[LLVMdev] recognizing DTORs and vptr updates in LLVM.

[Chris Lattner]

On Mar 20, 2012, at 12:51 AM, Duncan Sands wrote:

>>> Using instruction level metadata for this would be appropriate.  However, I
>>> also don't understand why a race on this is truly benign.
>> 
>> It isn't, really; calling it "benign" is deceptive.  It's just that
>> storing a pointer which is equal to the existing pointer stored at a
>> given address almost always makes the optimizer/codegen generate code
>> which can't trigger the race in a way which visibly misbehaves.
>> Therefore, as a heuristic users apparently want ThreadSanitizer to
>> ignore (or list separately) such races.
> 
> The gcc Ada front-end does this too, in quite a range of situations.  For
> example multiple threads racily initialize a pointer variable, but they all
> initialize to the same value.  The various valgrind based race detection
> tools all complain about this, which makes them much less useful than they
> might be for Ada.

FWIW, after thinking about this for awhile, I realize that we already have the tools to handle this: TBAA.  

It would be general goodness for clang to emit VTable loads and stores in their with their own TBAA type class (one that does not even alias "char*").  This would give us improved code quality, is straight-forward to reason about, is not "another knob" and would be a really easy for ASAN to use.

One issue is that TBAA is disabled in -O0 builds: I'd just make vtable TBAA information be produced when the optimizer is enabled or if ASAN is enabled.

-Chris