[MergeICmps] Handle chains with several complex BCE basic blocks.

 - Fix condition for detecting that a complex basic block was the first in
   the chain.
 - Add tests.

This was caught by buildbots when submitting rL324319.

llvm-svn: 324341

1 files changed, 4 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Scalar/MergeICmps.cpp b/llvm/lib/Transforms/Scalar/MergeICmps.cpp
index f045b62a922..745d29d14dd 100644
--- a/llvm/lib/Transforms/Scalar/MergeICmps.cpp
+++ b/llvm/lib/Transforms/Scalar/MergeICmps.cpp
@@ -278,7 +278,8 @@ BCECmpChain::BCECmpChain(const std::vector<BasicBlock *> &Blocks, PHINode &Phi)
   assert(!Blocks.empty() && "a chain should have at least one block");
   // Now look inside blocks to check for BCE comparisons.
   std::vector<BCECmpBlock> Comparisons;
-  for (BasicBlock *Block : Blocks) {
+  for (size_t BlockIdx = 0; BlockIdx < Blocks.size(); ++BlockIdx) {
+    BasicBlock *const Block = Blocks[BlockIdx];
     assert(Block && "invalid block");
     BCECmpBlock Comparison = visitCmpBlock(Phi.getIncomingValueForBlock(Block),
                                            Block, Phi.getParent());
@@ -289,7 +290,7 @@ BCECmpChain::BCECmpChain(const std::vector<BasicBlock *> &Blocks, PHINode &Phi)
     }
     if (Comparison.doesOtherWork()) {
       DEBUG(dbgs() << "block does extra work besides compare\n");
-      if (Comparisons.empty()) {  // First block.
+      if (BlockIdx == 0) {  // First block.
         // TODO(courbet): The first block can do other things, and we should
         // split them apart in a separate block before the comparison chain.
         // Right now we just discard it and make the chain shorter.
@@ -330,7 +331,7 @@ BCECmpChain::BCECmpChain(const std::vector<BasicBlock *> &Blocks, PHINode &Phi)
     DEBUG(dbgs() << "\n");
     Comparisons.push_back(Comparison);
   }
-  assert(!Comparisons.empty() && "chain with a single complex basic block");
+  assert(!Comparisons.empty() && "chain with no BCE basic blocks");
   EntryBlock_ = Comparisons[0].BB;
   Comparisons_ = std::move(Comparisons);
 #ifdef MERGEICMPS_DOT_ON