[clang][dataflow] Model pointer value for builtin functions.

[mboehme]

Summary

This fixes a false positive in the Crubit nullability verification.

clang/lib/Analysis/FlowSensitive/Transfer.cpp

case CK_NullToMemberPointer: {
       auto &Loc = Env.createStorageLocation(S->getType());
       Env.setStorageLocation(*S, Loc);
 
       auto &NullPointerVal =
           Env.getOrCreateNullPointerValue(S->getType()->getPointeeType());
       Env.setValue(Loc, NullPointerVal);
       break;
     }
-    case CK_FunctionToPointerDecay: {
+    case CK_FunctionToPointerDecay:
+    case CK_BuiltinFnToFnPtr: {
       StorageLocation *PointeeLoc =
           Env.getStorageLocation(*SubExpr, SkipPast::Reference);
       if (PointeeLoc == nullptr)
         break;
 
       auto &PointerLoc = Env.createStorageLocation(*S);
       auto &PointerVal = Env.create<PointerValue>(*PointeeLoc);
       Env.setStorageLocation(*S, PointerLoc);

clang/unittests/Analysis/FlowSensitive/TransferTest.cpp

runDataflow(
         auto &MemberP1 =
             getValueForDecl<PointerValue>(ASTCtx, Env, "member_p1");
         auto &MemberP2 =
             getValueForDecl<PointerValue>(ASTCtx, Env, "member_p2");
         EXPECT_EQ(&MemberP1.getPointeeLoc(), &MemberP2.getPointeeLoc());
       });
 }
 
+// Check that the pointer that a builtin function decays to is associated with
+// a value.
+TEST(TransferTest, BuiltinFunctionModeled) {
+  std::string Code = R"(
+    void target() {
+      __builtin_expect(0, 0);
+      // [[p]]
+    }
+  )";
+  runDataflow(
+      Code,
+      [](const llvm::StringMap<DataflowAnalysisState<NoopLattice>> &Results,
+         ASTContext &ASTCtx) {
+        using ast_matchers::selectFirst;
+        using ast_matchers::match;
+        using ast_matchers::traverse;
+        using ast_matchers::implicitCastExpr;
+        using ast_matchers::hasCastKind;
+
+        const Environment &Env = getEnvironmentAtAnnotation(Results, "p");
+
+        auto *ImplicitCast = selectFirst<ImplicitCastExpr>(
+            "implicit_cast",
+            match(traverse(TK_AsIs,
+                           implicitCastExpr(hasCastKind(CK_BuiltinFnToFnPtr))
+                               .bind("implicit_cast")),
+                  ASTCtx));
+
+        ASSERT_THAT(ImplicitCast, NotNull());
+        EXPECT_THAT(Env.getValueStrict(*ImplicitCast), NotNull());
+      });
+}
+
 } // namespace