[cfe-dev] Implementing blocks with try-finally semantics

[Reid Kleckner via cfe-dev]

Hi,

>From your example, it's not clear to me what you want this feature to do.
Do you want the finally block to execute:
1. When a C++ exception is thrown?
2. When normal control flow leaves the scope (goto, break, return)?
3. When a hardware trap occurs (div by zero, access violation, segv)?

3 is not implementable with LLVM today. It's not implemented properly even
for __try / __finally.

1 and 2 are simple and can be accomplished with regular C++ destructors.
You can do things like:
struct Cleanup {
  Cleanup(std::function<void()> f) : f(f) {}
  ~Cleanup() { f(); }
  std::function<void()> f;
};
void foo() {
  Cleanup c([&]() {
    .. // any finally code here
  });
  // any try body code here
}

Reid

On Thu, Jun 21, 2018 at 9:26 AM João Paulo Labegalini de Carvalho via
cfe-dev <cfe-dev at lists.llvm.org> wrote:

> Hi,
>
> I am working on a block annotation which should have a try-finally
> semantics. For example, given a code like this:
>
> *__speculate* *{*
>   if (x > 2) {
>     y = 0;
>   }
> *}*
>
> clang should produce something like
>
> __exec_mode = *__begin_spec*();
> if (__exec_mode == SW) {
>   if (__read_barrier(&x) > 2) {
>     __write_barrier(&y, 0);
>   }
>   *__end_spec*();
> } else {
>   if (x > 2) {
>     y = 0;
>   }
>   *__end_spec*();
> }
>
> such that __end_spec() is guaranteed to be called on every exit of the
> __speculate {}  block.
>
> It is my understanding that Clang implements try-finally statements as a
> Microsoft Specific Extension. However, I would not like to have such
> restriction, since such extension is only available for a reduced number of
> targets.
>
> My question is, how should I implement try-finally semantics? Through an
> IR pass (fixing up every exit with a call to __end_spec()) or it is
> possible to extend from CXXTryStmt (reusing some CodeGen code)?
> --
> João Paulo L. de Carvalho
> Computer Science |  IC-UNICAMP | Campinas , SP - Brazil
> jaopaulolc at gmail.com
> joao.carvalho at ic.unicamp.br
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>
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