Return "[LoopSimplifyCFG] Delete dead in-loop blocks"

The underlying bug that caused the revert should be fixed by rL348567.

Differential Revision: https://reviews.llvm.org/D54023

2 files changed, 58 insertions, 114 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp b/llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
index c370efa1207..383a3e64d3e 100644
--- a/llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
@@ -46,6 +46,8 @@ static cl::opt<bool> EnableTermFolding("enable-loop-simplifycfg-term-folding",
 
 STATISTIC(NumTerminatorsFolded,
           "Number of terminators folded to unconditional branches");
+STATISTIC(NumLoopBlocksDeleted,
+          "Number of loop blocks deleted");
 
 /// If \p BB is a switch or a conditional branch, but only one of its successors
 /// can be reached from this block in runtime, return this successor. Otherwise,
@@ -269,6 +271,27 @@ private:
            "All blocks that stay in loop should be live!");
   }
 
+  /// Delete loop blocks that have become unreachable after folding. Make all
+  /// relevant updates to DT and LI.
+  void deleteDeadLoopBlocks() {
+    DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
+    if (MSSAU)
+      MSSAU->removeBlocks(DeadLoopBlocks);
+    for (auto *BB : DeadLoopBlocks) {
+      assert(BB != L.getHeader() &&
+             "Header of the current loop cannot be dead!");
+      LLVM_DEBUG(dbgs() << "Deleting dead loop block " << BB->getName()
+                        << "\n");
+      if (LI.isLoopHeader(BB)) {
+        assert(LI.getLoopFor(BB) != &L && "Attempt to remove current loop!");
+        LI.erase(LI.getLoopFor(BB));
+      }
+      LI.removeBlock(BB);
+      DeleteDeadBlock(BB, &DTU);
+      ++NumLoopBlocksDeleted;
+    }
+  }
+
   /// Constant-fold terminators of blocks acculumated in FoldCandidates into the
   /// unconditional branches.
   void foldTerminators() {
@@ -358,15 +381,6 @@ public:
       return false;
     }
 
-    // TODO: Support deletion of dead loop blocks.
-    if (!DeadLoopBlocks.empty()) {
-      LLVM_DEBUG(dbgs() << "Give up constant terminator folding in loop "
-                        << L.getHeader()->getName()
-                        << ": we don't currently"
-                           " support deletion of dead in-loop blocks.\n");
-      return false;
-    }
-
     // TODO: Support dead loop exits.
     if (!DeadExitBlocks.empty()) {
       LLVM_DEBUG(dbgs() << "Give up constant terminator folding in loop "
@@ -377,7 +391,8 @@ public:
 
     // TODO: Support blocks that are not dead, but also not in loop after the
     // folding.
-    if (BlocksInLoopAfterFolding.size() != L.getNumBlocks()) {
+    if (BlocksInLoopAfterFolding.size() + DeadLoopBlocks.size() !=
+        L.getNumBlocks()) {
       LLVM_DEBUG(
           dbgs() << "Give up constant terminator folding in loop "
                  << L.getHeader()->getName()
@@ -397,6 +412,13 @@ public:
     // Make the actual transforms.
     foldTerminators();
 
+    if (!DeadLoopBlocks.empty()) {
+      LLVM_DEBUG(dbgs() << "Deleting " << DeadLoopBlocks.size()
+                    << " dead blocks in loop " << L.getHeader()->getName()
+                    << "\n");
+      deleteDeadLoopBlocks();
+    }
+
 #ifndef NDEBUG
     // Make sure that we have preserved all data structures after the transform.
     DT.verify();
diff --git a/llvm/test/Transforms/LoopSimplifyCFG/constant-fold-branch.ll b/llvm/test/Transforms/LoopSimplifyCFG/constant-fold-branch.ll
index 7998aa360dd..a06ada77bde 100644
--- a/llvm/test/Transforms/LoopSimplifyCFG/constant-fold-branch.ll
+++ b/llvm/test/Transforms/LoopSimplifyCFG/constant-fold-branch.ll
@@ -88,18 +88,12 @@ define i32 @dead_block_test_branch_loop(i32 %end) {
 ; CHECK-NEXT:  preheader:
 ; CHECK-NEXT:    br label [[HEADER:%.*]]
 ; CHECK:       header:
-; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[PREHEADER:%.*]] ], [ [[I_INC:%.*]], [[BACKEDGE:%.*]] ]
-; CHECK-NEXT:    br i1 true, label [[BACKEDGE]], label [[DEAD:%.*]]
-; CHECK:       dead:
-; CHECK-NEXT:    [[I_2:%.*]] = add i32 [[I]], 1
-; CHECK-NEXT:    br label [[BACKEDGE]]
-; CHECK:       backedge:
-; CHECK-NEXT:    [[I_1:%.*]] = phi i32 [ [[I]], [[HEADER]] ], [ [[I_2]], [[DEAD]] ]
-; CHECK-NEXT:    [[I_INC]] = add i32 [[I_1]], 1
+; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[PREHEADER:%.*]] ], [ [[I_INC:%.*]], [[HEADER]] ]
+; CHECK-NEXT:    [[I_INC]] = add i32 [[I]], 1
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[I_INC]], [[END:%.*]]
 ; CHECK-NEXT:    br i1 [[CMP]], label [[HEADER]], label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[I_INC_LCSSA:%.*]] = phi i32 [ [[I_INC]], [[BACKEDGE]] ]
+; CHECK-NEXT:    [[I_INC_LCSSA:%.*]] = phi i32 [ [[I_INC]], [[HEADER]] ]
 ; CHECK-NEXT:    ret i32 [[I_INC_LCSSA]]
 ;
 preheader:
@@ -129,22 +123,12 @@ define i32 @dead_block_test_switch_loop(i32 %end) {
 ; CHECK-NEXT:  preheader:
 ; CHECK-NEXT:    br label [[HEADER:%.*]]
 ; CHECK:       header:
-; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[PREHEADER:%.*]] ], [ [[I_INC:%.*]], [[BACKEDGE:%.*]] ]
-; CHECK-NEXT:    switch i32 1, label [[DEAD:%.*]] [
-; CHECK-NEXT:    i32 0, label [[DEAD]]
-; CHECK-NEXT:    i32 1, label [[BACKEDGE]]
-; CHECK-NEXT:    i32 2, label [[DEAD]]
-; CHECK-NEXT:    ]
-; CHECK:       dead:
-; CHECK-NEXT:    [[I_2:%.*]] = add i32 [[I]], 1
-; CHECK-NEXT:    br label [[BACKEDGE]]
-; CHECK:       backedge:
-; CHECK-NEXT:    [[I_1:%.*]] = phi i32 [ [[I]], [[HEADER]] ], [ [[I_2]], [[DEAD]] ]
-; CHECK-NEXT:    [[I_INC]] = add i32 [[I_1]], 1
+; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[PREHEADER:%.*]] ], [ [[I_INC:%.*]], [[HEADER]] ]
+; CHECK-NEXT:    [[I_INC]] = add i32 [[I]], 1
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[I_INC]], [[END:%.*]]
 ; CHECK-NEXT:    br i1 [[CMP]], label [[HEADER]], label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[I_INC_LCSSA:%.*]] = phi i32 [ [[I_INC]], [[BACKEDGE]] ]
+; CHECK-NEXT:    [[I_INC_LCSSA:%.*]] = phi i32 [ [[I_INC]], [[HEADER]] ]
 ; CHECK-NEXT:    ret i32 [[I_INC_LCSSA]]
 ;
 preheader:
@@ -175,18 +159,12 @@ define i32 @dead_block_propogate_test_branch_loop(i32 %end) {
 ; CHECK-NEXT:  preheader:
 ; CHECK-NEXT:    br label [[HEADER:%.*]]
 ; CHECK:       header:
-; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[PREHEADER:%.*]] ], [ [[I_INC:%.*]], [[BACKEDGE:%.*]] ]
-; CHECK-NEXT:    br i1 true, label [[BACKEDGE]], label [[DEAD:%.*]]
-; CHECK:       dead:
-; CHECK-NEXT:    [[I_2:%.*]] = add i32 [[I]], 1
-; CHECK-NEXT:    br label [[BACKEDGE]]
-; CHECK:       backedge:
-; CHECK-NEXT:    [[I_1:%.*]] = phi i32 [ [[I]], [[HEADER]] ], [ [[I_2]], [[DEAD]] ]
-; CHECK-NEXT:    [[I_INC]] = add i32 [[I_1]], 1
+; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[PREHEADER:%.*]] ], [ [[I_INC:%.*]], [[HEADER]] ]
+; CHECK-NEXT:    [[I_INC]] = add i32 [[I]], 1
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[I_INC]], [[END:%.*]]
 ; CHECK-NEXT:    br i1 [[CMP]], label [[HEADER]], label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[I_INC_LCSSA:%.*]] = phi i32 [ [[I_INC]], [[BACKEDGE]] ]
+; CHECK-NEXT:    [[I_INC_LCSSA:%.*]] = phi i32 [ [[I_INC]], [[HEADER]] ]
 ; CHECK-NEXT:    ret i32 [[I_INC_LCSSA]]
 ;
 preheader:
@@ -219,22 +197,12 @@ define i32 @dead_block_propogate_test_switch_loop(i32 %end) {
 ; CHECK-NEXT:  preheader:
 ; CHECK-NEXT:    br label [[HEADER:%.*]]
 ; CHECK:       header:
-; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[PREHEADER:%.*]] ], [ [[I_INC:%.*]], [[BACKEDGE:%.*]] ]
-; CHECK-NEXT:    switch i32 1, label [[DEAD:%.*]] [
-; CHECK-NEXT:    i32 0, label [[DEAD]]
-; CHECK-NEXT:    i32 1, label [[BACKEDGE]]
-; CHECK-NEXT:    i32 2, label [[DEAD]]
-; CHECK-NEXT:    ]
-; CHECK:       dead:
-; CHECK-NEXT:    [[I_2:%.*]] = add i32 [[I]], 1
-; CHECK-NEXT:    br label [[BACKEDGE]]
-; CHECK:       backedge:
-; CHECK-NEXT:    [[I_1:%.*]] = phi i32 [ [[I]], [[HEADER]] ], [ [[I_2]], [[DEAD]] ]
-; CHECK-NEXT:    [[I_INC]] = add i32 [[I_1]], 1
+; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[PREHEADER:%.*]] ], [ [[I_INC:%.*]], [[HEADER]] ]
+; CHECK-NEXT:    [[I_INC]] = add i32 [[I]], 1
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[I_INC]], [[END:%.*]]
 ; CHECK-NEXT:    br i1 [[CMP]], label [[HEADER]], label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[I_INC_LCSSA:%.*]] = phi i32 [ [[I_INC]], [[BACKEDGE]] ]
+; CHECK-NEXT:    [[I_INC_LCSSA:%.*]] = phi i32 [ [[I_INC]], [[HEADER]] ]
 ; CHECK-NEXT:    ret i32 [[I_INC_LCSSA]]
 ;
 preheader:
@@ -463,32 +431,19 @@ define i32 @dead_sub_loop_test_branch_loop(i32 %end) {
 ; CHECK-NEXT:  preheader:
 ; CHECK-NEXT:    br label [[HEADER:%.*]]
 ; CHECK:       header:
-; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[PREHEADER:%.*]] ], [ [[I_INC:%.*]], [[BACKEDGE:%.*]] ]
-; CHECK-NEXT:    br i1 true, label [[LIVE_PREHEADER:%.*]], label [[DEAD_PREHEADER:%.*]]
-; CHECK:       live_preheader:
+; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[PREHEADER:%.*]] ], [ [[I_INC:%.*]], [[EXIT_A:%.*]] ]
 ; CHECK-NEXT:    br label [[LIVE_LOOP:%.*]]
 ; CHECK:       live_loop:
-; CHECK-NEXT:    [[A:%.*]] = phi i32 [ 0, [[LIVE_PREHEADER]] ], [ [[A_INC:%.*]], [[LIVE_LOOP]] ]
+; CHECK-NEXT:    [[A:%.*]] = phi i32 [ 0, [[HEADER]] ], [ [[A_INC:%.*]], [[LIVE_LOOP]] ]
 ; CHECK-NEXT:    [[A_INC]] = add i32 [[A]], 1
 ; CHECK-NEXT:    [[CMP_A:%.*]] = icmp slt i32 [[A_INC]], [[END:%.*]]
-; CHECK-NEXT:    br i1 [[CMP_A]], label [[LIVE_LOOP]], label [[EXIT_A:%.*]]
+; CHECK-NEXT:    br i1 [[CMP_A]], label [[LIVE_LOOP]], label [[EXIT_A]]
 ; CHECK:       exit.a:
-; CHECK-NEXT:    br label [[BACKEDGE]]
-; CHECK:       dead_preheader:
-; CHECK-NEXT:    br label [[DEAD_LOOP:%.*]]
-; CHECK:       dead_loop:
-; CHECK-NEXT:    [[B:%.*]] = phi i32 [ 0, [[DEAD_PREHEADER]] ], [ [[B_INC:%.*]], [[DEAD_LOOP]] ]
-; CHECK-NEXT:    [[B_INC]] = add i32 [[B]], 1
-; CHECK-NEXT:    [[CMP_B:%.*]] = icmp slt i32 [[B_INC]], [[END]]
-; CHECK-NEXT:    br i1 [[CMP_B]], label [[DEAD_LOOP]], label [[EXIT_B:%.*]]
-; CHECK:       exit.b:
-; CHECK-NEXT:    br label [[BACKEDGE]]
-; CHECK:       backedge:
 ; CHECK-NEXT:    [[I_INC]] = add i32 [[I]], 1
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[I_INC]], [[END]]
 ; CHECK-NEXT:    br i1 [[CMP]], label [[HEADER]], label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[I_INC_LCSSA:%.*]] = phi i32 [ [[I_INC]], [[BACKEDGE]] ]
+; CHECK-NEXT:    [[I_INC_LCSSA:%.*]] = phi i32 [ [[I_INC]], [[EXIT_A]] ]
 ; CHECK-NEXT:    ret i32 [[I_INC_LCSSA]]
 ;
 preheader:
@@ -536,36 +491,19 @@ define i32 @dead_sub_loop_test_switch_loop(i32 %end) {
 ; CHECK-NEXT:  preheader:
 ; CHECK-NEXT:    br label [[HEADER:%.*]]
 ; CHECK:       header:
-; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[PREHEADER:%.*]] ], [ [[I_INC:%.*]], [[BACKEDGE:%.*]] ]
-; CHECK-NEXT:    switch i32 1, label [[DEAD_PREHEADER:%.*]] [
-; CHECK-NEXT:    i32 0, label [[DEAD_PREHEADER]]
-; CHECK-NEXT:    i32 1, label [[LIVE_PREHEADER:%.*]]
-; CHECK-NEXT:    i32 2, label [[DEAD_PREHEADER]]
-; CHECK-NEXT:    ]
-; CHECK:       live_preheader:
+; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[PREHEADER:%.*]] ], [ [[I_INC:%.*]], [[EXIT_A:%.*]] ]
 ; CHECK-NEXT:    br label [[LIVE_LOOP:%.*]]
 ; CHECK:       live_loop:
-; CHECK-NEXT:    [[A:%.*]] = phi i32 [ 0, [[LIVE_PREHEADER]] ], [ [[A_INC:%.*]], [[LIVE_LOOP]] ]
+; CHECK-NEXT:    [[A:%.*]] = phi i32 [ 0, [[HEADER]] ], [ [[A_INC:%.*]], [[LIVE_LOOP]] ]
 ; CHECK-NEXT:    [[A_INC]] = add i32 [[A]], 1
 ; CHECK-NEXT:    [[CMP_A:%.*]] = icmp slt i32 [[A_INC]], [[END:%.*]]
-; CHECK-NEXT:    br i1 [[CMP_A]], label [[LIVE_LOOP]], label [[EXIT_A:%.*]]
+; CHECK-NEXT:    br i1 [[CMP_A]], label [[LIVE_LOOP]], label [[EXIT_A]]
 ; CHECK:       exit.a:
-; CHECK-NEXT:    br label [[BACKEDGE]]
-; CHECK:       dead_preheader:
-; CHECK-NEXT:    br label [[DEAD_LOOP:%.*]]
-; CHECK:       dead_loop:
-; CHECK-NEXT:    [[B:%.*]] = phi i32 [ 0, [[DEAD_PREHEADER]] ], [ [[B_INC:%.*]], [[DEAD_LOOP]] ]
-; CHECK-NEXT:    [[B_INC]] = add i32 [[B]], 1
-; CHECK-NEXT:    [[CMP_B:%.*]] = icmp slt i32 [[B_INC]], [[END]]
-; CHECK-NEXT:    br i1 [[CMP_B]], label [[DEAD_LOOP]], label [[EXIT_B:%.*]]
-; CHECK:       exit.b:
-; CHECK-NEXT:    br label [[BACKEDGE]]
-; CHECK:       backedge:
 ; CHECK-NEXT:    [[I_INC]] = add i32 [[I]], 1
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[I_INC]], [[END]]
 ; CHECK-NEXT:    br i1 [[CMP]], label [[HEADER]], label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[I_INC_LCSSA:%.*]] = phi i32 [ [[I_INC]], [[BACKEDGE]] ]
+; CHECK-NEXT:    [[I_INC_LCSSA:%.*]] = phi i32 [ [[I_INC]], [[EXIT_A]] ]
 ; CHECK-NEXT:    ret i32 [[I_INC_LCSSA]]
 ;
 preheader:
@@ -898,18 +836,12 @@ define i32 @partial_sub_loop_test_branch_loop(i32 %end) {
 ; CHECK-NEXT:    [[J:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[J_INC:%.*]], [[OUTER_BACKEDGE:%.*]] ]
 ; CHECK-NEXT:    br label [[HEADER:%.*]]
 ; CHECK:       header:
-; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[OUTER_HEADER]] ], [ [[I_INC:%.*]], [[BACKEDGE:%.*]] ]
-; CHECK-NEXT:    br i1 true, label [[BACKEDGE]], label [[DEAD:%.*]]
-; CHECK:       dead:
-; CHECK-NEXT:    [[I_2:%.*]] = add i32 [[I]], 1
-; CHECK-NEXT:    br label [[BACKEDGE]]
-; CHECK:       backedge:
-; CHECK-NEXT:    [[I_1:%.*]] = phi i32 [ [[I]], [[HEADER]] ], [ [[I_2]], [[DEAD]] ]
-; CHECK-NEXT:    [[I_INC]] = add i32 [[I_1]], 1
+; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[OUTER_HEADER]] ], [ [[I_INC:%.*]], [[HEADER]] ]
+; CHECK-NEXT:    [[I_INC]] = add i32 [[I]], 1
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[I_INC]], [[END:%.*]]
 ; CHECK-NEXT:    br i1 [[CMP]], label [[HEADER]], label [[OUTER_BACKEDGE]]
 ; CHECK:       outer_backedge:
-; CHECK-NEXT:    [[I_INC_LCSSA:%.*]] = phi i32 [ [[I_INC]], [[BACKEDGE]] ]
+; CHECK-NEXT:    [[I_INC_LCSSA:%.*]] = phi i32 [ [[I_INC]], [[HEADER]] ]
 ; CHECK-NEXT:    [[J_INC]] = add i32 [[J]], 1
 ; CHECK-NEXT:    [[CMP_J:%.*]] = icmp slt i32 [[J_INC]], [[END]]
 ; CHECK-NEXT:    br i1 [[CMP_J]], label [[OUTER_HEADER]], label [[EXIT:%.*]]
@@ -958,22 +890,12 @@ define i32 @partial_sub_loop_test_switch_loop(i32 %end) {
 ; CHECK-NEXT:    [[J:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[J_INC:%.*]], [[OUTER_BACKEDGE:%.*]] ]
 ; CHECK-NEXT:    br label [[HEADER:%.*]]
 ; CHECK:       header:
-; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[OUTER_HEADER]] ], [ [[I_INC:%.*]], [[BACKEDGE:%.*]] ]
-; CHECK-NEXT:    switch i32 1, label [[DEAD:%.*]] [
-; CHECK-NEXT:    i32 0, label [[DEAD]]
-; CHECK-NEXT:    i32 1, label [[BACKEDGE]]
-; CHECK-NEXT:    i32 2, label [[DEAD]]
-; CHECK-NEXT:    ]
-; CHECK:       dead:
-; CHECK-NEXT:    [[I_2:%.*]] = add i32 [[I]], 1
-; CHECK-NEXT:    br label [[BACKEDGE]]
-; CHECK:       backedge:
-; CHECK-NEXT:    [[I_1:%.*]] = phi i32 [ [[I]], [[HEADER]] ], [ [[I_2]], [[DEAD]] ]
-; CHECK-NEXT:    [[I_INC]] = add i32 [[I_1]], 1
+; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[OUTER_HEADER]] ], [ [[I_INC:%.*]], [[HEADER]] ]
+; CHECK-NEXT:    [[I_INC]] = add i32 [[I]], 1
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[I_INC]], [[END:%.*]]
 ; CHECK-NEXT:    br i1 [[CMP]], label [[HEADER]], label [[OUTER_BACKEDGE]]
 ; CHECK:       outer_backedge:
-; CHECK-NEXT:    [[I_INC_LCSSA:%.*]] = phi i32 [ [[I_INC]], [[BACKEDGE]] ]
+; CHECK-NEXT:    [[I_INC_LCSSA:%.*]] = phi i32 [ [[I_INC]], [[HEADER]] ]
 ; CHECK-NEXT:    [[J_INC]] = add i32 [[J]], 1
 ; CHECK-NEXT:    [[CMP_J:%.*]] = icmp slt i32 [[J_INC]], [[END]]
 ; CHECK-NEXT:    br i1 [[CMP_J]], label [[OUTER_HEADER]], label [[EXIT:%.*]]