[llvm] r373429 - [Local] Simplify function removeUnreachableBlocks() to avoid (re-)computation.

[Florian Hahn via llvm-commits]

Author: fhahn
Date: Wed Oct  2 00:37:41 2019
New Revision: 373429

URL: http://llvm.org/viewvc/llvm-project?rev=373429&view=rev
Log:
[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 <rcor.cs at gmail.com>

Reviewers: efriedma, spatel, fhahn, xbolva00

Reviewed By: fhahn, xbolva00

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

Modified:
    llvm/trunk/lib/Transforms/Utils/Local.cpp

Modified: llvm/trunk/lib/Transforms/Utils/Local.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/Local.cpp?rev=373429&r1=373428&r2=373429&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/Local.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/Local.cpp Wed Oct  2 00:37:41 2019
@@ -2215,7 +2215,7 @@ void llvm::removeUnwindEdge(BasicBlock *
 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...
@@ -2223,14 +2223,14 @@ bool llvm::removeUnreachableBlocks(Funct
     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)
@@ -2249,13 +2249,6 @@ bool llvm::removeUnreachableBlocks(Funct
     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())
@@ -2263,9 +2256,6 @@ bool llvm::removeUnreachableBlocks(Funct
       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(Funct
     }
     if (!Deleted)
       return false;
+  } else {
+    for (auto *BB : DeadBlockSet)
+      BB->eraseFromParent();
   }
+
   return true;
 }