[llvm-dev] Replacing ptrtoint+inttoptr with addrspacecast

[Manuel Jacob via llvm-dev]

People have argued that eliminating a ptr -> int -> ptr cast sequence is 
wrong even if all pointers are in the same address space. Therefore the 
optimization currently performed by the optimizer is unsafe. See 
https://www.youtube.com/watch?v=CPokKMqAVdY

Additionally there might be additional reasons why replacing ptr -> int 
-> ptr cast sequence with addrspacecast is wrong. It depends on what the 
exact semantics of the addrspacecast is.

On 2019-07-15 20:34, Cashman, David via llvm-dev wrote:
> Given a ptrtoint followed by inttoptr:
> 
> %tmp = ptrtoint i16 addrspace(4)* %a to i32
> %b = inttoptr i32 %tmp to i8*
> 
> InstCombine will normally collapse them to a bitcast (or no-op, if the
> src and dst pointer types are the same).  But if the pointers are in
> different address spaces, as shown above, it won't, due to the
> following code in Instructions.cpp:
> 
>       // Cannot simplify if address spaces are different!
>       if (SrcTy->getPointerAddressSpace() != 
> DstTy->getPointerAddressSpace())
>         return 0;
> 
> Is this correct?  Given that the instructions above are effectively
> casting the pointer between two address spaces, can/should InstCombine
> replace the code above with an addrspacecast?  Is there something in
> the LLVM language spec that makes it illegal or undesirable?
> 
> Thanks,
> 
> David
> 
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