llvm dev - Aug 2008 - [LLVMdev] Modeling 16-bit pointer registers for an 8-bit target

Pointer size on our target is 16-bits, and we have two 16-bit registers
that can be used only to hold pointers (indirect addresses). 

 

All operations on the target are 8-bit operations, so it takes two 8-bit
loads to load these pointer registers. 

 

We want LLVM to automatically expand all types to 8-bit values.

 

The problem is that LLVM does not expand GlobalAddresses, which are
16-bit values.

This in turn means that you have to specify a 16-bit register class, and
once you specify a 16-bit register class, LLVM does not further expand
16-bit values.

 

Any suggestions on how to model this into LLVM?

 

TIA,

Sanjiv

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On Aug 6, 2008, at 3:54 AM, Sanjiv.Gupta at microchip.com wrote:
> Pointer size on our target is 16-bits, and we have two 16-bit  
> registers that can be used only to hold pointers (indirect addresses).
>
> All operations on the target are 8-bit operations, so it takes two 8- 
> bit loads to load these pointer registers.
>
> We want LLVM to automatically expand all types to 8-bit values.
>
> The problem is that LLVM does not expand GlobalAddresses, which are  
> 16-bit values.
> This in turn means that you have to specify a 16-bit register class,  
> and once you specify a 16-bit register class, LLVM does not further  
> expand 16-bit values.
I don't think there is code in Legalizer to expand GlobalAddress. But  
you can custom lower it. X86 custom lower GlobalAddress nodes for a  
different reason.

Evan
>
> Any suggestions on how to model this into LLVM?
>
> TIA,
> Sanjiv
> _______________________________________________
> 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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Legalize uses two primary predicates, isTypeLegal and isOperationLegal.
I guess your target tells Legalize that i16 as a legal type. The next  
step
is to tell legalize that loading a 16-bit GlobalAddress is not a legal
operation.

If you do that and it still doesn't work, it's a likely indication that
Legalize is making an assumption that isn't valid for your rather
unique architecture, and you should investigate correcting that  
assumption.

Dan

On Aug 6, 2008, at 3:54 AM, Sanjiv.Gupta at microchip.com wrote:
> Pointer size on our target is 16-bits, and we have two 16-bit  
> registers that can be used only to hold pointers (indirect addresses).
>
> All operations on the target are 8-bit operations, so it takes two 8- 
> bit loads to load these pointer registers.
>
> We want LLVM to automatically expand all types to 8-bit values.
>
> The problem is that LLVM does not expand GlobalAddresses, which are  
> 16-bit values.
> This in turn means that you have to specify a 16-bit register class,  
> and once you specify a 16-bit register class, LLVM does not further  
> expand 16-bit values.
>
> Any suggestions on how to model this into LLVM?
>
> TIA,
> Sanjiv
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

> 
> I don't think there is code in Legalizer to expand GlobalAddress. But
you
> can custom lower it. X86 custom lower GlobalAddress nodes for a
different
> reason.
> 
> Evan
> 
Hmmm...That means we have to make i16 as a legal type (since
GlobalAddresses are 16-bits) and custom lower all 16-bit operations to
8-bit operations. I was thinking to take advantage of the already
present ExpandOp infrastructure.

-Sanjiv