[cfe-dev] Code generated for pointer pair -> pointer + size -> pointer pair conversion

[Sean Silva]

On Sun, Jun 16, 2013 at 4:59 AM, Stephan Tolksdorf <st at quanttec.com> wrote:

> Hi,
>
> clang emits the following x64 code for `vector.data() + vector.size()`
> (where vector is a std::vector<int32> instance that contains two internal
> pointers that point to the beginning and the end of an array):
>
>   movq  (%rdi), %rcx  // rdi is a pointer to the vector
>   movq  8(%rdi), %rax
>   subq  %rcx, %rax
>   andq  $-4, %rax
>   addq  %rcx, %rax
>
> Is there any way to tell clang in the vector implementation that the array
> is aligned, so that it could reduce this code to a simple load
> `movq 8(%rdi), %rax`?
>
> This kind of optimization would be helpful for inlined code that converts
> back and forth between a pointer pair representation and pointer + size
> representation of an array reference.
>

This is really interesting. It seems we run into a similar problem with:

ArrayRef<int> arrayRefOpt(ArrayRef<int> A) {
  ArrayRef<int> B(A.data(), A.size());
  ArrayRef<int> C(B.begin(), B.end());
  ArrayRef<int> D(C.data(), C.size());
  ArrayRef<int> E(D.begin(), D.end());
  ArrayRef<int> F(E.data(), E.size());
  return D;
}

Which generates:
__Z11arrayRefOptN4llvm8ArrayRefIiEE:
       0:       55                                              pushq   %rbp
       1:       48 89 e5                                        movq
 %rsp, %rbp
       4:       48 8d 14 b5 00 00 00 00                         leaq
 (,%rsi,4), %rdx
       c:       48 c1 fa 02                                     sarq    $2,
%rdx
      10:       48 89 f8                                        movq
 %rdi, %rax
      13:       5d                                              popq    %rbp
      14:       c3                                              ret

Maybe this is bug-report-worthy?

-- Sean Silva
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