[llvm-commits] [llvm] r42051 - in /llvm/trunk: lib/VMCore/Verifier.cpp test/CodeGen/Generic/GC/lower_gcroot.ll test/Verifier/gcread-ptrptr.ll test/Verifier/gcroot-alloca.ll test/Verifier/gcroot-meta.ll test/Verifier/gcroot-ptrptr.ll test/Verifier/gcwrite-ptrptr.ll

[Duncan Sands]

Hi,

> >> +static Value *StripBitCasts(Value *Val) {
> >
> > how about using IntrinsicInst::StripPointerCasts instead?
> 
> Done. Note that, although there's tantalizing GEP-0 handling in  
> StripPointerCasts, it's still not really possible to use gcroot  
> without bitcasts:

the reason for the GEP handling, is that if you have a constant
pointer P to a struct S for which the first field is of type T,
and you constant bitcast P to a T*, then the optimizers will turn
this into a GEP-0 instead of a bitcast.  Thus it is unwise to assume
you'll always see a bitcast - it might be that you get passed a
GEP instead.

> 	%x_addr = alloca %class.Derived*
> 	%tmp0 = getelementptr %x_addr, i32 0, i32 0  ; Invalid! But nice try...

If x_addr pointed to a global variable rather than a local variable, this (or
something like it) should be valid.  Just to be clear: I'm not suggesting that you
can get rid of bitcasts - I'm pointing out that in obscure circumstances you
may be passed a GEP rather than a bitcast, even if you always generate bitcasts
in the front-end.  So you need to handle that, eg by using StripPointerCasts.

Ciao,

Duncan.