[PATCH] D31140: [LLVMbugs] [Bug 18710] Only generate .ARM.exidx and .ARM.extab when needed in EHABI

[Christian Bruel via cfe-commits]

Hi Jon


Le 03/21/2017 à 09:11 PM, Jonathan Roelofs a écrit :
>
>
> On 3/21/17 1:53 PM, Christian Bruel via Phabricator wrote:
>> chrib added a comment.
>>
>> In https://reviews.llvm.org/D31140#706411, @jroelofs wrote:
>>
>>> Can you clarify the logic here? It's my understanding that:
>>>
>>> `-fno-exceptions` does *not* imply `-fno-unwind-tables`
>>>
>>> however:
>>>
>>> `-fno-unwind-tables` *does* imply that exceptions cannot be used on 
>>> targets that require the tables to do unwinding.
>>
>> Yes, (bad things might happen or (std::terminate will be called, or 
>> destructors not called.)...
>>
>> But -f[no]-unwind-tables implies the UWTable attribute, not NoUwind 
>> attribute. To toggle NoUnwind, use -fno-exceptions
>>
>> And this is getting worse with .canunwind which means DoesNotThrow :)
>>
>> in my understanding,  the logic is as follow:
>>
>> Since "An exception cannot propagate through a function with a 
>> nounwind table. The exception handling runtime environment terminates 
>> the program if it encounters a nounwind table during exception 
>> processing." (ARM Information Center)
>>
>> The "nounwind" LLVM attribute, which means "Function does not throw" 
>> translates as the EXIDX_CANTUNWIND value in the exception table index 
>> table which needs to be created for the purpose (for the function)
>
> I think the problem is here, actually. "nounwind" implies "does not 
> throw", but "does not throw" really should not imply "nounwind". This 
> is something that ought to be clarified in the langref with the 
> addition of a "does not throw" attribute. Then the optimizer should be 
> fixed to deduce "does not throw" instead of "nounwind", and we can let 
> "nounwind" continue to imply .cantunwind.

Absolutely, renaming the nounwind attribute into a nothrow would make 
things lot clearer! That would fix the semantic (which is already a lot).

This has been already discussed here and there. e.g for reference this 
thread
http://lists.llvm.org/pipermail/llvm-dev/2014-February/070366.html

without any outcome, because the optimizer cannot deduce, without a 
context information from clang, if the environment supports EH or not.

One could imaging a global (LTO) analysis to find any cxa_throw in the 
IRs, but that would not pass the pre-compiled object barrier or dynamic 
dependencies.

So this context information could be well derived either from the 
language (! C) or the fno-exception flag. Unfortunately none of them is 
known to the optimizer.

The proposal was to make this information  of the form of an option 
(like here) or eventually an additional attribute of the form 
nocantunwind (or noeh, any suggestion welcome). So

attributes #0 = { nounwind nocantunwind}

means for the arm streamer dont' emit the .cantunwind directive required 
by the EHABI if the function cant take exceptions

>
>>
>> And of course without exception runtime environment (the test here) 
>> we don't need this table. So I can see 3 cases:
>>
>> - nounwind set :                    Generate .cantunwind directive 
>> and unwind table
>> - nounwind set but not EH   Do not generate the .cantunwind directive 
>> and do not emit the unwind table
>> - uwtable set                         Need to generate the unwind 
>> table (even without EH)
>>
>> The  disable-arm-cantunwind flag means: without EH support if the 
>> function does not throw, do dot generate the exception tables and the 
>> EXIDX_CANTUNWIND value.
>
> I'm not a big fan of this workaround flag. I'd rather see this fixed 
> by clarifying/fixing the semantics of the IR.
>

yes we can clarify the semantic of the IR. But that will not be enough 
to fix the problem.

nounwind implies emit .cantunwind which implies exception table
nounwind + NOEH implies don't emit the exception table

and we need this NOEH from clang. Now the question is how?

1) a new arm specific flag
2) a target independant no-exception flag
3) a nocantunwind attribute

But I'm not a big fan of adding a new attribute for a arm specific need...

Best Regards

Christian

>
> Jon
>
>>
>>
>> https://reviews.llvm.org/D31140
>>
>>
>>
>