'RE: [PATCH] RE: [cfe-dev] missing return statement for non-void functions in C++'

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List: cfe-commits
Subject: RE: [PATCH] RE: [cfe-dev] missing return statement for non-void functions in C++
From: Sjoerd Meijer via cfe-commits <cfe-commits () lists ! llvm ! org>
Date: 2015-08-18 11:26:36
Message-ID: 000501d0d9a8$be24a4b0$3a6dee10$ () arm ! com
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Please see updated patch file attached, which now includes a fixed/added regression \
test.

Sjoerd.

 

From: Sjoerd Meijer 
Sent: 17 August 2015 13:34
To: 'Gabriel Dos Reis'; Richard Smith
Cc: Marshall Clow; cfe-commits
Subject: RE: [PATCH] RE: [cfe-dev] missing return statement for non-void functions in \
C++

 

Agreed, let's get that right first.

The attached patch causes the program to trap for all optimizations levels, except \
when optimizing for size.

 

Sjoerd.

 

From: dosreis@gmail.com [mailto:dosreis@gmail.com] On Behalf Of Gabriel Dos Reis
Sent: 13 August 2015 21:51
To: Richard Smith
Cc: Sjoerd Meijer; Marshall Clow; cfe-commits
Subject: Re: [PATCH] RE: [cfe-dev] missing return statement for non-void functions in \
C++

 

Please make such programs crash early and often. They are a nightmare to maintain. \
Make them blow in the face of the original authors; not after they are gone.

 

-- Gaby

 


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<html xmlns:v="urn:schemas-microsoft-com:vml" \
xmlns:o="urn:schemas-microsoft-com:office:office" \
xmlns:w="urn:schemas-microsoft-com:office:word" \
xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" \
xmlns="http://www.w3.org/TR/REC-html40"><head><meta http-equiv=Content-Type \
content="text/html; charset=utf-8"><meta name=Generator content="Microsoft Word 14 \
(filtered medium)"><style></style></head><body lang=EN-GB link=blue vlink=purple><div \
class=WordSection1>Please see \
updated patch file attached, which now includes a fixed/added regression \
test.<o:p></o:p>Sjoerd.<o:p></o:p><o:p>&nbsp;</o:p><div><div \
style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0cm 0cm 0cm'>From: Sjoerd Meijer \
 Sent: 17 August 2015 13:34 To: 'Gabriel Dos Reis'; Richard \
Smith Cc: Marshall Clow; cfe-commits Subject: RE: [PATCH] RE: \
[cfe-dev] missing return statement for non-void functions in \
C++<o:p></o:p></div></div><o:p>&nbsp;</o:p>Agreed, \
let's get that right first.<o:p></o:p>The \
attached patch causes the program to trap for all optimizations levels, except when \
optimizing for size.<o:p></o:p><o:p>&nbsp;</o:p>Sjoerd.<o:p></o:p><o:p>&nbsp;</o:p>From: <a \
href="mailto:dosreis@gmail.com">dosreis@gmail.com</a> [<a \
href="mailto:dosreis@gmail.com">mailto:dosreis@gmail.com</a>] On Behalf Of \
Gabriel Dos Reis Sent: 13 August 2015 21:51 To: Richard \
Smith Cc: Sjoerd Meijer; Marshall Clow; cfe-commits Subject: Re: \
[PATCH] RE: [cfe-dev] missing return statement for non-void functions in \
C++<o:p></o:p><o:p>&nbsp;</o:p><div>Please make such programs crash early and often.&nbsp; They are a \
nightmare to maintain.&nbsp; Make them blow in the face of the original authors; not \
after they are gone.<o:p></o:p><div><o:p>&nbsp;</o:p></div><div>-- \
Gaby<o:p></o:p><div><o:p>&nbsp;</o:p></div></div></div></div></body></html>


["0001-Always-trap-for-missing-return-statements-except-whe.patch" (application/octet-stream)]

From 64fb3ce15a22d100d79847a074b31c64f24cd75d Mon Sep 17 00:00:00 2001
From: Sjoerd Meijer <sjoerd.meijer@arm.com>
Date: Mon, 17 Aug 2015 11:38:35 +0100
Subject: [PATCH] Always trap for missing return statements, except when
 optimizing for size.

---
 lib/CodeGen/CodeGenFunction.cpp | 9 ++++++++-
 test/CodeGenCXX/return.cpp | 6 +++++-
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/lib/CodeGen/CodeGenFunction.cpp b/lib/CodeGen/CodeGenFunction.cpp
index 9bf4161..3135c1a 100644
--- a/lib/CodeGen/CodeGenFunction.cpp
+++ b/lib/CodeGen/CodeGenFunction.cpp
@@ -926,7 +926,14 @@ void CodeGenFunction::GenerateCode(GlobalDecl GD, llvm::Function \
*Fn,  EmitCheck(std::make_pair(IsFalse, SanitizerKind::Return),
                 "missing_return", EmitCheckSourceLocation(FD->getLocation()),
                 None);
-    } else if (CGM.getCodeGenOpts().OptimizationLevel == 0) {
+    } 
+
+    // A missing return statement provides opportunities for optimizing away
+    // unreachable control flow, i.e it is a code-size optimization opportunity. 
+    // However, not returning from a function is potentially unsafe and also not 
+    // so user-friendly. Therefore, we trap for all optimizations levels, except 
+    // when optimizing for size.
+    if (!CGM.getCodeGenOpts().OptimizeSize) {
       EmitTrapCall(llvm::Intrinsic::trap);
     }
     Builder.CreateUnreachable();
diff --git a/test/CodeGenCXX/return.cpp b/test/CodeGenCXX/return.cpp
index 5c1cfda..ff4fbee 100644
--- a/test/CodeGenCXX/return.cpp
+++ b/test/CodeGenCXX/return.cpp
@@ -1,5 +1,6 @@
 // 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 \
-Oz -o - %s | FileCheck %s --check-prefix=CHECK-OPT-SIZE  
 // CHECK:     @_Z9no_return
 // CHECK-OPT: @_Z9no_return
@@ -7,6 +8,9 @@ int no_return() {
   // CHECK:      call void @llvm.trap
   // CHECK-NEXT: unreachable
 
-  // CHECK-OPT-NOT: call void @llvm.trap
+  // CHECK-OPT: call void @llvm.trap
   // CHECK-OPT:     unreachable
+
+  // CHECK-OPT-SIZE-NOT: call void @llvm.trap
+  // CHECK-OPT-SIZE:     unreachable
 }
-- 
1.9.1


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