[PATCH] Canonicalize addrspacecast between different pointer types.

[Michele Scandale]

On 11/18/2013 03:05 AM, Matt Arsenault wrote:
>
> On Nov 17, 2013, at 5:12 AM, Michele Scandale <michele.scandale at gmail.com> wrote:
>
>>
>>   I would like to better understand the motivation of this patch: what are the benefit obtained doing a split between the change of address space and the pointer type within the new address space?
>>   Being a bitcast just a reintepretation of the input value (no change in the bits) this behavior has been included in the addrspacecast instruction (nobody reported any objection on this).
>>
>
> The addrspacecast instruction’s purpose is to only change the address space. Changing the type of the pointer is a logically separate operation. If separated out, the bitcast could potentially be combined with other instructions that use the value to simplify things. This is similar to how inttoptr / ptrtoint behave. You can use a different size integer than the pointer size, but instcombine splits it into the 2 logically steps of reinterpreting as an integer, and then extending or truncating to the real pointer size.
>

Originally the /addrspacecast/ was intended to replace every pointer to pointer 
cast when the two have different address spaces: 
http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-August/064674.html

I'm not convinced too much by the potential benefit of the semantic split you 
propose: I can understand the case of inttoptr and ptrtoint, but with the 
addrspacecast we are in a different case.
Exposing a change of pointer type doesn't seem to me useful: if I need to 
reinterpret it to a non pointer type an explicit /ptrtoint/ would be required, 
otherwise an explicit /bitcast/ would be present, but this can be fused in the 
current /addrspacecast/ itself.
Do you have an example that would allow me to understand this?

Probably it would be enough to add the missing optimizations in the InstCombiner.

BTW, if we want to have a strong separation of the change of address space from 
the rest, I would prefer a radically different solution: a restriction of the 
current /addrspacecast/ instruction semantic to allow only pointers or vector of 
pointer with the same base-type.

-Michele

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