[llvm-dev] Aliasing and forwarding optimization

[Hiroshi Yamauchi via llvm-dev]

If you see the LLVM IR output, the second line is lowered into a load/store
of an int. Maybe that's why.

https://godbolt.org/z/u3VyAM

I'm not sure what the strict aliasing rules say about this, though.

HTH

On Thu, Jun 18, 2020 at 10:16 PM Venkataramanan Kumar via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> ----Snip--
> struct st1{
>   int a;
> };
> struct st2{
>   int b;
> };
> struct st {
>   struct st1 obj1;
>   struct st2 obj2;
> }Obj;
>
> int test1(struct st1 * ptr1 , struct st2 * ptr2, struct st2 *ptr3) {
>   ptr1->a = 10;
>   *ptr3 = *ptr2;
>   return ptr1->a;
> }
> --Snip---
>
> For the above case GCC is able to store forward the value 10 to the return
> place.
> LLVM is not doing this.
> GCC
> https://godbolt.org/z/FCjCXy
> LLVM
> https://godbolt.org/z/TFgnig
>
> My understanding is that under strict aliasing rules accessing objects of
> different types don't alias.
> In this case we are accessing  "struct st2" object and "int" type
> object. so aliasing should not prevent the forwarding of store 10 to the
> return place .
>
> Can someone please clarify this?
>
>
> regards,
> Venkat.
>
> :
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