[LLVMdev] C embedded extensions and LLVM

Christopher Lamb christopher.lamb at gmail.com
Wed Nov 21 15:25:12 PST 2007

On Nov 11, 2007, at 9:52 AM, Chris Lattner wrote:

> On Nov 10, 2007, at 11:07 PM, Christopher Lamb wrote:
>> I've been playing around with clang/LLVM looking at adding partial  
>> support for the draft technical report for embedded C extensions  
>> (TR18037, http://www.open-std.org/jtc1/sc22/wg14/www/docs/ 
>> n1169.pdf), specifically named address spaces.
>> Named address spaces need to be tracked in LLVM in essentially all  
>> the same places that alignment is tracked,
> Others addressed the other questions, one (surprising?) thing I'd  
> recommend:
> Unlike alignment and volatility, I think that the address space  
> qualifier should be represented explicitly in the type system.  The  
> reason for this is primarily that pointers to different address  
> spaces are really very different sorts of beasties: for example  
> they can be codegen'd to have different sizes.  Any property that  
> affects how the value is stored in registers needs to be in the  
> type instead of on the load/store instruction.  Also, unlike  
> volatile, it is not common to cast a pointer between two different  
> address spaces.
> The good thing about this is that I think it will make it  
> substantially easier to update the various llvm optimizations if  
> you do this.  The meat of project boils down to adding a new  
> address space qualifier field to PointerType, making sure  
> PointerType takes this field into account when it is being uniqued,  
> and adding the address space qualifier to things like global variable.
> Does this sound reasonable?

I've come across a hitch. Store instructions do not reference the  
pointer type in the .bc format, only the stored type. The .bc reader  
constructs the pointer type from the stored value's type. This means  
that the address space information doesn't come along for the ride.

I see three solutions:

1) Change how stores are written/read in .bc to store the pointer  
type rather than the stored type. This is the most straight forward,  
but I think it also breaks .bc compatibility in a way that's  
impossible to work around. There's no way to differentiate the new  
and old forms.

2) Have an extended record form of stores that carries the address  
space information for the pointer type which then gets restored by  
the reader. This preserves backwards compatibility, but is kind of ugly.

3) Store address space information on all types (not just pointers),  
but it only really affects how pointers are handled. This ensures  
that address spaces go wherever the type goes. This is pretty  
invasive, and I'd like to avoid that overhead if at all possible.

My suggestion would be 2 for now with an intention to change to 1 in  
LLVM 3.0.

Christopher Lamb

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