[llvm-dev] llvm.memcpy for struct copy

[Jakub (Kuba) Kuderski via llvm-dev]

Hi Ma,

how can I  transform the llvm.memcpy into data move loop IR and eliminate
> the bitcast instruction ?
>

I'm not sure why you are concerned about memcpy and bitcasts, but if you
call MCpyInst->getSource() and MCpyInst->getDest() it will look through
casts  and give you the 'true' source/destination.

If you want to get rid of memcpy altogether, you can take a look at this
pass:
https://github.com/seahorn/seahorn/blob/master/lib/Transforms/Scalar/PromoteMemcpy.cc
.

Best,
Kuba

On Tue, Jan 30, 2018 at 3:22 AM, ma jun via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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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>


-- 
Jakub Kuderski
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