From 167b0529be766a85db3c0811607ac60f821670ff Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo@fhahn.com>
Date: Wed, 2 Oct 2019 07:37:41 +0000
Subject: [PATCH] [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@gmail.com>

Reviewers: efriedma, spatel, fhahn, xbolva00

Reviewed By: fhahn, xbolva00

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

llvm-svn: 373429
---
 llvm/lib/Transforms/Utils/Local.cpp | 26 ++++++++++----------------
 1 file changed, 10 insertions(+), 16 deletions(-)

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 2285b6c..eabf4bf 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -2215,7 +2215,7 @@ 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...
@@ -2223,14 +2223,14 @@ bool llvm::removeUnreachableBlocks(Function &F, LazyValueInfo *LVI,
     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(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())
@@ -2263,9 +2256,6 @@ bool llvm::removeUnreachableBlocks(Function &F, LazyValueInfo *LVI,
       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;
 }
 
-- 
2.7.4