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

1 files changed, 97 insertions, 4 deletions

diff --git a/clang/test/CodeGenCXX/return.cpp b/clang/test/CodeGenCXX/return.cpp
index 5c1cfdaeed7..6166cccd724 100644
--- a/clang/test/CodeGenCXX/return.cpp
+++ b/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
 }