[Local] Simplify function removeUnreachableBlocks() to avoid (re-)computation.

Two small changes in llvm::removeUnreachableBlocks() to avoid unnecessary (re-)computation.

First, replace the use of count() with find(), which has better time complexity.

Second, because we have already computed the set of dead blocks, replace the second loop over all basic blocks to a loop only over the already computed dead blocks. This simplifies the loop and avoids recomputation.

Patch by Rodrigo Caetano Rocha <[email protected]>

Reviewers: efriedma, spatel, fhahn, xbolva00

Reviewed By: fhahn, xbolva00

Differential Revision: https://reviews.llvm.org/D68191

llvm-svn: 373429

Author: fhahn

llvm/lib/Transforms/Utils/Local.cpp

@@ -2215,22 +2215,22 @@ void llvm::removeUnwindEdge(BasicBlock *BB, DomTreeUpdater *DTU) {
 bool llvm::removeUnreachableBlocks(Function &F, LazyValueInfo *LVI,
                                    DomTreeUpdater *DTU,
                                    MemorySSAUpdater *MSSAU) {
-  SmallPtrSet<BasicBlock*, 16> Reachable;
+  SmallPtrSet<BasicBlock *, 16> Reachable;
   bool Changed = markAliveBlocks(F, Reachable, DTU);
 
   // If there are unreachable blocks in the CFG...
   if (Reachable.size() == F.size())
     return Changed;
 
   assert(Reachable.size() < F.size());
-  NumRemoved += F.size()-Reachable.size();
+  NumRemoved += F.size() - Reachable.size();
 
   SmallSetVector<BasicBlock *, 8> DeadBlockSet;
-  for (Function::iterator I = ++F.begin(), E = F.end(); I != E; ++I) {
-    auto *BB = &*I;
-    if (Reachable.count(BB))
+  for (BasicBlock &BB : F) {
+    // Skip reachable basic blocks
+    if (Reachable.find(&BB) != Reachable.end())
       continue;
-    DeadBlockSet.insert(BB);
+    DeadBlockSet.insert(&BB);
   }
 
   if (MSSAU)
@@ -2249,23 +2249,13 @@ bool llvm::removeUnreachableBlocks(Function &F, LazyValueInfo *LVI,
     if (LVI)
       LVI->eraseBlock(BB);
     BB->dropAllReferences();
-  }
-  for (Function::iterator I = ++F.begin(); I != F.end();) {
-    auto *BB = &*I;
-    if (Reachable.count(BB)) {
-      ++I;
-      continue;
-    }
     if (DTU) {
       // Remove the terminator of BB to clear the successor list of BB.
       if (BB->getTerminator())
         BB->getInstList().pop_back();
       new UnreachableInst(BB->getContext(), BB);
       assert(succ_empty(BB) && "The successor list of BB isn't empty before "
                                "applying corresponding DTU updates.");
-      ++I;
-    } else {
-      I = F.getBasicBlockList().erase(I);
     }
   }
 
@@ -2281,7 +2271,11 @@ bool llvm::removeUnreachableBlocks(Function &F, LazyValueInfo *LVI,
     }
     if (!Deleted)
       return false;
+  } else {
+    for (auto *BB : DeadBlockSet)
+      BB->eraseFromParent();
   }
+
   return true;
 }