llvm dev - Sep 2012 - [LLVMdev] Bitcasts between pointers with different address spaces

Should LLVM make bitcasts between pointers with different address spaces
illegal?

This will require a small clarification in the documentation and an assertion
check added to the verifier, but I think this would be a good approach.

The reason being is that in different address spaces, pointers are not always
the same size.

This could be limited to make it legal only if the size of the pointer in source
and destination address spaces are equivalent, but that seems like more of a
work-around than a proper solution.

Ideas?
Micah
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Hi,

I don't think we should make bit casts between pointers with different
address spaces illegal.  Address spaces are not required to be disjoint.  In the
N1169 spec, it says
   A non-null pointer into an address space A can be cast to a pointer into
another address space B, but such a cast is undefined if the source pointer does
not point to a location in B.

If the address spaces overlap, one should be able to bticast between them. 

-- Mon Ping


On Sep 7, 2012, at 10:47 AM, Villmow, Micah wrote:
> Should LLVM make bitcasts between pointers with different address spaces
illegal?
>  
> This will require a small clarification in the documentation and an
assertion check added to the verifier, but I think this would be a good
approach.
>  
> The reason being is that in different address spaces, pointers are not
always the same size.
>  
> This could be limited to make it legal only if the size of the pointer in
source and destination address spaces are equivalent, but that seems like more
of a work-around than a proper solution.
>  
> Ideas?
> Micah
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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What about the part about address spaces with pointers of different sizes?

Micah

From: Mon Ping Wang [mailto:monping at apple.com]
Sent: Friday, September 07, 2012 3:19 PM
To: Villmow, Micah
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Bitcasts between pointers with different address spaces

Hi,

I don't think we should make bit casts between pointers with different
address spaces illegal.  Address spaces are not required to be disjoint.  In the
N1169 spec, it says
   A non-null pointer into an address space A can be cast to a pointer into
another address space B, but such a cast is undefined if the source pointer does
not point to a location in B.

If the address spaces overlap, one should be able to bticast between them.

-- Mon Ping


On Sep 7, 2012, at 10:47 AM, Villmow, Micah wrote:


Should LLVM make bitcasts between pointers with different address spaces
illegal?

This will require a small clarification in the documentation and an assertion
check added to the verifier, but I think this would be a good approach.

The reason being is that in different address spaces, pointers are not always
the same size.

This could be limited to make it legal only if the size of the pointer in source
and destination address spaces are equivalent, but that seems like more of a
work-around than a proper solution.

Ideas?
Micah
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LLVMdev at cs.uiuc.edu<mailto:LLVMdev at cs.uiuc.edu>        
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Mon Ping Wang wrote:
> Hi,
>
> I don't think we should make bit casts between pointers with different
> address spaces illegal. Address spaces are not required to be disjoint.
> In the N1169 spec, it says
> A non-null pointer into an address space A can be cast to a pointer into
> another address space B, but such a cast is undefined if the source
> pointer does not point to a location in B.
Use a ptrtoint + inttoptr to implement the cast.
> If the address spaces overlap, one should be able to bticast between them.
I am not okay with having the verifier accept or reject a bitcast based 
on the contents of the TargetData. Given that we intend to permit 
different address spaces to have different sized pointers, we'll need to 
reject all cross address-space bitcasts and casts will need to go 
through ptrtoint+inttoptr which makes the potential for truncation and 
extension of the value explicit.

Is that acceptable? In particular, it still permits you to have 
overlapping address spaces, right?

Nick
> On Sep 7, 2012, at 10:47 AM, Villmow, Micah wrote:
>
>> Should LLVM make bitcasts between pointers with different address
>> spaces illegal?
>> This will require a small clarification in the documentation and an
>> assertion check added to the verifier, but I think this would be a
>> good approach.
>> The reason being is that in different address spaces, pointers are not
>> always the same size.
>> This could be limited to make it legal only if the size of the pointer
>> in source and destination address spaces are equivalent, but that
>> seems like more of a work-around than a proper solution.
>> Ideas?
>> Micah
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu <mailto:LLVMdev at
cs.uiuc.edu>http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev