[ItaniumCXXABI] Add -fassume-nothrow-exception-dtor to assume that all exception objects' destructors are non-throwing

Link: https://lists.llvm.org/pipermail/cfe-dev/2021-August/068740.html ("[Excep

[ItaniumCXXABI] Add -fassume-nothrow-exception-dtor to assume that all exception objects' destructors are non-throwing

Link: https://lists.llvm.org/pipermail/cfe-dev/2021-August/068740.html ("[Exception Handling] Could we mark __cxa_end_catch as nounwind conditionally?"
Link: https://github.com/llvm/llvm-project/issues/57375

A catch handler calls `__cxa_begin_catch` and `__cxa_end_catch`. For a catch-all
clause or a catch clause matching a record type, we:

* assume that the exception object may have a throwing destructor
* emit `invoke void @__cxa_end_catch` (as the call is not marked as the `nounwind` attribute).
* emit a landing pad to destroy local variables and call `_Unwind_Resume`

```
struct A { ~A(); };
struct B { int x; };
void opaque();
void foo() {
A a;
try { opaque(); } catch (...) { } // the exception object has an unknown type and may throw
try { opaque(); } catch (B b) { } // B::~B is nothrow, but we do not utilize this
}
```

Per C++ [dcl.fct.def.coroutine], a coroutine's function body implies a `catch (...)`.
Our code generation pessimizes even simple code, like:
```
UserFacing foo() {
A a;
opaque();
co_return;
// For `invoke void @__cxa_end_catch()`, the landing pad destroys the
// promise_type and deletes the coro frame.
}
```

Throwing destructors are typically discouraged. In many environments, the
destructors of exception objects are guaranteed to never throw, making our
conservative code generation approach seem wasteful.

Furthermore, throwing destructors tend not to work well in practice:

* GCC does not emit call site records for the region containing `__cxa_end_catch`. This has been a long time, since 2000.
* If a catch-all clause catches an exception object that throws, both GCC and Clang using libstdc++ leak the allocated exception object.

To avoid code generation pessimization, add an opt-in driver option
-fassume-nothrow-exception-dtor to assume that `__cxa_end_catch` calls have the
`nounwind` attribute. This implies that thrown exception objects' destructors
will never throw.

To detect misuses, diagnose throw expressions with a potentially-throwing
destructor. Technically, it is possible that a potentially-throwing destructor
never throws when called transitively by `__cxa_end_catch`, but these cases seem
rare enough to justify a relaxed mode.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D108905

show more ...

Revert "[Driver][test] Replace ^//$ with empty string"

This reverts commit 4817b7729a1846b709ec02b98bfe11b0125f8e8f.

It caused some `^/\n` and had some objection about its readability improvement.

[Driver][test] Replace ^//$ with empty string

The convention does not add //\n. Having all RUN/CHECK lines separated by //\n
makes editor movement difficult (e.g. { } in Vim).

[PS5] Disable exceptions by default

Added a test to check that exception flags are not passed by default on PS4.

This patch adds an extra test case to clang-exception-flags.cpp.
No functional change intended.

Patch by Wolfgang Pieb!

Added a test to check that exception flags are not passed by default on PS4.

This patch adds an extra test case to clang-exception-flags.cpp.
No functional change intended.

Patch by Wolfgang Pieb!

Reviewers: filcab, alexr

Differential Revision: http://reviews.llvm.org/D7298

llvm-svn: 228394

show more ...

Force an unknown target for the default case to make it work.

Summary:
This test was failing if LLVM_DEFAULT_TARGET_TRIPLE=x86_64-scei-ps4 due to
its behavior for exceptions.
Force an --unknown trip

Force an unknown target for the default case to make it work.

Summary:
This test was failing if LLVM_DEFAULT_TARGET_TRIPLE=x86_64-scei-ps4 due to
its behavior for exceptions.
Force an --unknown triple for the default case.

Reviewers: echristo, chandlerc

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7422

llvm-svn: 228337

show more ...

Add a test case that would have caught the bug fixed in r126640.

I've only covered C++'s flags here, we should probably do something
similar for ObjC flags.

llvm-svn: 126641