[LLVMdev] PointerTypes with AddressSpace

[Alireza.Moshtaghi at microchip.com]

Thank you Chris and Christopher,
I agree... the Embedded C Language Extensions report provides a good
foundation to build on, and what it proposes as far as Address Space is
probably a super set of what we have in our existing compiler (and
probably would like to keep) so no conflict there. I also agree that
regardless of how we would like to deal with pointers, the same
extensions must be applied to LLVM. 
I think it all boils down to whether you think it is time to incorporate
these extensions into LLVM IR and how long do you think it will take to
do so?

Regards
Ali. 

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
On Behalf Of Chris Lattner
Sent: Wednesday, September 12, 2007 11:07 PM
To: LLVM Developers Mailing List
Subject: Re: [LLVMdev] PointerTypes with AddressSpace


On Sep 12, 2007, at 6:41 PM, <Alireza.Moshtaghi at microchip.com>  
<Alireza.Moshtaghi at microchip.com> wrote:

> Chris,
> Extending LLVM IR to support PointerTypes that take an address  
> space is
> what I was hoping to avoid. However, if we want to do things right,  
> that
> is probably the way to go. Now that we got here, let me write some  
> of my
> thoughts on this and solicit your input:

Okay, I agree that it's the right way to go.  Also, being able to  
eventually the Embedded C specification as Christopher points out  
seems very useful :).

> --- 1) Syntax extension:
> In our existing compiler for 8-bit microcontrollers, we have  
> introduced
> rom and ram qualifiers (with ram being the default one) that can be
> applied to any type for example:
> rom int a;        //integer in program memory
> rom int *a;       //ram pointer to integer in rom
> int * rom a;      //rom pointer to integer in ram
> rom int * rom a;  //rom pointer to integer in rom
> Is something similar to the above what you also envision?

As far as C syntax goes, I have no preference.  I think that  
following Embedded C makes the most sense.

> --- 2) Automatic pointers:
> This is what we don't have in our existing compiler, but many  
> people are
> asking for it. Would it be possible in LLVM to treat pointers as  
> general
> all the way to code generation, and then decide its Address Space  
> based
> on the following criteria? (we should be able to do so in an LLVM pass
> because at code generation time we have the full view of the program)
> -- a) Address Space of the pointer is the Address Space of the  
> variable 
> eg: ptr = &var; //AddSp of ptr becomes AddSp of var
> -- b) Address Space of the pointer is the address Space of the pointer
> eg: ptr1 = ptr2; //AddSp of ptr1 becomes AddSp of ptr2
> -- c) Conflicts inside functions are not resolvable and should  
> generate
> diagnostic.
> eg:
> void f(void){
>     generalPtr = romPtr;
>     //some code
>     generalPtr = ramPtr; // non resolvable conflict
> }

This basically amounts to type inference.  If you want this, it would  
have to be implemented in the front-end, not in at the LLVM level  
(you lose too much to give useful error reports etc).

Type inference is very nice, but it is not in the spirit of C at  
all.  C is very explicit (to a fault perhaps).

> -- d) Conflicts at the function interface will spawn a new function
> eg:
> void inc(int *a){
>     (*a)++;
> }
> void g(void){
>     inc(romPointer); // this will spawn an f with rom pointer
>     inc(ramPointer); // this will spawn an f with ram pointer
> }
>
> In the case of (2) we still need rom and ram qualifiers to declare
> variables in the intended Address Space, however the impact on the  
> front
> end will probably be reduced.
> A combination of (1) and (2) would probably be ideal.

This again is a front-end issue.  It sounds like you want generic  
functions ala C++ templates.  If you go down this path, you are  
basically designing your own c-like language, you're not doing a  
simple C extension (which is what Embedded C is).

Regardless of whether you choose to make your own language or use  
Embedded C, the LLVM support should be the same though.

-Chris

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