[LLVMdev] Exact meaning of byval

[Richard Osborne]

Duncan Sands wrote:
> Hi, byval means that the parameter setup code for performing a call
> copies the struct onto the stack, in an appropriate position relative
> to the other call parameters.
>
>   
>> I'm particularly confused by the "between the caller and the callee" part. The
>> way I see this, the responsibility for the copying should be with either the
>> caller or the callee,
>>     
>
> It is with the caller.  It would be nice to do an explicit memcpy in the caller,
> but the problem is that the value has to end up in a very precise place on the
> stack and there is no way of representing this in the LLVM IR.
>   
>> not somewhere in between. In particular, I think byval
>> could either mean:
>> 	a) The callee is not allowed to modify the argument. If the original code
>> 	modifies the argument, the callee should make a copy and modify that
>> 	instead.
>>     
>
> It is the same as passing anything else (for example an integer) by value:
> the callee can modify the value if it likes, but this doesn't cause any
> changes to the value the callee has.  That's because, just like for an
> integer, a copy is passed to the callee (in a register or on the stack;
> for byval arguments it's always on the stack).
>
>   
>> 	b) The caller will always pass in a copy of the object, which the callee is
>> 	free to modify and will be thrown away afterwards.
>>     
>
> This is how it is done.
>
>   
>> In both cases, it seems that byval argument must always be a valid pointer.
>>     
>
> Correct.  I thought this was stated in the LangRef?
>
>   
>> From the code, I suspect that option b is the case. I would think that option
>> a is the better option, since it can prevent copies when the callee doesn't
>> even modify the value (but this stems from C ABI or something?)
>>     
>
> If the callee doesn't modify the argument, then it might be possible to drop
> the byval attribute (you also have to worry about aliasing etc, but some IPO
> pass could do it).
>
> Ciao,
>
> Duncan.
In that case if you did want the callee to copy the argument am I right 
in thinking that the Frontend would have to pass a pointer and add an 
explicit memcpy in the callee?

Richard