[llvm-dev] llvm.memcpy for struct copy

[Craig Topper via llvm-dev]

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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