[llvm-dev] llvm.memcpy for struct copy

[ma jun via llvm-dev]

Hi Craig
    Thank you very much ！

2018-01-30 16:11 GMT+08:00 Craig Topper <craig.topper at gmail.com>:

> The pointers must always be i8* the alignment is independent and is
> controlled by the attributes on the arguments in the call to memcpy.
>
> ~Craig
>
> On Mon, Jan 29, 2018 at 11:45 PM, ma jun <jun.parser at gmail.com> wrote:
>
>> 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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