[PATCH] D64886: [Use] Preserve original uselist order in Use::swap().

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: efriedma, rnk, dblaikie, reames.
Herald added a subscriber: hiraditya.
Herald added a project: LLVM.
fhahn added a child revision: D64884: [PHINode] Preserve use-list order when removing incoming values..

There is no need to remove and add the swapped values to their
respective use lists. We can just adjust thread the swapped values in
the uselist, preserving the original order.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D64886

Files:
  llvm/include/llvm/IR/Value.h
  llvm/lib/IR/Use.cpp
  llvm/test/Transforms/LoopReroll/nonconst_lb.ll


Index: llvm/test/Transforms/LoopReroll/nonconst_lb.ll
===================================================================
--- llvm/test/Transforms/LoopReroll/nonconst_lb.ll
+++ llvm/test/Transforms/LoopReroll/nonconst_lb.ll
@@ -56,7 +56,7 @@
 ; CHECK:   %4 = add i32 %3, 3
 ; CHECK:   br label %for.body
 
-; CHECK: for.body:                                         ; preds = %for.body, %for.body.preheader
+; CHECK: for.body:                                         ; preds = %for.body.preheader, %for.body
 ; CHECK:   %indvar = phi i32 [ 0, %for.body.preheader ], [ %indvar.next, %for.body ]
 ; CHECK:   %5 = add i32 %m, %indvar
 ; CHECK:   %arrayidx = getelementptr inbounds i32, i32* %B, i32 %5
Index: llvm/lib/IR/Use.cpp
===================================================================
--- llvm/lib/IR/Use.cpp
+++ llvm/lib/IR/Use.cpp
@@ -17,21 +17,39 @@
   if (Val == RHS.Val)
     return;
 
-  if (Val)
-    removeFromList();
+  assert(Val && RHS.Val);
 
   Value *OldVal = Val;
+  Use **OldPrev = Prev.getPointer();
+  Use *OldNext = Next;
   if (RHS.Val) {
-    RHS.removeFromList();
     Val = RHS.Val;
-    Val->addUse(*this);
+    Prev.setPointer(RHS.Prev.getPointer());
+    Next = RHS.Next;
+
+    // Adjust use list pointers to point to this instead of RHS.
+    if (Next)
+      Next->setPrev(&Next);
+    if (Val->UseList == &RHS)
+      Val->UseList = this;
+    else
+      (*Prev.getPointer()) = this;
   } else {
     Val = nullptr;
   }
 
   if (OldVal) {
     RHS.Val = OldVal;
-    RHS.Val->addUse(RHS);
+    RHS.Prev.setPointer(OldPrev);
+    RHS.Next = OldNext;
+
+    // Adjust use list pointers to point to RHS instead of this.
+    if (RHS.Next)
+      RHS.Next->setPrev(&RHS.Next);
+    if (RHS.Val->UseList == this)
+      RHS.Val->UseList = &RHS;
+    else
+      (*RHS.Prev.getPointer()) = &RHS;
   } else {
     RHS.Val = nullptr;
   }
Index: llvm/include/llvm/IR/Value.h
===================================================================
--- llvm/include/llvm/IR/Value.h
+++ llvm/include/llvm/IR/Value.h
@@ -77,6 +77,7 @@
 
   friend class ValueAsMetadata; // Allow access to IsUsedByMD.
   friend class ValueHandleBase;
+  friend class Use;
 
   const unsigned char SubclassID;   // Subclass identifier (for isa/dyn_cast)
   unsigned char HasValueHandle : 1; // Has a ValueHandle pointing to this?


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