ; Test that regular LTO will analyze IR, detect unreachable functions and discard unreachable functions
 ; when finding virtual call targets.
 ; In this test case, the unreachable function is the virtual deleting destructor of an abstract class.
+; RUN: opt -S -passes=wholeprogramdevirt -whole-program-visibility -pass-remarks=wholeprogramdevirt -wholeprogramdevirt-keep-unreachable-function=false %s 2>&1 | FileCheck %s --check-prefix=DEVIRT
 
-; RUN: opt -S -passes=wholeprogramdevirt -whole-program-visibility -pass-remarks=wholeprogramdevirt %s 2>&1 | FileCheck %s
-
-; CHECK: remark: tmp.cc:21:3: single-impl: devirtualized a call to _ZN7DerivedD0Ev
-; CHECK: remark: <unknown>:0:0: devirtualized _ZN7DerivedD0Ev
+; DEVIRT: remark: tmp.cc:21:3: single-impl: devirtualized a call to _ZN7DerivedD0Ev
+; DEVIRT: remark: <unknown>:0:0: devirtualized _ZN7DerivedD0Ev
 
 source_filename = "tmp.cc"
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"