Revert "Do actual DCE in LoopUnroll (try 3)"

This reverts commit b6320eeb8622f05e4a5d4c7f5420523357490fca
as it causes clang to assert; see
https://reviews.llvm.org/rGb6320eeb8622f05e4a5d4c7f5420523357490fca.

1 files changed, 15 insertions, 9 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUnroll.cpp b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
index 395d2af..20bf5a25 100644
--- a/llvm/lib/Transforms/Utils/LoopUnroll.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
@@ -213,27 +213,33 @@ void llvm::simplifyLoopAfterUnroll(Loop *L, bool SimplifyIVs, LoopInfo *LI,
 
     // Aggressively clean up dead instructions that simplifyLoopIVs already
     // identified. Any remaining should be cleaned up below.
-    RecursivelyDeleteTriviallyDeadInstructions(DeadInsts);
+    while (!DeadInsts.empty()) {
+      Value *V = DeadInsts.pop_back_val();
+      if (Instruction *Inst = dyn_cast_or_null<Instruction>(V))
+        RecursivelyDeleteTriviallyDeadInstructions(Inst);
+    }
   }
 
-  // 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++;
+
       if (Value *V = SimplifyInstruction(Inst, {DL, nullptr, DT, AC}))
         if (LI->replacementPreservesLCSSAForm(Inst, V))
           Inst->replaceAllUsesWith(V);
       if (isInstructionTriviallyDead(Inst))
-        DeadInsts.emplace_back(Inst);
+        BB->getInstList().erase(Inst);
     }
-    // We can't do recursive deletion until we're done iterating, as we might
-    // have a phi which (potentially indirectly) uses instructions later in
-    // the block we're iterating through.
-    RecursivelyDeleteTriviallyDeadInstructions(DeadInsts);
   }
+
+  // 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