[PATCH] D29252: [NewGVN] Update dominator tree for unreachable blocks.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.

The dominator tree has to be updated after all instructions for an
unreachable block have been removed.

This patch should fix all NewGVN test failures with EXPENSIVE_CHECKS.


https://reviews.llvm.org/D29252

Files:
  lib/Transforms/Scalar/NewGVN.cpp
  test/Transforms/NewGVN/2007-07-25-NestedLoop.ll
  test/Transforms/NewGVN/2007-07-30-PredIDom.ll
  test/Transforms/NewGVN/cond_br.ll


Index: test/Transforms/NewGVN/cond_br.ll
===================================================================
--- test/Transforms/NewGVN/cond_br.ll
+++ test/Transforms/NewGVN/cond_br.ll
@@ -1,4 +1,4 @@
-; RUN: opt -basicaa -newgvn -S < %s | FileCheck %s
+; RUN: opt -basicaa -newgvn -verify-dom-info -S < %s | FileCheck %s
 @y = external global i32
 @z = external global i32
 
Index: test/Transforms/NewGVN/2007-07-30-PredIDom.ll
===================================================================
--- test/Transforms/NewGVN/2007-07-30-PredIDom.ll
+++ test/Transforms/NewGVN/2007-07-30-PredIDom.ll
@@ -1,4 +1,4 @@
-; RUN: opt < %s -newgvn | llvm-dis
+; RUN: opt < %s -newgvn -verify-dom-info | llvm-dis
 
 	%"struct.Block::$_16" = type { i32 }
 	%struct.Exp = type { %struct.Exp_*, i32, i32, i32, %struct.Exp*, %struct.Exp*, %"struct.Exp::$_10", %"struct.Block::$_16", %"struct.Exp::$_12" }
Index: test/Transforms/NewGVN/2007-07-25-NestedLoop.ll
===================================================================
--- test/Transforms/NewGVN/2007-07-25-NestedLoop.ll
+++ test/Transforms/NewGVN/2007-07-25-NestedLoop.ll
@@ -1,4 +1,4 @@
-; RUN: opt < %s -newgvn | llvm-dis
+; RUN: opt < %s -newgvn -verify-dom-info | llvm-dis
 
 	%struct.TypHeader = type { i32, %struct.TypHeader**, [3 x i8], i8 }
 
Index: lib/Transforms/Scalar/NewGVN.cpp
===================================================================
--- lib/Transforms/Scalar/NewGVN.cpp
+++ lib/Transforms/Scalar/NewGVN.cpp
@@ -1876,10 +1876,43 @@
   for (auto &BB : make_filter_range(F, UnreachableBlockPred)) {
     DEBUG(dbgs() << "We believe block " << getBlockName(&BB)
                  << " is unreachable\n");
+    std::vector<BasicBlock *> OldSuccessors(succ_begin(&BB), succ_end(&BB));
+
     deleteInstructionsInBlock(&BB);
+
+    // Update immediate dominator of successors of BB, which have only a single
+    // predecessor after removing all instruction from BB.
+    for (auto *Succ : OldSuccessors) {
+      if (BasicBlock *OnlyPred = Succ->getSinglePredecessor()) {
+        if (DT->getNode(OnlyPred))
+          DT->changeImmediateDominator(Succ, OnlyPred);
+      }
+    }
     Changed = true;
   }
 
+  // Erase unreachable blocks from the dominator tree.
+  for (auto &BB : make_filter_range(F, UnreachableBlockPred)) {
+    DomTreeNode *DTN = DT->getNode(&BB);
+    if (!DTN)
+      continue;
+
+    // If BB has a predecessor in the CFG, we have to keep it in the dominator
+    //  tree.
+    if (pred_begin(&BB) != pred_end(&BB))
+      continue;
+
+    SmallVector<DomTreeNode *, 8> Children(DTN->begin(), DTN->end());
+    // Temporarily change the immediate dominator for all nodes immediately
+    // dominated by BB, so we can erase BB from the dominator tree. Those
+    // nodes should be unreachable as well and will be deleted in future
+    // iterations of the outer loop.
+    for (auto *DI : Children)
+      DT->changeImmediateDominator(DI, DT->getRootNode());
+
+    DT->eraseNode(&BB);
+  }
+
   cleanupTables();
   return Changed;
 }


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