Revert "Do actual DCE in LoopUnroll"

This reverts commit 9d1a61e695eb01298e26c76867d65592f1e1968c.

I'd missed some review feedback, and had missed updating an aarch64 test.  Reverting while I fix both.

1 files changed, 8 insertions, 9 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUnroll.cpp b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
index 9a1f812..893b918 100644
--- a/llvm/lib/Transforms/Utils/LoopUnroll.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
@@ -220,10 +220,10 @@ void llvm::simplifyLoopAfterUnroll(Loop *L, bool SimplifyIVs, LoopInfo *LI,
     }
   }
 
-  // At this point, the code is well formed.  Perform constprop, instsimplify,
-  // and dce.
+  // At this point, the code is well formed.  We now do a quick sweep over the
+  // inserted code, doing constant propagation and dead code elimination as we
+  // go.
   const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
-  SmallVector<WeakTrackingVH, 16> DeadInsts;
   for (BasicBlock *BB : L->getBlocks()) {
     for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;) {
       Instruction *Inst = &*I++;
@@ -232,15 +232,14 @@ void llvm::simplifyLoopAfterUnroll(Loop *L, bool SimplifyIVs, LoopInfo *LI,
         if (LI->replacementPreservesLCSSAForm(Inst, V))
           Inst->replaceAllUsesWith(V);
       if (isInstructionTriviallyDead(Inst))
-        DeadInsts.emplace_back(Inst);
+        BB->getInstList().erase(Inst);
     }
   }
 
-  while (!DeadInsts.empty()) {
-    Value *V = DeadInsts.pop_back_val();
-    if (Instruction *Inst = dyn_cast_or_null<Instruction>(V))
-      RecursivelyDeleteTriviallyDeadInstructions(Inst);
-  }
+  // TODO: after peeling or unrolling, previously loop variant conditions are
+  // likely to fold to constants, eagerly propagating those here will require
+  // fewer cleanup passes to be run.  Alternatively, a LoopEarlyCSE might be
+  // appropriate.
 }
 
 /// Unroll the given loop by Count. The loop must be in LCSSA form.  Unrolling