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, 29 insertions, 0 deletions

diff --git a/clang/test/CodeGenObjCXX/return.mm b/clang/test/CodeGenObjCXX/return.mm
new file mode 100644
index 00000000000..53343e12b2f
--- /dev/null
+++ b/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