[cfe-dev] devirtualisation appears to crash clang on covariant functions on ARM

[David Tweed]

Hi,

| Killing all frontend devirtualization of destructors is a bit
over-the-top.

| Note that we shouldn't actually need a conversion here because, while
| destructors do generally return 'this', the correctness of that return
value
| is not guaranteed for *virtual* destructor calls, basically so that we
don't
| have to thunk them just to maintain this optimization.  So I'd like to
know
| what's actually introducing a dependence on the return value.


To recap what's happening: on a platform using the ARM ABI (at least a linux
version) that test is crashing clang due to an assertion in the module
verifier. I don't understand exactly what's happening, but the error message
from the verifier (which can be obtained running the test on x86 with the
-triple armv7l-unknown-linux-gnueabihf)

Function return type does not match operand type of return inst!
  ret %"struct.Test6::D"* %this1
 %"struct.Test6::A"*Broken module found, compilation aborted!

is to do with matching up return instruction types and type signature return
types is seeing a D* (derived class) returned where it wants an A* (parent
class). What I don't know is "which" signature this is: there'd be other
breakage everywhere if the actual D's destructor had a wrong type, so I'm
thinking it's either the signature of the method apparently being called
(A's specific destructor which returns an A*), or some sort of
combined-signature for the virtual function as a whole. Single stepping
through, _at the point the devirtualisation decision is taken_ both
functions have the return type code for void (as they would in "generic
C++") so the devirtualisation decision gets taken.

In terms of what to do: I'm of the opinion that having code that can crash
the compiler is not a good behaviour. However, I lack the expertise to
resolve the underlying problem here properly in the time I've got allocated.
Suppressing devirtualisation of destructors _only when using the ARM CXXABI_
(which I've got a patch to do) at least stops the compiler crashing, and so
would be my preferred intermediate solution. But I'm very open to other
ideas or ways to improve the situation on the ARM platform for this case.

Many thanks,
Dave