[CodeExtractor] Add a few debug lines to understand why a region is not extracted

The CodeExtractor is not smart enough to compute which basic block is
the entry of a region. Instead it relies on the order of the list
of basic blocks that is handed to it and assumes that the entry
is the first block in the list.

Without the additional debug information, it is hard to understand
why a valid region does not get extracted, because we would miss
that the order of in the list just doesn't match what the CodeExtractor
wants.

NFC

llvm-svn: 358471

1 files changed, 8 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Utils/CodeExtractor.cpp b/llvm/lib/Transforms/Utils/CodeExtractor.cpp
index d3bcfd56841..61c1e9313c2 100644
--- a/llvm/lib/Transforms/Utils/CodeExtractor.cpp
+++ b/llvm/lib/Transforms/Utils/CodeExtractor.cpp
@@ -209,6 +209,9 @@ buildExtractionBlockSet(ArrayRef<BasicBlock *> BBs, DominatorTree *DT,
       llvm_unreachable("Repeated basic blocks in extraction input");
   }
 
+  LLVM_DEBUG(dbgs() << "Region front block: " << Result.front()->getName()
+                    << '\n');
+
   for (auto *BB : Result) {
     if (!isBlockValidForExtraction(*BB, Result, AllowVarArgs, AllowAlloca))
       return {};
@@ -226,9 +229,11 @@ buildExtractionBlockSet(ArrayRef<BasicBlock *> BBs, DominatorTree *DT,
     // the subgraph which is being extracted.
     for (auto *PBB : predecessors(BB))
       if (!Result.count(PBB)) {
-        LLVM_DEBUG(
-            dbgs() << "No blocks in this region may have entries from "
-                      "outside the region except for the first block!\n");
+        LLVM_DEBUG(dbgs() << "No blocks in this region may have entries from "
+                             "outside the region except for the first block!\n"
+                          << "Problematic source BB: " << BB->getName() << "\n"
+                          << "Problematic destination BB: " << PBB->getName()
+                          << "\n");
         return {};
       }
   }