From: Chris Lattner <sabre@nondot.org>
Date: Tue, 24 Jun 2003 21:49:45 +0000 (+0000)
Subject: Do not mark ALL terminators live if any instruciton in the block is live.  We only
X-Git-Url: http://plrg.eecs.uci.edu/git/?a=commitdiff_plain;h=99c91e021a41150ac55edf91d1339b5c58a122dc;p=oota-llvm.git

Do not mark ALL terminators live if any instruciton in the block is live.  We only
want to mark it live if it is an unconditional branch.  This fixes bug:
ADCE/2002-05-28-Crash.ll and makes this pass _much_ more useful.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@6887 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Transforms/Scalar/ADCE.cpp b/lib/Transforms/Scalar/ADCE.cpp
index 15ae7677f22..38dcff1beaa 100644
--- a/lib/Transforms/Scalar/ADCE.cpp
+++ b/lib/Transforms/Scalar/ADCE.cpp
@@ -105,8 +105,11 @@ void ADCE::markBlockAlive(BasicBlock *BB) {
              bind_obj(this, &ADCE::markTerminatorLive));
   }
   
-  // If this basic block is live, then the terminator must be as well!
-  markTerminatorLive(BB);
+  // If this basic block is live, and it ends in an unconditional branch, then
+  // the branch is alive as well...
+  if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
+    if (BI->isUnconditional())
+      markTerminatorLive(BB);
 }
 
 // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
@@ -248,6 +251,7 @@ bool ADCE::doADCE() {
       NewEntry->getInstList().push_back(new BranchInst(&Func->front()));
       Func->getBasicBlockList().push_front(NewEntry);
       AliveBlocks.insert(NewEntry);    // This block is always alive!
+      LiveSet.insert(NewEntry->getTerminator());  // The branch is live
     }
     
     // Loop over all of the alive blocks in the function.  If any successor
@@ -260,6 +264,23 @@ bool ADCE::doADCE() {
         BasicBlock *BB = I;
         TerminatorInst *TI = BB->getTerminator();
       
+        // If the terminator instruction is alive, but the block it is contained
+        // in IS alive, this means that this terminator is a conditional branch
+        // on a condition that doesn't matter.  Make it an unconditional branch
+        // to ONE of the successors.  This has the side effect of dropping a use
+        // of the conditional value, which may also be dead.
+        if (!LiveSet.count(TI)) {
+          assert(TI->getNumSuccessors() > 1 && "Not a conditional?");
+          BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI);
+
+          // Remove entries from PHI nodes to avoid confusing ourself later...
+          for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
+              TI->getSuccessor(i)->removePredecessor(BB);
+
+          BB->getInstList().erase(TI);
+          TI = NB;
+        }
+
         // Loop over all of the successors, looking for ones that are not alive.
         // We cannot save the number of successors in the terminator instruction
         // here because we may remove them if we don't have a postdominator...