[polly] r219275 - [Fix] Ignore forwarding alias sets in the alias set tracker.

[Johannes Doerfert]

On 10/08, Tobias Grosser wrote:
> On 08/10/2014 04:23, Johannes Doerfert wrote:
> >Author: jdoerfert
> >Date: Tue Oct  7 21:23:48 2014
> >New Revision: 219275
> >
> >URL: http://llvm.org/viewvc/llvm-project?rev=219275&view=rev
> >Log:
> >[Fix] Ignore forwarding alias sets in the alias set tracker.
> >
> >Modified:
> >     polly/trunk/lib/Analysis/ScopInfo.cpp
> >
> >Modified: polly/trunk/lib/Analysis/ScopInfo.cpp
> >URL: http://llvm.org/viewvc/llvm-project/polly/trunk/lib/Analysis/ScopInfo.cpp?rev=219275&r1=219274&r2=219275&view=diff
> >==============================================================================
> >--- polly/trunk/lib/Analysis/ScopInfo.cpp (original)
> >+++ polly/trunk/lib/Analysis/ScopInfo.cpp Tue Oct  7 21:23:48 2014
> >@@ -1294,7 +1294,7 @@ bool Scop::buildAliasGroups(AliasAnalysi
> >
> >    SmallVector<AliasGroupTy, 4> AliasGroups;
> >    for (AliasSet &AS : AST) {
> >-    if (AS.isMustAlias())
> >+    if (AS.isMustAlias() || AS.isForwardingAliasSet())
> >        continue;
> >      AliasGroupTy AG;
> >      for (auto PR : AS)
> 
> This change unbroke the asserts on our build bots. This is nice!
> 
> Could you explain me what forwarding alias sets are and why they are safe to
> ignore? Specifically, why do we know that they will not contain
> any pointers (except the forward reference).
See the link.

> I was quickly browsing the alias set tracker code, but I did not find the
> corresponding documentation.
http://llvm.org/doxygen/classllvm_1_1AliasSet.html#a059eb44c592d8d82ea2a59069bc09ee3

> Also, was there a reason you did not commit a test case? I attached you
> the one I used, in case you did not manage to reduce one.
The test case for this is so unstable, I couldn't reproduce it with an
llvm from yesterday. The forwarding alias sets are also a pure artifact
of the alias set tracker and for the last 11 years we are supposed to
ignore them when iterating over an AST, I just didn't know.
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