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

[Krzysztof Parzyszek via llvm-dev]

This is the standard way of copying memory in the IR. Backends can 
expand the memcpy into loads/stores if they want.

-Krzysztof

On 4/18/2018 12:38 PM, edA-qa mort-ora-y via llvm-dev wrote:
> Yes, I understand that as well (it's what I'm trying to recreate in my
> language now).
> 
> I'm really wondering why it does the copy, since from what I can tell it
> could just as easily cast the original value and do the load without the
> memcpy operation.
> 
> That is, the question is about the memcpy and extra alloca -- I
> understand what it's doing, just not why it's doing it this way.
> 
> 
> On 18/04/18 19:33, Krzysztof Parzyszek via llvm-dev wrote:
>> 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
>>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
> 

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, 
hosted by The Linux Foundation