Fix AliasSetTracker bug

[Prazek]

Summary

AliasSetTracker was crashing when the UnknownInstruction
was reduced to Value (llvm.invariant.group.barrier(null) to null)

[sanjoy]

Hi Piotr,

I've been looking at this bug too. Unfortunately changing the cast to dyn_cast_or_null is not sufficient -- once the ValueHandle has been RAUW'ed to something else, you've lost the instruction, and so you have an incomplete list of UnknownInsts.

[Prazek]

In D32654#741104, @sanjoy wrote:

Hi Piotr,

I've been looking at this bug too. Unfortunately changing the cast to dyn_cast_or_null is not sufficient -- once the ValueHandle has been RAUW'ed to something else, you've lost the instruction, and so you have an incomplete list of UnknownInsts.

I haven't got that deep into that. Are you working on it right now, or do you have some ideas how to fix it?

[sanjoy]

Yes, we need a VH that does not track RAUW but nulls itself out on deletion. I had a fix that I had to revert and I'm currently working on figuring out a way to reland it.

[Prazek]

In D32654#741128, @sanjoy wrote:

Yes, we need a VH that does not track RAUW but nulls itself out on deletion. I had a fix that I had to revert and I'm currently working on figuring out a way to reland it.

Awesome, I will wait with the ConstantFolder patch that breaks then

[Prazek]

If you will have fix, please add this patch as dependency https://reviews.llvm.org/D32423

include/llvm/Analysis/AliasSetTracker.h

class AliasSet : public ilist_node<AliasSet> {
   void dropRef(AliasSetTracker &AST) {
     assert(RefCount >= 1 && "Invalid reference count detected!");
     if (--RefCount == 0)
       removeFromTracker(AST);
   }
 
   Instruction *getUnknownInst(unsigned i) const {
     assert(i < UnknownInsts.size());
-    return cast_or_null<Instruction>(UnknownInsts[i]);
+    return dyn_cast_or_null<Instruction>(UnknownInsts[i]);
   }
 
 public:
   /// Accessors...
   bool isRef() const { return Access & RefAccess; }
   bool isMod() const { return Access & ModAccess; }
   bool isMustAlias() const { return Alias == SetMustAlias; }
   bool isMayAlias()  const { return Alias == SetMayAlias; }

test/Transforms/LICM/2017-28-04-invariant.group.ll

+; RUN: opt -licm < %s
+
+; This test check if nothing explodes if invariant.group.barrier of null is
+; reduced to null.
+; Function Attrs: nounwind uwtable
+define fastcc void @_ZN12_GLOBAL__N_117GlobalISelEmitter3runERN4llvm11raw_ostreamE() unnamed_addr align 2 {
+entry:
+  switch i8 undef, label %if.end8.i.i [
+    i8 0, label %if.then.i.i3
+    i8 1, label %if.then4.i.i
+  ]
+
+if.then.i.i3:                                     ; preds = %entry
+  br label %for.body
+
+if.then4.i.i:                                     ; preds = %entry
+  unreachable
+
+if.end8.i.i:                                      ; preds = %entry
+  unreachable
+
+for.body:                                         ; preds = %for.cond66.i.i.for.end151.i.i_crit_edge, %if.then.i.i3
+  br label %for.body69.i.i
+
+for.body69.i.i:                                   ; preds = %for.body69.i.i, %for.body
+  %0 = call i8* @llvm.invariant.group.barrier(i8* null) #2, !noalias !1
+  br i1 undef, label %for.cond66.i.i.for.end151.i.i_crit_edge, label %for.body69.i.i
+
+for.cond66.i.i.for.end151.i.i_crit_edge:          ; preds = %for.body69.i.i
+  br label %for.body
+}
+
+; Function Attrs: inaccessiblememonly nounwind
+declare i8* @llvm.invariant.group.barrier(i8*) #1
+
+attributes #1 = { inaccessiblememonly nounwind }
+attributes #2 = { nounwind }
+
+!1 = !{!2}
+!2 = distinct !{!2, !3, !"_ZN4llvm5Error11takePayloadEv: %agg.result"}
+!3 = distinct !{!3, !"_ZN4llvm5Error11takePayloadEv"}