[Utils] Skip RemoveRedundantDbgInstrs in MergeBlockIntoPredecessor (PR47746)

This patch changes MergeBlockIntoPredecessor to skip the call to
RemoveRedundantDbgInstrs, in effect partially reverting D71480 due to
some compile-time issues spotted in LoopUnroll and SimplifyCFG.

The call to RemoveRedundantDbgInstrs appears to have changed the
worst-case behavior of the merging utility. Loosely speaking, it seems
to have gone from O(#phis) to O(#insts).

It might not be possible to mitigate this by scanning a block to
determine whether there are any debug intrinsics to remove, since such a
scan costs O(#insts).

So: skip the call to RemoveRedundantDbgInstrs. There's surprisingly
little fallout from this, and most of it can be addressed by doing
RemoveRedundantDbgInstrs later. The exception is (the block-local
version of) SimplifyCFG, where it might just be too expensive to call
RemoveRedundantDbgInstrs.

Differential Revision: https://reviews.llvm.org/D88928

(cherry picked from commit 5a3ef55)

Author: vedantk

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -622,6 +622,7 @@ bool CodeGenPrepare::eliminateFallThrough(Function &F) {
   for (auto &Block : llvm::make_range(std::next(F.begin()), F.end()))
     Blocks.push_back(&Block);
 
+  SmallSet<WeakTrackingVH, 16> Preds;
   for (auto &Block : Blocks) {
     auto *BB = cast_or_null<BasicBlock>(Block);
     if (!BB)
@@ -640,8 +641,16 @@ bool CodeGenPrepare::eliminateFallThrough(Function &F) {
 
       // Merge BB into SinglePred and delete it.
       MergeBlockIntoPredecessor(BB);
+      Preds.insert(SinglePred);
     }
   }
+
+  // (Repeatedly) merging blocks into their predecessors can create redundant
+  // debug intrinsics.
+  for (auto &Pred : Preds)
+    if (auto *BB = cast_or_null<BasicBlock>(Pred))
+      RemoveRedundantDbgInstrs(BB);
+
   return Changed;
 }
 

llvm/lib/Transforms/Utils/BasicBlockUtils.cpp

@@ -285,11 +285,6 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
   // Add unreachable to now empty BB.
   new UnreachableInst(BB->getContext(), BB);
 
-  // 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())
     PredBB->takeName(BB);

llvm/lib/Transforms/Utils/LoopRotationUtils.cpp

@@ -575,7 +575,10 @@ bool LoopRotate::rotateLoop(Loop *L, bool SimplifiedLatch) {
     // connected by an unconditional branch.  This is just a cleanup so the
     // emitted code isn't too gross in this common case.
     DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
-    MergeBlockIntoPredecessor(OrigHeader, &DTU, LI, MSSAU);
+    BasicBlock *PredBB = OrigHeader->getUniquePredecessor();
+    bool DidMerge = MergeBlockIntoPredecessor(OrigHeader, &DTU, LI, MSSAU);
+    if (DidMerge)
+      RemoveRedundantDbgInstrs(PredBB);
 
     if (MSSAU && VerifyMemorySSA)
       MSSAU->getMemorySSA()->verifyMemorySSA();

llvm/test/Transforms/SimplifyCFG/hoist-dbgvalue-inlined.ll

@@ -8,6 +8,7 @@ init:
 
 ; CHECK:  %vala = load i64, i64* %ptr
 ; CHECK-NEXT:  call void @llvm.dbg.value(metadata i64 %vala, metadata [[MD:![0-9]*]]
+; CHECK-NEXT:  call void @llvm.dbg.value(metadata i64 %vala, metadata [[MD]]
 ; CHECK-NEXT:  %valbmasked = and i64 %vala, 1
 
 a:                                              ; preds = %init
@@ -26,6 +27,8 @@ test.exit:                                      ; preds = %a, %b
   ret i64 %retv
 }
 
+; CHECK: [[MD]] = !DILocalVariable(name: "var"
+
 ; Function Attrs: nounwind readnone speculatable
 declare void @llvm.dbg.value(metadata, metadata, metadata) #0