On Jul 19, 2011, at 11:29 PM, Chris Lattner wrote:
> Author: lattner
> Date: Wed Jul 20 01:29:00 2011
> New Revision: 135572
>
> URL: http://llvm.org/viewvc/llvm-project?rev=135572&view=rev
> Log:
> fix rdar://9780211 - Clang crashes with an assertion failure building
> WKView.mm from WebKit
>
> This is something of a hack, the problem is as follows:
>
> 1. we instantiate both copied of RetainPtr with the two different argument
> types
> (an id and protocol-qualified id).
> 2. We refer to the ctor of one of the instantiations when introducing global
> "x",
> this causes us to emit an llvm::Function for a prototype whose "this" has
> type
> "RetainPtr<id<bork> >*".
> 3. We refer to the ctor of the other instantiation when introducing global
> "y",
> however, because it *mangles to the same name as the other ctor* we just use
> a bitcasted version of the llvm::Function we previously emitted.
> 4. We emit deferred declarations, causing us to emit the body of the ctor,
> however
> the body we emit is for RetainPtr<id>, which expects its 'this' to have an
> IR
> type of "RetainPtr<id>*".
>
> Because of the mangling collision, we don't have this case, and explode.
>
> This is really some sort of weird AST invariant violation or something, but
> hey
> a bitcast makes the pain go away.
It's fundamental brokenness in the ObjC++ ABI. Essentially, ObjC considers id
and id<P> to be distinct types, but GCC gave them the same name mangling, which
means that the language's notion of identity differs from the ABI's notion of
identity, and a long string of bugs will inevitably follow. For example, we
used to crash on code like this in Objective-C++:
void f(id) { }
void f(id<P>) { }
because those two function types are distinct, yet they mangle to the same
thing, causing CodeGen to assert. Now we have special-case logic in Sema to
call the second a redefinition of the first, but the only way to actually solve
this problem in the long term is to change ObjC or change the ABI.
- Doug
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