[CodeGen] Teach SimplifyPersonality about the updated LandingPadInst

[vsk]

Summary

When uses of personality functions were moved from LandingPadInst to Function, we forgot to update SimplifyPersonality(). This patch corrects that.

Note: SimplifyPersonality() is an optimization which replaces personality functions with the default C++ personality when possible. Without this update, some ObjC++ projects fail to link against C++ libraries (seeing as the exception ABI had effectively changed).

[rjmccall]

lib/CodeGen/CGException.cpp

276 ↗

(On Diff #34391)

if (!F)

You should test this by adding an uncommon use of the personality function. Just declare it as a global function (the type doesn't really matter) and take its address somewhere.

[vsk]

Thanks for the review.

• Addressed if (!U) bug.
• Added test which loads a personality function, confirmed that we crash without the proper `if (!F)' check.

[rjmccall]

LGTM, thanks.

[vsk]

Committed r247421

[rnk]

I'm confused. I thought SimplifyPersonalityFunction was an optimization, but somehow it caused link failures? Why do you think this was an ABI break?

test/CodeGenObjCXX/exception-cxx.mm

8 ↗

(On Diff #34408)

Don't you want to test the 'catch (int e)' case? That introduces interesting uses of the selector that catch-all doesn't have.

[vsk]

I think 'optimization' is a bit of a misnomer. There's a comment in this code that reads: "Can't do the optimization if it has non-C++ uses", so that's why I picked up the word. Without SimplifyPersonality(), some objective c++ code can no longer link against c++ libraries.

Consider the following test:
$ echo "#include <stdexcept>" "\n" 'void foo() { throw std::runtime_error("foo"); }' | clang -x objective-cxx -S -emit-llvm - -o - | grep personality

After we moved personality from landingpadinst to functions, the output of the test changes (from gxx_personality to objc_personality). This commit just takes us back to the original behavior.

The original comment, "Otherwise, it has to be a landingpad instruction.", is wrong after David's patch.

[vsk]

Addressed Reid's comment w.r.t exception object type.

[rnk]

Right, I understand the behavior change, I'm just wondering why it results in link failures. There isn't a ton of public info about how ObjC++ EH interacts with C++ EH.

[vsk]

Ah, ok. We have some objective-c++ code which calls into a boost routine which throws an exception. That results in an undefined reference to ___objc_personality_v0, because the boost library we linked against doesn't have ___objc_personality_v0.

Should the compiler have found the ___objc_personality_v0 symbol in this case regardless?

[rjmccall]

In D12743#244375, @vsk wrote:

Ah, ok. We have some objective-c++ code which calls into a boost routine which throws an exception. That results in an undefined reference to ___objc_personality_v0, because the boost library we linked against doesn't have ___objc_personality_v0.

Should the compiler have found the ___objc_personality_v0 symbol in this case regardless?

It's pretty straightforward. Sometimes people write code in ObjC++ files that's really pure C++. Such code is generally compiled with -fexceptions because that's the default, and so it has cleanups, and those cleanups require us to pick a personality function. When they do so, they generally don't link against the ObjC runtime, and so __objc_personality_v0 isn't found. The workaround here is to recognize that they're not actually catching ObjC exception types and just quietly degrade to use the C++ personality.

It is probably technically an optimization, because it removes some overhead from double-parsing the exception tables (because the ObjC personality delegates to the C++ personality, instead of being tightly integrated with it), but that's not the real reason we do it.

cfe/trunk/test/CodeGenObjCXX/exception-cxx.mm

 // RUN: %clang_cc1 -x objective-c++ -triple x86_64-apple-darwin10 -emit-llvm -fcxx-exceptions -fexceptions -fobjc-exceptions -o - %s | FileCheck %s
 
 // rdar://problem/22155434
 namespace test0 {
   void foo() {
     try {
       throw 0;
-    } catch (...) {
+    } catch (int e) {
       return;
     }
   }
 // CHECK: define void @_ZN5test03fooEv() #0 personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*)
 }

cfe/trunk/test/CodeGenObjCXX/personality-abuse.mm

 // RUN: %clang_cc1 -x objective-c++ -triple x86_64-apple-darwin10 -emit-llvm -fcxx-exceptions -fexceptions -fobjc-exceptions -o - %s | FileCheck %s
 
 extern "C" {
   int __objc_personality_v0();
 }
 
 void *abuse_personality_func() {
   return (void *)&__objc_personality_v0;
 }
 
 void foo() {
   try {
     throw 0;
-  } catch (...) {
+  } catch (int e) {
     return;
   }
 }
 
 // CHECK: define void @_Z3foov() #1 personality i8* bitcast (i32 ()* @__objc_personality_v0 to i8*)