[cfe-dev] RFC: allowing EH constructs, but not generating handlers/cleanups

[Aaron Ballman via cfe-dev]

On Wed, Mar 22, 2017 at 8:45 PM, Hubert Tong via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> On Wed, Mar 22, 2017 at 7:21 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>>
>>
>> On 03/22/2017 06:17 PM, Hubert Tong via cfe-dev wrote:
>>
>> -fno-exceptions causes EH constructs such as try/catch,
>> function-try-blocks, and throw expressions to be diagnosed with a severe
>> error. That is, -f[no-]exceptions controls a language-support property.
>>
>> For experiments in evaluating the cost of (unused) exception handling, and
>> for cases where components are being deployed with the expectation that
>> exceptions will not be thrown, etc. it is useful to allow EH constructs in
>> the source, but to generate code with no handlers or cleanups.
>>
>> I think that it still makes sense under such a mode to allow throw
>> expressions to throw. This is consistent with -fno-exceptions still calling
>> __cxa_bad_cast and __cxa_bad_typeid (which GCC also does, but not with
>> __cxa_throw_bad_array_new_length).
>>
>> That is, the proposed semantics is essentially the same as for
>> -fno-exceptions, except to treat try/catch and function-try-blocks as just
>> normal code introducing no handlers, and to allow throw expressions.
>>
>> -fno-exceptions already suppresses creation of cleanups for unwinding and
>> the ignoring of exception specifications.
>>
>> I like to think of this as -fignore-exceptions. I have a use for this, and
>> I would like to know what people think.
>>
>>
>> To summarize, code compiled with -fignore-exceptions can throw, but can't
>> catch anything. All exceptions just unwind through all such code. Is that
>> right?
>
> That depends on what you mean by "unwind". Exceptions will go past all such
> code without executing destructors.

Will throwing always result in calling std::terminate()? If so, does
that call happen immediately upon executing the throw statement?

Or, if you have one TU compiled with -fignore-exceptions that throws,
and a second TU with -fno-ignore-exceptions that catches the exception
thrown from the first TU, can the exception still be caught?

~Aaron

>
>>
>>
>>  -Hal
>>
>>
>> -- HT
>>
>>
>>
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>>
>> --
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>
>
>
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