[llvm] r341398 - [SimpleLoopUnswitch] remove a chain of dead blocks at once

[Fedor Sergeev via llvm-commits]

Author: fedor.sergeev
Date: Tue Sep  4 13:19:41 2018
New Revision: 341398

URL: http://llvm.org/viewvc/llvm-project?rev=341398&view=rev
Log:
[SimpleLoopUnswitch] remove a chain of dead blocks at once

Recent change to deleteDeadBlocksFromLoop was not enough to
fix all the problems related to dead blocks after nontrivial
unswitching of switches.

We need to delete all the dead blocks that were created during
unswitching, otherwise we will keep having problems with phi's
or dead blocks.

This change removes all the dead blocks that are reachable from the loop,
not trying to track whether these blocks are newly created by unswitching
or not. While not completely correct, we are unlikely to get loose but
reachable dead blocks that do not belong to our loop nest.

It does fix all the failures currently known, in particular PR38778.

Reviewed By: asbirlea
Differential Revision: https://reviews.llvm.org/D51519

Modified:
    llvm/trunk/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
    llvm/trunk/test/Transforms/SimpleLoopUnswitch/delete-dead-blocks.ll

Modified: llvm/trunk/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp?rev=341398&r1=341397&r2=341398&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp Tue Sep  4 13:19:41 2018
@@ -1378,25 +1378,25 @@ static void
 deleteDeadBlocksFromLoop(Loop &L,
                          SmallVectorImpl<BasicBlock *> &ExitBlocks,
                          DominatorTree &DT, LoopInfo &LI) {
-  // Find all the dead blocks, and remove them from their successors.
-  SmallVector<BasicBlock *, 16> DeadBlocks;
-  for (BasicBlock *BB : ExitBlocks)
-    if (!DT.isReachableFromEntry(BB)) {
-      for (BasicBlock *SuccBB : successors(BB))
+  // Find all the dead blocks tied to this loop, and remove them from their
+  // successors.
+  SmallPtrSet<BasicBlock *, 16> DeadBlockSet;
+
+  // Start with loop/exit blocks and get a transitive closure of reachable dead
+  // blocks.
+  SmallVector<BasicBlock *, 16> DeathCandidates(ExitBlocks.begin(),
+                                                ExitBlocks.end());
+  DeathCandidates.append(L.blocks().begin(), L.blocks().end());
+  while (!DeathCandidates.empty()) {
+    auto *BB = DeathCandidates.pop_back_val();
+    if (!DeadBlockSet.count(BB) && !DT.isReachableFromEntry(BB)) {
+      for (BasicBlock *SuccBB : successors(BB)) {
         SuccBB->removePredecessor(BB);
-      DeadBlocks.push_back(BB);
-    }
-
-  for (Loop *ParentL = &L; ParentL; ParentL = ParentL->getParentLoop())
-    for (BasicBlock *BB : ParentL->blocks())
-      if (!DT.isReachableFromEntry(BB)) {
-        for (BasicBlock *SuccBB : successors(BB))
-          SuccBB->removePredecessor(BB);
-        DeadBlocks.push_back(BB);
+        DeathCandidates.push_back(SuccBB);
       }
-
-  SmallPtrSet<BasicBlock *, 16> DeadBlockSet(DeadBlocks.begin(),
-                                             DeadBlocks.end());
+      DeadBlockSet.insert(BB);
+    }
+  }
 
   // Filter out the dead blocks from the exit blocks list so that it can be
   // used in the caller.
@@ -1405,7 +1405,7 @@ deleteDeadBlocksFromLoop(Loop &L,
 
   // Walk from this loop up through its parents removing all of the dead blocks.
   for (Loop *ParentL = &L; ParentL; ParentL = ParentL->getParentLoop()) {
-    for (auto *BB : DeadBlocks)
+    for (auto *BB : DeadBlockSet)
       ParentL->getBlocksSet().erase(BB);
     llvm::erase_if(ParentL->getBlocksVector(),
                    [&](BasicBlock *BB) { return DeadBlockSet.count(BB); });
@@ -1430,7 +1430,7 @@ deleteDeadBlocksFromLoop(Loop &L,
   // Remove the loop mappings for the dead blocks and drop all the references
   // from these blocks to others to handle cyclic references as we start
   // deleting the blocks themselves.
-  for (auto *BB : DeadBlocks) {
+  for (auto *BB : DeadBlockSet) {
     // Check that the dominator tree has already been updated.
     assert(!DT.getNode(BB) && "Should already have cleared domtree!");
     LI.changeLoopFor(BB, nullptr);

Modified: llvm/trunk/test/Transforms/SimpleLoopUnswitch/delete-dead-blocks.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SimpleLoopUnswitch/delete-dead-blocks.ll?rev=341398&r1=341397&r2=341398&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/SimpleLoopUnswitch/delete-dead-blocks.ll (original)
+++ llvm/trunk/test/Transforms/SimpleLoopUnswitch/delete-dead-blocks.ll Tue Sep  4 13:19:41 2018
@@ -43,3 +43,59 @@ get_out:
 get_out2:
   unreachable
 }
+
+;
+; This comes from PR38778
+; CHECK-LABEL: @Test2
+define void @Test2(i32) {
+header:
+  br label %loop
+loop:
+  switch i32 %0, label %continue [
+    i32 -2147483648, label %check
+    i32 98, label %guarded1
+    i32 99, label %guarded2
+  ]
+; CHECK-NOT: {{^}}guarded1:
+guarded1:
+  br i1 undef, label %continue, label %leave
+guarded2:
+  br label %continue
+check:
+  %val = add i32 0, 1
+  br i1 undef, label %continue, label %leave
+continue:
+  br label %loop
+leave:
+  %local = phi i32 [ 0, %guarded1 ], [ %val, %check ]
+  ret void
+}
+
+;
+; Yet another test from PR38778
+;
+; CHECK-LABEL: @Test3
+define void @Test3(i32) {
+header:
+  br label %outer
+outer:
+  %bad_input.i = icmp eq i32 %0, -2147483648
+  br label %inner
+inner:
+  br i1 %bad_input.i, label %overflow, label %switchme
+overflow:
+  br label %continue
+switchme:
+  switch i32 %0, label %continue [
+    i32 88, label %go_out
+    i32 99, label %case2
+  ]
+; CHECK-NOT: {{^}}case2:
+case2:
+  br label %continue
+continue:
+  %local_11_92 = phi i32 [ 0, %switchme ], [ 18, %case2 ], [ 0, %overflow ]
+  br i1 undef, label %outer, label %inner
+go_out:
+  unreachable
+}