Use WeakVH instead of WeakTrackingVH in AliasSetTracker's UnkownInsts

[sanjoy]

Summary

In cases where an instruction (a call site, say) is RAUW'ed with some
other value (this is possible via the returned attribute, amongst
other things), we want the slot in UnknownInsts to point to the
original Instruction we wanted to track, not the value it got replaced
by.

Fixes PR32587.

[sanjoy]

ping! (on this and dependencies)

[davide]

test/Analysis/AliasSet/unknown-inst-tracking.ll

1

Can you add a comment to explain what you're testing here? I would also add some more CHECK lines as I'm not a fan of no-crash tests, but up to you.

1–2

I wish there was a way to trigger this with a single pass, but even when I reduced the testcase I wasn't able to find it.

include/llvm/Analysis/AliasSetTracker.h

class AliasSet : public ilist_node<AliasSet> {
   };
 
   // Doubly linked list of nodes.
   PointerRec *PtrList, **PtrListEnd;
   // Forwarding pointer.
   AliasSet *Forward;
 
   /// All instructions without a specific address in this alias set.
-  /// In rare cases this vector can have a null'ed out WeakTrackingVH
+  /// In rare cases this vector can have a null'ed out WeakVH
   /// instances (can happen if some other loop pass deletes an
   /// instruction in this list).
-  std::vector<WeakTrackingVH> UnknownInsts;
+  std::vector<WeakVH> UnknownInsts;
 
   /// Number of nodes pointing to this AliasSet plus the number of AliasSets
   /// forwarding to it.
   unsigned RefCount : 27;
 
   // Signifies that this set should be considered to alias any pointer.
   // Use when the tracker holding this set is saturated.
   unsigned AliasAny : 1;

test/Analysis/AliasSet/unknown-inst-tracking.ll

+; RUN: opt -S -licm -loop-unswitch < %s | FileCheck %s
+
+@global = external global i32
+
+declare i32 @f_1(i8, i32 returned)
+
+define i32 @f_0() {
+; CHECK-LABEL: @f_0(
+bb:
+  br label %bb1
+
+bb1:                                              ; preds = %bb3, %bb
+  %tmp = load i32, i32* @global
+  %tmp2 = select i1 false, i16 1, i16 0
+  br label %bb3
+
+bb3:                                              ; preds = %bb3, %bb1
+  %tmp4 = phi i8 [ 0, %bb1 ], [ %tmp6, %bb3 ]
+  %tmp5 = icmp eq i16 %tmp2, 0
+  %tmp6 = select i1 %tmp5, i8 %tmp4, i8 1
+  %tmp7 = tail call i32 @f_1(i8 %tmp6, i32 1)
+  br i1 false, label %bb1, label %bb3
+}