Generalize MergeBlockIntoPredecessor. Replace uses of MergeBasicBlockIntoOnlyPred.

Summary:
Two utils methods have essentially the same functionality. This is an attempt to merge them into one.
1. lib/Transforms/Utils/Local.cpp : MergeBasicBlockIntoOnlyPred
2. lib/Transforms/Utils/BasicBlockUtils.cpp : MergeBlockIntoPredecessor

Prior to the patch:
1. MergeBasicBlockIntoOnlyPred
Updates either DomTree or DeferredDominance
Moves all instructions from Pred to BB, deletes Pred
Asserts BB has single predecessor
If address was taken, replace the block address with constant 1 (?)

2. MergeBlockIntoPredecessor
Updates DomTree, LoopInfo and MemoryDependenceResults
Moves all instruction from BB to Pred, deletes BB
Returns if doesn't have a single predecessor
Returns if BB's address was taken

After the patch:
Method 2. MergeBlockIntoPredecessor is attempting to become the new default:
Updates DomTree or DeferredDominance, and LoopInfo and MemoryDependenceResults
Moves all instruction from BB to Pred, deletes BB
Returns if doesn't have a single predecessor
Returns if BB's address was taken

Uses of MergeBasicBlockIntoOnlyPred that need to be replaced:

1. lib/Transforms/Scalar/LoopSimplifyCFG.cpp
Updated in this patch. No challenges.

2. lib/CodeGen/CodeGenPrepare.cpp
Updated in this patch.
  i. eliminateFallThrough is straightforward, but I added using a temporary array to avoid the iterator invalidation.
  ii. eliminateMostlyEmptyBlock(s) methods also now use a temporary array for blocks
Some interesting aspects:
  - Since Pred is not deleted (BB is), the entry block does not need updating.
  - The entry block was being updated with the deleted block in eliminateMostlyEmptyBlock. Added assert to make obvious that BB=SinglePred.
  - isMergingEmptyBlockProfitable assumes BB is the one to be deleted.
  - eliminateMostlyEmptyBlock(BB) does not delete BB on one path, it deletes its unique predecessor instead.
  - adding some test owner as subscribers for the interesting tests modified:
    test/CodeGen/X86/avx-cmp.ll
    test/CodeGen/AMDGPU/nested-loop-conditions.ll
    test/CodeGen/AMDGPU/si-annotate-cf.ll
    test/CodeGen/X86/hoist-spill.ll
    test/CodeGen/X86/2006-11-17-IllegalMove.ll

3. lib/Transforms/Scalar/JumpThreading.cpp
Not covered in this patch. It is the only use case using the DeferredDominance.
I would defer to Brian Rzycki to make this replacement.

Reviewers: chandlerc, spatel, davide, brzycki, bkramer, javed.absar

Subscribers: qcolombet, sanjoy, nemanjai, nhaehnle, jlebar, tpr, kbarton, RKSimon, wmi, arsenm, llvm-commits

Differential Revision: https://reviews.llvm.org/D48202

llvm-svn: 335183

1 files changed, 30 insertions, 23 deletions

diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index 595fcf70766..743acc5af30 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -516,8 +516,16 @@ bool CodeGenPrepare::runOnFunction(Function &F) {
 bool CodeGenPrepare::eliminateFallThrough(Function &F) {
   bool Changed = false;
   // Scan all of the blocks in the function, except for the entry block.
-  for (Function::iterator I = std::next(F.begin()), E = F.end(); I != E;) {
-    BasicBlock *BB = &*I++;
+  // Use a temporary array to avoid iterator being invalidated when
+  // deleting blocks.
+  SmallVector<WeakTrackingVH, 16> Blocks;
+  for (auto &Block : llvm::make_range(std::next(F.begin()), F.end()))
+    Blocks.push_back(&Block);
+
+  for (auto &Block : Blocks) {
+    auto *BB = cast_or_null<BasicBlock>(Block);
+    if (!BB)
+      continue;
     // If the destination block has a single pred, then this is a trivial
     // edge, just collapse it.
     BasicBlock *SinglePred = BB->getSinglePredecessor();
@@ -528,17 +536,10 @@ bool CodeGenPrepare::eliminateFallThrough(Function &F) {
     BranchInst *Term = dyn_cast<BranchInst>(SinglePred->getTerminator());
     if (Term && !Term->isConditional()) {
       Changed = true;
-      LLVM_DEBUG(dbgs() << "To merge:\n" << *SinglePred << "\n\n\n");
-      // Remember if SinglePred was the entry block of the function.
-      // If so, we will need to move BB back to the entry position.
-      bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
-      MergeBasicBlockIntoOnlyPred(BB, nullptr);
-
-      if (isEntry && BB != &BB->getParent()->getEntryBlock())
-        BB->moveBefore(&BB->getParent()->getEntryBlock());
+      LLVM_DEBUG(dbgs() << "To merge:\n" << *BB << "\n\n\n");
 
-      // We have erased a block. Update the iterator.
-      I = BB->getIterator();
+      // Merge BB into SinglePred and delete it.
+      MergeBlockIntoPredecessor(BB);
     }
   }
   return Changed;
@@ -591,9 +592,17 @@ bool CodeGenPrepare::eliminateMostlyEmptyBlocks(Function &F) {
   }
 
   bool MadeChange = false;
+  // Copy blocks into a temporary array to avoid iterator invalidation issues
+  // as we remove them.
   // Note that this intentionally skips the entry block.
-  for (Function::iterator I = std::next(F.begin()), E = F.end(); I != E;) {
-    BasicBlock *BB = &*I++;
+  SmallVector<WeakTrackingVH, 16> Blocks;
+  for (auto &Block : llvm::make_range(std::next(F.begin()), F.end()))
+    Blocks.push_back(&Block);
+
+  for (auto &Block : Blocks) {
+    BasicBlock *BB = cast_or_null<BasicBlock>(Block);
+    if (!BB)
+      continue;
     BasicBlock *DestBB = findDestBlockOfMergeableEmptyBlock(BB);
     if (!DestBB ||
         !isMergingEmptyBlockProfitable(BB, DestBB, Preheaders.count(BB)))
@@ -762,15 +771,13 @@ void CodeGenPrepare::eliminateMostlyEmptyBlock(BasicBlock *BB) {
   // just collapse it.
   if (BasicBlock *SinglePred = DestBB->getSinglePredecessor()) {
     if (SinglePred != DestBB) {
-      // Remember if SinglePred was the entry block of the function.  If so, we
-      // will need to move BB back to the entry position.
-      bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
-      MergeBasicBlockIntoOnlyPred(DestBB, nullptr);
-
-      if (isEntry && BB != &BB->getParent()->getEntryBlock())
-        BB->moveBefore(&BB->getParent()->getEntryBlock());
-
-      LLVM_DEBUG(dbgs() << "AFTER:\n" << *DestBB << "\n\n\n");
+      assert(SinglePred == BB &&
+             "Single predecessor not the same as predecessor");
+      // Merge DestBB into SinglePred/BB and delete it.
+      MergeBlockIntoPredecessor(DestBB);
+      // Note: BB(=SinglePred) will not be deleted on this path.
+      // DestBB(=its single successor) is the one that was deleted.
+      LLVM_DEBUG(dbgs() << "AFTER:\n" << *SinglePred << "\n\n\n");
       return;
     }
   }