Add -f[no-]strict-return flag that can be used to avoid undefined behaviour

in non-void functions that fall off at the end without returning a value when
compiling C++.

Clang uses the new compiler flag to determine when it should treat control flow
paths that fall off the end of a non-void function as unreachable. If
-fno-strict-return is on, the code generator emits the ureachable and trap
IR only when the function returns either a record type with a non-trivial
destructor or another non-trivially copyable type.

The primary goal of this flag is to avoid treating falling off the end of a
non-void function as undefined behaviour. The burden of undefined behaviour
is placed on the caller instead: if the caller ignores the returned value then
the undefined behaviour is avoided. This kind of behaviour is useful in
several cases, e.g. when compiling C code in C++ mode.

rdar://13102603

Differential Revision: https://reviews.llvm.org/D27163

llvm-svn: 290960

clang/include/clang/Driver/Options.td

@@ -1331,6 +1331,12 @@ def funique_section_names : Flag <["-"], "funique-section-names">,
 def fno_unique_section_names : Flag <["-"], "fno-unique-section-names">,
   Group<f_Group>, Flags<[CC1Option]>;
 
+def fstrict_return : Flag<["-"], "fstrict-return">, Group<f_Group>,
+  Flags<[CC1Option]>,
+  HelpText<"Always treat control flow paths that fall off the end of a non-void"
+           "function as unreachable">;
+def fno_strict_return : Flag<["-"], "fno-strict-return">, Group<f_Group>,
+  Flags<[CC1Option]>;
 
 def fdebug_types_section: Flag <["-"], "fdebug-types-section">, Group<f_Group>,
   Flags<[CC1Option]>, HelpText<"Place debug types in their own section (ELF Only)">;

clang/include/clang/Frontend/CodeGenOptions.def

@@ -251,6 +251,10 @@ CODEGENOPT(DiagnosticsWithHotness, 1, 0)
 /// Whether copy relocations support is available when building as PIE.
 CODEGENOPT(PIECopyRelocations, 1, 0)
 
+/// Whether we should use the undefined behaviour optimization for control flow
+/// paths that reach the end of a function without executing a required return.
+CODEGENOPT(StrictReturn, 1, 1)
+
 #undef CODEGENOPT
 #undef ENUM_CODEGENOPT
 #undef VALUE_CODEGENOPT

clang/lib/CodeGen/CodeGenFunction.cpp

@@ -1049,6 +1049,19 @@ QualType CodeGenFunction::BuildFunctionArgList(GlobalDecl GD,
   return ResTy;
 }
 
+static bool
+shouldUseUndefinedBehaviorReturnOptimization(const FunctionDecl *FD,
+                                             const ASTContext &Context) {
+  QualType T = FD->getReturnType();
+  // Avoid the optimization for functions that return a record type with a
+  // trivial destructor or another trivially copyable type.
+  if (const RecordType *RT = T.getCanonicalType()->getAs<RecordType>()) {
+    if (const auto *ClassDecl = dyn_cast<CXXRecordDecl>(RT->getDecl()))
+      return !ClassDecl->hasTrivialDestructor();
+  }
+  return !T.isTriviallyCopyableType(Context);
+}
+
 void CodeGenFunction::GenerateCode(GlobalDecl GD, llvm::Function *Fn,
                                    const CGFunctionInfo &FnInfo) {
   const FunctionDecl *FD = cast<FunctionDecl>(GD.getDecl());
@@ -1127,17 +1140,23 @@ void CodeGenFunction::GenerateCode(GlobalDecl GD, llvm::Function *Fn,
   //   function call is used by the caller, the behavior is undefined.
   if (getLangOpts().CPlusPlus && !FD->hasImplicitReturnZero() && !SawAsmBlock &&
       !FD->getReturnType()->isVoidType() && Builder.GetInsertBlock()) {
+    bool ShouldEmitUnreachable =
+        CGM.getCodeGenOpts().StrictReturn ||
+        shouldUseUndefinedBehaviorReturnOptimization(FD, getContext());
     if (SanOpts.has(SanitizerKind::Return)) {
       SanitizerScope SanScope(this);
       llvm::Value *IsFalse = Builder.getFalse();
       EmitCheck(std::make_pair(IsFalse, SanitizerKind::Return),
                 SanitizerHandler::MissingReturn,
                 EmitCheckSourceLocation(FD->getLocation()), None);
-    } else if (CGM.getCodeGenOpts().OptimizationLevel == 0) {
-      EmitTrapCall(llvm::Intrinsic::trap);
+    } else if (ShouldEmitUnreachable) {
+      if (CGM.getCodeGenOpts().OptimizationLevel == 0)
+        EmitTrapCall(llvm::Intrinsic::trap);
+    }
+    if (SanOpts.has(SanitizerKind::Return) || ShouldEmitUnreachable) {
+      Builder.CreateUnreachable();
+      Builder.ClearInsertionPoint();
     }
-    Builder.CreateUnreachable();
-    Builder.ClearInsertionPoint();
   }
 
   // Emit the standard function epilogue.

clang/lib/Driver/Tools.cpp

@@ -4462,6 +4462,9 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   if (Args.hasFlag(options::OPT_fstrict_enums, options::OPT_fno_strict_enums,
                    false))
     CmdArgs.push_back("-fstrict-enums");
+  if (!Args.hasFlag(options::OPT_fstrict_return, options::OPT_fno_strict_return,
+                    true))
+    CmdArgs.push_back("-fno-strict-return");
   if (Args.hasFlag(options::OPT_fstrict_vtable_pointers,
                    options::OPT_fno_strict_vtable_pointers,
                    false))

clang/lib/Frontend/CompilerInvocation.cpp

@@ -602,6 +602,7 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
   Opts.NoDwarfDirectoryAsm = Args.hasArg(OPT_fno_dwarf_directory_asm);
   Opts.SoftFloat = Args.hasArg(OPT_msoft_float);
   Opts.StrictEnums = Args.hasArg(OPT_fstrict_enums);
+  Opts.StrictReturn = !Args.hasArg(OPT_fno_strict_return);
   Opts.StrictVTablePointers = Args.hasArg(OPT_fstrict_vtable_pointers);
   Opts.UnsafeFPMath = Args.hasArg(OPT_menable_unsafe_fp_math) ||
                       Args.hasArg(OPT_cl_unsafe_math_optimizations) ||

clang/test/CodeGenCXX/return.cpp

@@ -1,12 +1,105 @@
-// RUN: %clang_cc1 -emit-llvm -triple %itanium_abi_triple -o - %s | FileCheck %s
-// RUN: %clang_cc1 -emit-llvm -triple %itanium_abi_triple -O -o - %s | FileCheck %s --check-prefix=CHECK-OPT
+// RUN: %clang_cc1 -emit-llvm -triple %itanium_abi_triple -std=c++11 -o - %s | FileCheck --check-prefixes=CHECK,CHECK-COMMON %s
+// RUN: %clang_cc1 -emit-llvm -triple %itanium_abi_triple -std=c++11 -O -o - %s | FileCheck %s --check-prefixes=CHECK-OPT,CHECK-COMMON
+// RUN: %clang_cc1 -emit-llvm -triple %itanium_abi_triple -std=c++11 -fno-strict-return -o - %s | FileCheck %s --check-prefixes=CHECK-NOSTRICT,CHECK-COMMON
+// RUN: %clang_cc1 -emit-llvm -triple %itanium_abi_triple -std=c++11 -fno-strict-return -Wno-return-type -o - %s | FileCheck %s --check-prefixes=CHECK-NOSTRICT,CHECK-COMMON
+// RUN: %clang_cc1 -emit-llvm -triple %itanium_abi_triple -std=c++11 -fno-strict-return -O -o - %s | FileCheck %s --check-prefixes=CHECK-NOSTRICT-OPT,CHECK-COMMON
 
-// CHECK:     @_Z9no_return
-// CHECK-OPT: @_Z9no_return
+// CHECK-COMMON-LABEL: @_Z9no_return
 int no_return() {
   // CHECK:      call void @llvm.trap
   // CHECK-NEXT: unreachable
 
   // CHECK-OPT-NOT: call void @llvm.trap
   // CHECK-OPT:     unreachable
+
+  // -fno-strict-return should not emit trap + unreachable but it should return
+  // an undefined value instead.
+
+  // CHECK-NOSTRICT: entry:
+  // CHECK-NOSTRICT-NEXT: alloca
+  // CHECK-NOSTRICT-NEXT: load
+  // CHECK-NOSTRICT-NEXT: ret i32
+  // CHECK-NOSTRICT-NEXT: }
+
+  // CHECK-NOSTRICT-OPT: entry:
+  // CHECK-NOSTRICT-OPT: ret i32 undef
+}
+
+enum Enum {
+  A, B
+};
+
+// CHECK-COMMON-LABEL: @_Z27returnNotViableDontOptimize4Enum
+int returnNotViableDontOptimize(Enum e) {
+  switch (e) {
+  case A: return 1;
+  case B: return 2;
+  }
+  // Undefined behaviour optimization shouldn't be used when -fno-strict-return
+  // is turned on, even if all the enum cases are covered in this function.
+
+  // CHECK-NOSTRICT-NOT: call void @llvm.trap
+  // CHECK-NOSTRICT-NOT: unreachable
+}
+
+struct Trivial {
+  int x;
+};
+
+// CHECK-NOSTRICT-LABEL: @_Z7trivialv
+Trivial trivial() {
+  // This function returns a trivial record so -fno-strict-return should avoid
+  // the undefined behaviour optimization.
+
+  // CHECK-NOSTRICT-NOT: call void @llvm.trap
+  // CHECK-NOSTRICT-NOT: unreachable
+}
+
+struct NonTrivialCopy {
+  NonTrivialCopy(const NonTrivialCopy &);
+};
+
+// CHECK-NOSTRICT-LABEL: @_Z14nonTrivialCopyv
+NonTrivialCopy nonTrivialCopy() {
+  // CHECK-NOSTRICT-NOT: call void @llvm.trap
+  // CHECK-NOSTRICT-NOT: unreachable
+}
+
+struct NonTrivialDefaultConstructor {
+  int x;
+
+  NonTrivialDefaultConstructor() { }
+};
+
+// CHECK-NOSTRICT-LABEL: @_Z28nonTrivialDefaultConstructorv
+NonTrivialDefaultConstructor nonTrivialDefaultConstructor() {
+  // CHECK-NOSTRICT-NOT: call void @llvm.trap
+  // CHECK-NOSTRICT-NOT: unreachable
+}
+
+// Functions that return records with non-trivial destructors should always use
+// the -fstrict-return optimization.
+
+struct NonTrivialDestructor {
+  ~NonTrivialDestructor();
+};
+
+// CHECK-NOSTRICT-LABEL: @_Z20nonTrivialDestructorv
+NonTrivialDestructor nonTrivialDestructor() {
+  // CHECK-NOSTRICT: call void @llvm.trap
+  // CHECK-NOSTRICT-NEXT: unreachable
+}
+
+// The behavior for lambdas should be identical to functions.
+// CHECK-COMMON-LABEL: @_Z10lambdaTestv
+void lambdaTest() {
+  auto lambda1 = []() -> int {
+  };
+  lambda1();
+
+  // CHECK: call void @llvm.trap
+  // CHECK-NEXT: unreachable
+
+  // CHECK-NOSTRICT-NOT: call void @llvm.trap
+  // CHECK-NOSTRICT-NOT: unreachable
 }

clang/test/CodeGenObjCXX/return.mm

@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 -emit-llvm -fblocks -triple x86_64-apple-darwin -fstrict-return -o - %s | FileCheck %s
+// RUN: %clang_cc1 -emit-llvm -fblocks -triple x86_64-apple-darwin -fstrict-return -O -o - %s | FileCheck %s
+
+@interface I
+@end
+
+@implementation I
+
+- (int)method {
+}
+
+@end
+
+enum Enum {
+  a
+};
+
+int (^block)(Enum) = ^int(Enum e) {
+  switch (e) {
+  case a:
+    return 1;
+  }
+};
+
+// Ensure that both methods and blocks don't use the -fstrict-return undefined
+// behaviour optimization.
+
+// CHECK-NOT: call void @llvm.trap
+// CHECK-NOT: unreachable

clang/test/Driver/clang_f_opts.c

@@ -477,3 +477,8 @@
 // CHECK-NEW-PM-NOT: -fno-experimental-new-pass-manager
 // CHECK-NO-NEW-PM: -fno-experimental-new-pass-manager
 // CHECK-NO-NEW-PM-NOT: -fexperimental-new-pass-manager
+
+// RUN: %clang -### -S -fstrict-return %s 2>&1 | FileCheck -check-prefix=CHECK-STRICT-RETURN %s
+// RUN: %clang -### -S -fno-strict-return %s 2>&1 | FileCheck -check-prefix=CHECK-NO-STRICT-RETURN %s
+// CHECK-STRICT-RETURN-NOT: "-fno-strict-return"
+// CHECK-NO-STRICT-RETURN: "-fno-strict-return"