llvm dev - Jun 2012 - [LLVMdev] mapping a virtual register to a specific address

Does LLVM allow mapping a virtual register to a specific memory location? If
not, how do we extract information on where the virtual registers are mapped to?

Thanks,
Amruth

On Fri, Jun 22, 2012 at 2:46 PM, amruth.rd <amruth.rd at
knights.ucf.edu>wrote:
>
> Does LLVM allow mapping a virtual register to a specific memory location?
> If not, how do we extract information on where the virtual registers are
> mapped to?
>
In general, you do not know where a virtual register is mapped, and the
mapping is dependent on the back-end.  It could be allocated or spilled
onto the stack, or it may live entirely within a physical register (e.g. no
address).

>
> Thanks,
> Amruth
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>


-- 

Thanks,

Justin Holewinski
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On 6/22/12 2:04 PM, Justin Holewinski wrote:
> On Fri, Jun 22, 2012 at 2:46 PM, amruth.rd <amruth.rd at knights.ucf.edu
> <mailto:amruth.rd at knights.ucf.edu>> wrote:
>
>
>     Does LLVM allow mapping a virtual register to a specific memory
>     location? If not, how do we extract information on where the
>     virtual registers are mapped to?
>
>
> In general, you do not know where a virtual register is mapped, and 
> the mapping is dependent on the back-end.  It could be allocated or 
> spilled onto the stack, or it may live entirely within a physical 
> register (e.g. no address).
To elaborate on what Justin said, at the LLVM IR level, you don't know 
where a virtual register will live after code generation (or even if it 
will live in the same "place;" the code generator is free to allocate
it
to a register for part of its life and a stack spill slot for another 
part of its life).

You may be able to get this information if you write a MachineFunction 
pass or some other code that is run by LLVM after code generation and 
register allocation (because you can see the generated code).  However, 
I don't know enough about MachineFunction passes to be sure that it's 
possible; that's just where I'd look if I were you.

-- John T.
>
>     Thanks,
>     Amruth
>
>
>
>     _______________________________________________
>     LLVM Developers mailing list
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>     http://llvm.cs.uiuc.edu
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>
>
>
>
> -- 
>
> Thanks,
>
> Justin Holewinski
>
>
>
> _______________________________________________
> 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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On Fri, Jun 22, 2012 at 3:16 PM, amruth.rd <amruth.rd at
knights.ucf.edu>wrote:
> Thanks Justin. It was really helpful.
>
> Can i use "alloca" instruction to allocate memory at a particular
address
> in the stack? Is it possible to do that? As far as I know, alloca does not
> take any address as an operand.
>
The only control alloca gives you over the address is alignment.    If you
really need an address, you can do as John suggested and write a
MachineFunctionPass that executes towards the end of codegen to see where
in the stack it is allocated.  Also keep in mind that there is not
necessarily a one-to-one mapping from virtual register to physical
register.  An LLVM IR register can hold a complex type that will take
multiple physical registers to hold.

If you want to get the address at run-time, you can also pass the alloca'd
pointer to some user function:

%a = alloca i32
call void @foo(i32* %a)   // Call foo with the stack pointer

>
> Regards,
> Amruth
> ________________________________________
> From: Justin Holewinski [justin.holewinski at gmail.com]
> Sent: Friday, June 22, 2012 3:04 PM
> To: amruth.rd
> Cc: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] mapping a virtual register to a specific address
>
> On Fri, Jun 22, 2012 at 2:46 PM, amruth.rd <amruth.rd at knights.ucf.edu
> <mailto:amruth.rd at knights.ucf.edu>> wrote:
>
> Does LLVM allow mapping a virtual register to a specific memory location?
> If not, how do we extract information on where the virtual registers are
> mapped to?
>
> In general, you do not know where a virtual register is mapped, and the
> mapping is dependent on the back-end.  It could be allocated or spilled
> onto the stack, or it may live entirely within a physical register (e.g. no
> address).
>
>
> Thanks,
> Amruth
>
>
>
> _______________________________________________
> 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
>
>
>
> --
>
> Thanks,
>
> Justin Holewinski
>
>
>

-- 

Thanks,

Justin Holewinski
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