[PATCH] D42114: Teach ConstantFoldTerminator to preserve DomTree

[Matt Arsenault via Phabricator via llvm-commits]

arsenm created this revision.
arsenm added a reviewer: kuhar.
Herald added a subscriber: wdng.

The pass I'm working on only cares about the BranchInst
handling, which I know works. I don't currently have a way
to test the switch handling, so alternatively I could
just assert there is no DomTree to update for now.


https://reviews.llvm.org/D42114

Files:
  include/llvm/Transforms/Utils/Local.h
  lib/Transforms/Utils/Local.cpp


Index: lib/Transforms/Utils/Local.cpp
===================================================================
--- lib/Transforms/Utils/Local.cpp
+++ lib/Transforms/Utils/Local.cpp
@@ -100,7 +100,8 @@
 /// conditions and indirectbr addresses this might make dead if
 /// DeleteDeadConditions is true.
 bool llvm::ConstantFoldTerminator(BasicBlock *BB, bool DeleteDeadConditions,
-                                  const TargetLibraryInfo *TLI) {
+                                  const TargetLibraryInfo *TLI,
+                                  DominatorTree *DT) {
   TerminatorInst *T = BB->getTerminator();
   IRBuilder<> Builder(T);
 
@@ -123,6 +124,9 @@
       // Replace the conditional branch with an unconditional one.
       Builder.CreateBr(Destination);
       BI->eraseFromParent();
+
+      if (DT)
+        DT->deleteEdge(BB, OldDest);
       return true;
     }
 
@@ -159,6 +163,8 @@
       TheOnlyDest = SI->case_begin()->getCaseSuccessor();
     }
 
+    SmallVector<DominatorTree::UpdateType, 8> Updates;
+
     // Figure out which case it goes to.
     for (auto i = SI->case_begin(), e = SI->case_end(); i != e;) {
       // Found case matching a constant operand?
@@ -194,6 +200,12 @@
         }
         // Remove this entry.
         DefaultDest->removePredecessor(SI->getParent());
+
+        if (DT) {
+          Updates.push_back({DominatorTree::Delete, SI->getParent(),
+                DefaultDest});
+        }
+
         i = SI->removeCase(i);
         e = SI->case_end();
         continue;
@@ -222,18 +234,26 @@
       Builder.CreateBr(TheOnlyDest);
       BasicBlock *BB = SI->getParent();
 
+
       // Remove entries from PHI nodes which we no longer branch to...
       for (BasicBlock *Succ : SI->successors()) {
         // Found case matching a constant operand?
         if (Succ == TheOnlyDest)
           TheOnlyDest = nullptr; // Don't modify the first branch to TheOnlyDest
-        else
+        else {
           Succ->removePredecessor(BB);
+
+          if (DT)
+            Updates.push_back({DominatorTree::Delete, BB, Succ});
+        }
       }
 
       // Delete the old switch.
       Value *Cond = SI->getCondition();
       SI->eraseFromParent();
+
+      if (DT)
+        DT->applyUpdates(Updates);
       if (DeleteDeadConditions)
         RecursivelyDeleteTriviallyDeadInstructions(Cond, TLI);
       return true;
Index: include/llvm/Transforms/Utils/Local.h
===================================================================
--- include/llvm/Transforms/Utils/Local.h
+++ include/llvm/Transforms/Utils/Local.h
@@ -117,7 +117,8 @@
 /// conditions and indirectbr addresses this might make dead if
 /// DeleteDeadConditions is true.
 bool ConstantFoldTerminator(BasicBlock *BB, bool DeleteDeadConditions = false,
-                            const TargetLibraryInfo *TLI = nullptr);
+                            const TargetLibraryInfo *TLI = nullptr,
+                            DominatorTree *DT = nullptr);
 
 //===----------------------------------------------------------------------===//
 //  Local dead code elimination.


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