[BasicBlockUtils] Fix dbg.value elimination problem in MergeBlockIntoPredecessor

Summary:
In commit d60f34c20a2f31335c8d5626e (llvm-svn 317128,
PR35113) MergeBlockIntoPredecessor was changed into
discarding some dbg.value intrinsics referring to
PHI values, post-splice due to loop rotation.

That elimination of dbg.value intrinsics did not
consider which dbg.value to keep depending on the
context (e.g. if the variable is changing its value
several times inside the basic block).

In the past that hasn't been such a big problem since
CodeGenPrepare::placeDbgValues has moved the dbg.value
to be next to the PHI node anyway. But after commit
00e238896cd8ad3a7d7 CodeGenPrepare isn't doing that
any longer, so we need to be more careful when avoiding
duplicate dbg.value intrinsics in MergeBlockIntoPredecessor.

This patch replaces the code that tried to avoid duplicate
dbg.values by using the RemoveRedundantDbgInstrs helper.

Reviewers: aprantl, jmorse, vsk

Reviewed By: aprantl, vsk

Subscribers: jholewinski, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71480

1 files changed, 4 insertions, 14 deletions

diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index d5faa4b5c33..c9eb4abfa21 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -284,20 +284,10 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
   // Add unreachable to now empty BB.
   new UnreachableInst(BB->getContext(), BB);
 
-  // Eliminate duplicate dbg.values describing the entry PHI node post-splice.
-  for (auto Incoming : IncomingValues) {
-    if (isa<Instruction>(*Incoming)) {
-      SmallVector<DbgValueInst *, 2> DbgValues;
-      SmallDenseSet<std::pair<DILocalVariable *, DIExpression *>, 2>
-          DbgValueSet;
-      llvm::findDbgValues(DbgValues, Incoming);
-      for (auto &DVI : DbgValues) {
-        auto R = DbgValueSet.insert({DVI->getVariable(), DVI->getExpression()});
-        if (!R.second)
-          DVI->eraseFromParent();
-      }
-    }
-  }
+  // Eliminate duplicate/redundant dbg.values. This seems to be a good place to
+  // do that since we might end up with redundant dbg.values describing the
+  // entry PHI node post-splice.
+  RemoveRedundantDbgInstrs(PredBB);
 
   // Inherit predecessors name if it exists.
   if (!PredBB->hasName())