[llvm-dev] llvm.memcpy for struct copy

[ma jun via llvm-dev]

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)
  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
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>>
>>
>
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