[PATCH v4][C][ADA] use function descriptors instead of trampolines in C

[Szabolcs Nagy]

On 17/12/2018 18:22, Uecker, Martin wrote:
> Am Montag, den 17.12.2018, 15:25 +0000 schrieb Szabolcs Nagy:
>> On 16/12/2018 22:45, Uecker, Martin wrote:
>>> Am Sonntag, den 16.12.2018, 09:13 -0700 schrieb Jeff Law:
>>>> Ultimately using function descriptors is an ABI breaking choice and we
>>>> might declare that function descriptors imply higher function
>>>> alignments.  
>>>
>>> Increasing the alignment is not an ABI breaking change.
>>
>> increasing alignment _requirement_ is an abi breaking change.
> 
> You are right. The idea was to increase the minimum alignment
> to always be compatible with code compiled with 
> "-fno-trampolines" but without actually requiring the
> alignment for other code as long as "-fno-trampolines"
> is not given. 
> 
> 
>> and it's not clear who would benefit from the new abi:
>>
>> - it affects everything that does indirect calls (if alignment
>> requirement is increased then in addition everything that has
>> functions whose address may be taken), so it can easily affect
>> existing handwritten asm and it definitely requires the rebuild
>> of the c runtime to support this abi (i think it even requires
>> asm changes there if you allow a thread or makecontext start
>> function to be a nested function).
>>
>> - it makes indirect calls more expensive everywhere, even if
>> nested functions are not used.
> 
> Yes, transition to "-fno-trampolines" by default would be a
> major undertaking and the cost for
> indirect calls might not
> be acceptable. I was not proposing this.
> 
>> i think to fix the executable stack problem in practice, the
>> new nested function mechanism should only require the rebuild
>> of code that actually contains nested functions and thus have
>> no abi or performance impact on code that never uses them.
> 
> My use case is to activate '-fno-trampolines' for some
> project which use nested functions internally. This works
> just fine with existing code because 1) no pointers to nested
> functions escape 2) the default alignment on the existing
> code is high enough.
> 
> This is a practical fix, but only when you are careful and
> activate on a case by case. Of course, it is not a full solution
> to the general problem. 

i see.
i think that's not a common use-case.
i'd expect nested function pointers to often escape
(as callbacks to extern library function calls).

>> i believe this can be achieved by some restrictions on nested
>> function usage in a way that covers most practical use-cases:
>> e.g. only allowing one active parent function call frame per
>> thread, no recursive calls to it, the nested function must be
>> invoked in the same thread as the parent using the same stack,
>> etc. (then the new mechanism can be used safely if nested
>> functions are known to follow the restrictions, the compiler
>> may even emit code to check the constraints at runtime.)
> 
> So a thread_local static variable for storing the static
> chain?

something like that, but the more i think about it the
harder it seems: the call site of the nested function
may not be under control of the nested function writer,
in particular the nested function may be called on a
different thread, and extern library apis are unlikely
to provide guarantees about this, so in general if a
nested function escapes into an extern library then
this cannot be relied on, which limits my original
idea again to cases where there is no escape (which i
think is not that useful).