[llvm-dev] llvm.memcpy for struct copy

[ma jun via llvm-dev]

Hi


2018-01-30 15:36 GMT+08:00 ma jun <jun.parser at gmail.com>:

> Hi
>     Thanks !
>     so  for  this example
> void foo(X &src, X &dst) {
>   dst = src;
> }
> and the IR:
>
> define void @foo(X&, X&)(%struct.X* dereferenceable(8), %struct.X*
> dereferenceable(8)) #0 {
>   %3 = alloca %struct.X*, align 8
>   %4 = alloca %struct.X*, align 8
>   store %struct.X* %0, %struct.X** %3, align 8
>   store %struct.X* %1, %struct.X** %4, align 8
>   %5 = load %struct.X*, %struct.X** %3, align 8
>   %6 = load %struct.X*, %struct.X** %4, align 8
>   %7 = bitcast %struct.X* %6 to i8*
>   %8 = bitcast %struct.X* %5 to i8*
>   call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %7, i8* align 4 %8, i64
> 8, i1 false)
>

also  since the dst and src are 4 byte align , can we use the IR below:

%7 = bitcast %struct.X* %6 to i32*

%8 = bitcast %struct.X* %5 to i32*

call void @llvm.memcpy.p0i32.p0i32.i64(i32* align 4 %7, i32* align 4 %8, i64
 8, i1 false)


>   ret void
> }
>
> how can I  transform the llvm.memcpy into data move loop IR and eliminate
> the bitcast instruction ?
>
> Regards
> Jun
>
>
> 2018-01-30 15:24 GMT+08:00 Craig Topper <craig.topper at gmail.com>:
>
>> The i8 type in the pointers doesn't matter a whole lot. There's a long
>> term plan to remove the type from all pointers in llvm IR.
>>
>> Yes, clang will use memcpy for struct copies. You can see example IR here
>> https://godbolt.org/g/8gQ18m. You'll see that the struct pointers are
>> bitcasted to i8* before the call.
>>
>> ~Craig
>>
>> On Mon, Jan 29, 2018 at 11:12 PM, ma jun via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>>
>>> Hi all
>>>      I'm new here, and I have some question about llvm.memcpy intrinsic.
>>>      why does llvm.memcpy intrinsic only support i8*  for first two
>>> arguments?  and does clang will also transform struct copy into llvm.memcpy
>>> ? what format does IR looks like?
>>>      Thanks !
>>>
>>>      Regards
>>>      Jun
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
>
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