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

3 files changed, 27 insertions, 4 deletions

diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index a954f487d1e..7cab13de923 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/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.
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index 01a17dbf517..874799b1101 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/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))
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index a0682e26e70..ca4a7655a37 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/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) ||