[cfe-commits] [patch] Qualifiers refactor

[Chris Lattner]

On Sep 23, 2009, at 2:04 AM, John McCall wrote:

> The attached patch refactors how qualifiers are represented in clang.
>
> Currently, QualType uses the bottom three bits of a type pointer to  
> represent the const, volatile, and restrict qualifiers.  Extended  
> qualifiers are represented using an ExtQualType object, which is an  
> ordinary node in the type hierarchy --- except that it isn't.  Most  
> common operations on types have to explicitly check for these  
> special qualifier nodes and handle them differently.
>
> This patch changes QualType to store only two qualifiers directly on  
> the pointer;  the third bit is repurposed to signal whether the  
> pointer is a Type or an ExtQuals object (which holds, but is not, a  
> Type).  This makes it very cheap to decide whether a type carries  
> any qualifiers, as well as to compute the unqualified version of a  
> type.
>
> Because there are only two remaining bits, it's necessary to store  
> 'volatile' on the ExtQuals object, 'const' being ubiquitous and  
> 'restrict' being common in certain system headers.  This could be  
> avoided by using 16-byte alignment on Types, which would be  
> extremely straightforward to do under the patch (modulo any possible  
> required changes to the allocator, which I haven't investigated).
>
> This patch also audits many of the uses of qualifiers in clang,  
> adjusting them to handle arbitrary qualifiers (or be augmentable for  
> future qualifiers) when that was reasonably straightforward.

I haven't had a chance to look at the patch yet, but the approach  
makes a lot of sense to me.

-Chris