llvm dev - Dec 2009 - [LLVMdev] Problem with code generated for call using stdcall convention

I too have stumbled over this.

Wouldn't it be a good idea to add a check for this to the function 
verifier pass?


Hans

Anton Korobeynikov wrote:
> Hello
> 
>> I naively though it would pick up the call convention from the
declaration
>> as there was no error like we have when the wrong number of parameters
is
>> used.
> This is not an error, you're just invoking undefined behaviour.
>

Hi,
> Wouldn't it be a good idea to add a check for this to the function 
> verifier pass?
no, because it is valid.  The problem is that the optimizers can expose
this kind of thing.  For example, imagine you have an indirect call, where
the calling convention is CC_X.  Suppose the optimizers are able to deduce
the function that is being called, say F, and turn the call into a direct
call to F.  If the calling convention for F is different, say CC_Y, then
having the verifier reject the bitcode because CC_X != CC_Y would result
in a compiler crash.  So you would have to teach all kind of optimizers
to be careful because they might turn a valid program into an invalid one.
That's too painful.

Ciao,

Duncan.

Hans Wennborg wrote:
> I too have stumbled over this.
>
> Wouldn't it be a good idea to add a check for this to the function
> verifier pass?
No. This is a FAQ: http://llvm.org/docs/FAQ.html#callconvwrong

Nick
>
>
> Hans
>
> Anton Korobeynikov wrote:
>> Hello
>>
>>> I naively though it would pick up the call convention from the
declaration
>>> as there was no error like we have when the wrong number of
parameters is
>>> used.
>> This is not an error, you're just invoking undefined behaviour.
>>
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>

On Thu, Dec 10, 2009 at 12:21 PM, Nick Lewycky <nicholas at mxc.ca>
wrote:
> Hans Wennborg wrote:
>> I too have stumbled over this.
>>
>> Wouldn't it be a good idea to add a check for this to the function
>> verifier pass?
>
> No. This is a FAQ: http://llvm.org/docs/FAQ.html#callconvwrong
IMO you could move the cc to the type and still replace mismatched
calls with unreachable.  Are there other design considerations for not
doing this?

Reid