[cfe-dev] Adding memory spaces to types

Chris Lattner clattner at apple.com
Sat Nov 10 18:43:33 PST 2007


On Nov 10, 2007, at 1:59 AM, 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), in particular: memory spaces.

Nice!

> It's been fairly simple to thread memory space ID's through LLVM so  
> far, but I'm new to FE's and the language from the TR has left me  
> wondering about the best way to implement this in clang. From TR18037:
>
> Clause 6.2.5 - Types, replace the second sentence of paragraph 25  
> with:
>
> Each unqualified type has several qualified versions of its type,38)  
> corresponding to the combinations
> of one, two, or all three of the const, volatile, and restrict  
> qualifiers, and all combinations
> of any subset of these three qualifiers with one address space  
> qualifier.  (Syntactically, an address
> space qualifier is an address space name, so there is an address  
> space qualifier for each visible
> address space name.)
>
> The question I have is, how to track this info without adding memory  
> space id's to QualType, which seems
> (1) infeasible given the implementation of QualType as a smart  
> pointer with only a few bits for additional data, and
> (2) would loose the performance benefit of the current QualType  
> implementation (and thus the whole purpose of QualTypes existence,  
> it seems) if QualType were made extensible.
>
> My first thought was to create a new Type subclass called  
> MemSpacedType that would essentially just be used to store the  
> memory space ID in addition to the QualType of the underlying type.  
> Is this the way to go? I'm deep in new territory and need some  
> seasoned advice.

Yep, I think this is a very reasonable way to go.  QualType itself is  
just an optimization for representing types.  Instead of having  
Type*'s everywhere, and having a "ConstType" type and "RestrictType"  
type (that wrapped some other type), the information is encoded into  
QualType.

However,  this optimization for CVR qualifiers doesn't impact other  
"qualifiers".  It would be very reasonable to have an  
AddressSpaceQualifiedType class, which takes an address space ID and a  
QualType.   This combines the space/time efficiency niceties of  
QualType with the generality of having explicit classes for all of  
these.

-Chris
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