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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