Reapply: [ADCE][Dominators] Teach ADCE to preserve dominators

Summary:
This patch teaches ADCE to preserve both DominatorTrees and PostDominatorTrees.

I didn't notice any performance impact when bootstrapping clang with this patch.

The patch was originally committed in r311039 and reverted in r311049.
This revision fixes the problem with not adding a dependency on the
DominatorTreeWrapperPass for the LegacyPassManager.

Reviewers: dberlin, chandlerc, sanjoy, davide, grosser, brzycki

Reviewed By: davide

Subscribers: grandinj, zhendongsu, llvm-commits, david2050

Differential Revision: https://reviews.llvm.org/D35869

llvm-svn: 311057

include/llvm/Support/GenericDomTreeConstruction.h

@@ -914,7 +914,12 @@ struct SemiNCAInfo {
     if (!FromTN) return;
 
     const TreeNodePtr ToTN = DT.getNode(To);
-    assert(ToTN && "To already unreachable -- there is no edge to delete");
+    if (!ToTN) {
+      DEBUG(dbgs() << "\tTo (" << BlockNamePrinter(To)
+                   << ") already unreachable -- there is no edge to delete\n");
+      return;
+    }
+
     const NodePtr NCDBlock = DT.findNearestCommonDominator(From, To);
     const TreeNodePtr NCD = DT.getNode(NCDBlock);
 

lib/Transforms/Scalar/ADCE.cpp

@@ -27,6 +27,7 @@
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/DebugInfoMetadata.h"
+#include "llvm/IR/Dominators.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/InstIterator.h"
 #include "llvm/IR/Instructions.h"
@@ -89,6 +90,10 @@ struct BlockInfoType {
 
 class AggressiveDeadCodeElimination {
   Function &F;
+
+  // ADCE does not use DominatorTree per se, but it updates it to preserve the
+  // analysis.
+  DominatorTree &DT;
   PostDominatorTree &PDT;
 
   /// Mapping of blocks to associated information, an element in BlockInfoVec.
@@ -157,9 +162,10 @@ class AggressiveDeadCodeElimination {
   void makeUnconditional(BasicBlock *BB, BasicBlock *Target);
 
 public:
-  AggressiveDeadCodeElimination(Function &F, PostDominatorTree &PDT)
-      : F(F), PDT(PDT) {}
-  bool performDeadCodeElimination();
+ AggressiveDeadCodeElimination(Function &F, DominatorTree &DT,
+                               PostDominatorTree &PDT)
+     : F(F), DT(DT), PDT(PDT) {}
+ bool performDeadCodeElimination();
 };
 }
 
@@ -557,14 +563,31 @@ void AggressiveDeadCodeElimination::updateDeadRegions() {
     }
     assert((PreferredSucc && PreferredSucc->PostOrder > 0) &&
            "Failed to find safe successor for dead branch");
+
+    // Collect removed successors to update the (Post)DominatorTrees.
+    SmallPtrSet<BasicBlock *, 4> RemovedSuccessors;
     bool First = true;
     for (auto *Succ : successors(BB)) {
-      if (!First || Succ != PreferredSucc->BB)
+      if (!First || Succ != PreferredSucc->BB) {
         Succ->removePredecessor(BB);
-      else
+        RemovedSuccessors.insert(Succ);
+      } else
         First = false;
     }
     makeUnconditional(BB, PreferredSucc->BB);
+
+    // Inform the dominators about the deleted CFG edges.
+    for (auto *Succ : RemovedSuccessors) {
+      // It might have happened that the same successor appeared multiple times
+      // and the CFG edge wasn't really removed.
+      if (Succ != PreferredSucc->BB) {
+        DEBUG(dbgs() << "ADCE: Removing (Post)DomTree edge " << BB->getName()
+                     << " -> " << Succ->getName() << "\n");
+        DT.deleteEdge(BB, Succ);
+        PDT.deleteEdge(BB, Succ);
+      }
+    }
+
     NumBranchesRemoved += 1;
   }
 }
@@ -609,6 +632,9 @@ void AggressiveDeadCodeElimination::makeUnconditional(BasicBlock *BB,
   InstInfo[NewTerm].Live = true;
   if (const DILocation *DL = PredTerm->getDebugLoc())
     NewTerm->setDebugLoc(DL);
+
+  InstInfo.erase(PredTerm);
+  PredTerm->eraseFromParent();
 }
 
 //===----------------------------------------------------------------------===//
@@ -617,13 +643,16 @@ void AggressiveDeadCodeElimination::makeUnconditional(BasicBlock *BB,
 //
 //===----------------------------------------------------------------------===//
 PreservedAnalyses ADCEPass::run(Function &F, FunctionAnalysisManager &FAM) {
+  auto &DT = FAM.getResult<DominatorTreeAnalysis>(F);
   auto &PDT = FAM.getResult<PostDominatorTreeAnalysis>(F);
-  if (!AggressiveDeadCodeElimination(F, PDT).performDeadCodeElimination())
+  if (!AggressiveDeadCodeElimination(F, DT, PDT).performDeadCodeElimination())
     return PreservedAnalyses::all();
 
   PreservedAnalyses PA;
   PA.preserveSet<CFGAnalyses>();
   PA.preserve<GlobalsAA>();
+  PA.preserve<DominatorTreeAnalysis>();
+  PA.preserve<PostDominatorTreeAnalysis>();
   return PA;
 }
 
@@ -637,14 +666,23 @@ struct ADCELegacyPass : public FunctionPass {
   bool runOnFunction(Function &F) override {
     if (skipFunction(F))
       return false;
+
+    auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
     auto &PDT = getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
-    return AggressiveDeadCodeElimination(F, PDT).performDeadCodeElimination();
+    return AggressiveDeadCodeElimination(F, DT, PDT)
+        .performDeadCodeElimination();
   }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
+    // We require DominatorTree here only to update and thus preserve it.
+    AU.addRequired<DominatorTreeWrapperPass>();
     AU.addRequired<PostDominatorTreeWrapperPass>();
     if (!RemoveControlFlowFlag)
       AU.setPreservesCFG();
+    else {
+      AU.addPreserved<DominatorTreeWrapperPass>();
+      AU.addPreserved<PostDominatorTreeWrapperPass>();
+    }
     AU.addPreserved<GlobalsAAWrapperPass>();
   }
 };
@@ -653,6 +691,7 @@ struct ADCELegacyPass : public FunctionPass {
 char ADCELegacyPass::ID = 0;
 INITIALIZE_PASS_BEGIN(ADCELegacyPass, "adce",
                       "Aggressive Dead Code Elimination", false, false)
+INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
 INITIALIZE_PASS_DEPENDENCY(PostDominatorTreeWrapperPass)
 INITIALIZE_PASS_END(ADCELegacyPass, "adce", "Aggressive Dead Code Elimination",
                     false, false)

test/Transforms/ADCE/domtree-DoubleDeletion.ll

@@ -0,0 +1,39 @@
+; RUN: opt < %s -gvn -simplifycfg -adce | llvm-dis
+; RUN: opt < %s -gvn -simplifycfg -adce -verify-dom-info | llvm-dis
+
+; This test makes sure that the DominatorTree properly handles
+; deletion of edges that go to forward-unreachable regions.
+; In this case, %land.end is already forward unreachable when
+; the DT gets informed about the deletion of %entry -> %land.end.
+
+@a = common global i32 0, align 4
+
+define i32 @main() {
+entry:
+  %retval = alloca i32, align 4
+  store i32 0, i32* %retval, align 4
+  %0 = load i32, i32* @a, align 4
+  %cmp = icmp ne i32 %0, 1
+  br i1 %cmp, label %land.rhs, label %land.end4
+
+land.rhs:                                         ; preds = %entry
+  %1 = load i32, i32* @a, align 4
+  %tobool = icmp ne i32 %1, 0
+  br i1 %tobool, label %land.rhs1, label %land.end
+
+land.rhs1:                                        ; preds = %land.rhs
+  br label %land.end
+
+land.end:                                         ; preds = %land.rhs1, %land.rhs
+  %2 = phi i1 [ false, %land.rhs ], [ true, %land.rhs1 ]
+  %land.ext = zext i1 %2 to i32
+  %conv = trunc i32 %land.ext to i16
+  %conv2 = sext i16 %conv to i32
+  %tobool3 = icmp ne i32 %conv2, 0
+  br label %land.end4
+
+land.end4:                                        ; preds = %land.end, %entry
+  %3 = phi i1 [ false, %entry ], [ %tobool3, %land.end ]
+  %land.ext5 = zext i1 %3 to i32
+  ret i32 0
+}

test/Transforms/ADCE/unreachable.ll

@@ -0,0 +1,18 @@
+; RUN: opt < %s -adce -simplifycfg | llvm-dis
+; RUN: opt < %s -passes=adce | llvm-dis
+
+define i32 @Test(i32 %A, i32 %B) {
+BB1:
+        br label %BB4
+
+BB2:            ; No predecessors!
+        br label %BB3
+
+BB3:            ; preds = %BB4, %BB2
+        %ret = phi i32 [ %X, %BB4 ], [ %B, %BB2 ]               ; <i32> [#uses=1]
+        ret i32 %ret
+
+BB4:            ; preds = %BB1
+        %X = phi i32 [ %A, %BB1 ]               ; <i32> [#uses=1]
+        br label %BB3
+}