[LLVMdev] MemoryDependence and volatile loads

[Philip Reames]

For anyone interested in the topic of this email, I have a patch up for 
review which tries to be more precise w.r.t. volatile loads encountered 
when looking for dependencies of a non-volatile load. See 
http://reviews.llvm.org/D6901

On 01/07/2015 06:57 PM, Philip Reames wrote:
> What are the intended semantics of a volatile load when considered as 
> a possible dependency?
>
> Currently, we have a check for isVolatile with a nice comment about 
> being overly conservative.  However, this code turns out to be 
> completely dead.  :)  Instead, this case is handled by the 
> !isUnordered block above*.  In the case I'm looking at (load PRE in 
> GVN), we fail the queryInst test and return the same result as we 
> would have in the isVolatile block, but there are cases where we allow 
> volatile loads to be bypassed.  (Specifically, when we can tell the 
> queryInst isn't volatile.)
It turns out that the code isn't quite dead.  It's just only used if a) 
your asking a block local dependence question and b) your query 
instruction passes the atomic checks.  In that case, the isVolatile 
check executed and gives you a conservative result.  It is dead if 
called from GVN though.
>
> When I make it so that a volatile load is treated like a normal load, 
> none of the tests fail.  It also results in load PRE bypassing 
> volatile loads which is good for the case I'm looking at, but the lack 
> of any test failures disturbed me a bit.  :)
>
> I *think* it's perfectly legal to consider a volatile load as a normal 
> load *unless* the query instruction we're testing against is itself 
> volatile.  Another way of phrasing this is that volatiles are only 
> ordered with respects to other volatiles.  Do others agree?
>
> * For the sake of forthrightness, I was one of the (several) reviewers 
> on change that added this and none of us noticed the possible issue.
>
> Philip
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev