[llvm-dev] Why does clang do a memcpy? Is the cast not enough? (ABI function args)

[Krzysztof Parzyszek via llvm-dev]

It is a matter of the calling convention. It would specify what structs 
are passed in registers, and which are passed through stack.

-Krzysztof

On 4/18/2018 12:28 PM, edA-qa mort-ora-y via llvm-dev wrote:
> I understand it's passing by value, that's what I'm testing here. The
> question is why does it copy the data rather than just casting and
> loading values from the original variable (%v) ?  It seems like the
> copying is unnecessary.
> 
> Not all struct's result in the copy, only certain forms -- others are
> just cast directly as I was expecting. I'm just not clear on what the
> differences are, and whether I need to do the same thing.
> 
> 
> On 18/04/18 19:13, Dimitry Andric wrote:
>> On 18 Apr 2018, at 18:40, edA-qa mort-ora-y via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>> I'm implementing function arguments and tested this code in C:
>>>
>>>      // clang -emit-llvm ll_struct_arg.c -S -o /dev/tty
>>>      typedef struct vpt_data {
>>>          char a;
>>>          int b;
>>>          float c;
>>>      } vpt_data;
>>>
>>>      void vpt_test( vpt_data vd ) {
>>>      }
>>>
>>>      int main() {
>>>          vpt_data v;
>>>          vpt_test(v);
>>>      }
>>>
>>> This emits an odd LLVM structure that casts to the desired struct type,
>>> but also memcpy's to a temporary structure. I'm unsure of why the memcpy
>>> is done as opposed to just casting directly?
>> Because you are passing the parameter by value?  It *should* copy the
>> data.  In this particular case it will probably be elided if you turn on
>> optimization, but it is more logical to pass structs via a const
>> reference or pointer.
>>
>> -Dimitry
>>
> 
> 
> 
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