From e2c6d131d12c779a410740e0a90545def75e0f48 Mon Sep 17 00:00:00 2001
From: Dan Gohman <gohman@apple.com>
Date: Sat, 14 Aug 2010 00:29:42 +0000
Subject: Teach SimplifyCFG how to simplify indirectbr instructions.  -
 Eliminate redundant successors.  - Convert an indirectbr with one successor
 into a direct branch.

Also, generalize SimplifyCFG to be able to be run on a function entry block.
It knows quite a few simplifications which are applicable to the entry
block, and it only needs a few checks to avoid trouble with the entry block.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@111060 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/Utils/Local.cpp       |  3 ++
 lib/Transforms/Utils/SimplifyCFG.cpp | 53 +++++++++++++++++++++++++++---------
 2 files changed, 43 insertions(+), 13 deletions(-)

(limited to 'lib/Transforms/Utils')

diff --git a/lib/Transforms/Utils/Local.cpp b/lib/Transforms/Utils/Local.cpp
index 8e9113871f..52f0499f39 100644
--- a/lib/Transforms/Utils/Local.cpp
+++ b/lib/Transforms/Utils/Local.cpp
@@ -490,6 +490,9 @@ static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
 /// rewriting all the predecessors to branch to the successor block and return
 /// true.  If we can't transform, return false.
 bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB) {
+  assert(BB != &BB->getParent()->getEntryBlock() &&
+         "TryToSimplifyUncondBranchFromEmptyBlock called on entry block!");
+
   // We can't eliminate infinite loops.
   BasicBlock *Succ = cast<BranchInst>(BB->getTerminator())->getSuccessor(0);
   if (BB == Succ) return false;
diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index 36a245ff8c..db719a0ce4 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1724,12 +1724,12 @@ bool SimplifyCFGOpt::run(BasicBlock *BB) {
 
   assert(BB && BB->getParent() && "Block not embedded in function!");
   assert(BB->getTerminator() && "Degenerate basic block encountered!");
-  assert(&BB->getParent()->getEntryBlock() != BB &&
-         "Can't Simplify entry block!");
 
-  // Remove basic blocks that have no predecessors... or that just have themself
-  // as a predecessor.  These are unreachable.
-  if (pred_begin(BB) == pred_end(BB) || BB->getSinglePredecessor() == BB) {
+  // Remove basic blocks that have no predecessors (except the entry block)...
+  // or that just have themself as a predecessor.  These are unreachable.
+  if ((pred_begin(BB) == pred_end(BB) &&
+       &BB->getParent()->getEntryBlock() != BB) ||
+      BB->getSinglePredecessor() == BB) {
     DEBUG(dbgs() << "Removing BB: \n" << *BB);
     DeleteDeadBlock(BB);
     return true;
@@ -1880,8 +1880,9 @@ bool SimplifyCFGOpt::run(BasicBlock *BB) {
       while (isa<DbgInfoIntrinsic>(BBI))
         ++BBI;
       if (BBI->isTerminator()) // Terminator is the only non-phi instruction!
-        if (TryToSimplifyUncondBranchFromEmptyBlock(BB))
-          return true;
+        if (BB != &BB->getParent()->getEntryBlock())
+          if (TryToSimplifyUncondBranchFromEmptyBlock(BB))
+            return true;
       
     } else {  // Conditional branch
       if (isValueEqualityComparison(BI)) {
@@ -2049,12 +2050,37 @@ bool SimplifyCFGOpt::run(BasicBlock *BB) {
       }
 
       // If this block is now dead, remove it.
-      if (pred_begin(BB) == pred_end(BB)) {
+      if (pred_begin(BB) == pred_end(BB) &&
+          BB != &BB->getParent()->getEntryBlock()) {
         // We know there are no successors, so just nuke the block.
         M->getBasicBlockList().erase(BB);
         return true;
       }
     }
+  } else if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(BB->getTerminator())) {
+    // Eliminate redundant destinations.
+    SmallPtrSet<Value *, 8> Succs;
+    for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) {
+      BasicBlock *Dest = IBI->getDestination(i);
+      if (!Succs.insert(Dest)) {
+        Dest->removePredecessor(BB);
+        IBI->removeDestination(i);
+        --i; --e;
+        Changed = true;
+      }
+    } 
+
+    if (IBI->getNumDestinations() == 0) {
+      // If the indirectbr has no successors, change it to unreachable.
+      new UnreachableInst(IBI->getContext(), IBI);
+      IBI->eraseFromParent();
+      Changed = true;
+    } else if (IBI->getNumDestinations() == 1) {
+      // If the indirectbr has one successor, change it to a direct branch.
+      BranchInst::Create(IBI->getDestination(0), IBI);
+      IBI->eraseFromParent();
+      Changed = true;
+    }
   }
 
   // Merge basic blocks into their predecessor if there is only one distinct
@@ -2068,12 +2094,15 @@ bool SimplifyCFGOpt::run(BasicBlock *BB) {
   // is a conditional branch, see if we can hoist any code from this block up
   // into our predecessor.
   pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
-  BasicBlock *OnlyPred = *PI++;
-  for (; PI != PE; ++PI)  // Search all predecessors, see if they are all same
-    if (*PI != OnlyPred) {
+  BasicBlock *OnlyPred = 0;
+  for (; PI != PE; ++PI) { // Search all predecessors, see if they are all same
+    if (!OnlyPred)
+      OnlyPred = *PI;
+    else if (*PI != OnlyPred) {
       OnlyPred = 0;       // There are multiple different predecessors...
       break;
     }
+  }
   
   if (OnlyPred)
     if (BranchInst *BI = dyn_cast<BranchInst>(OnlyPred->getTerminator()))
@@ -2172,8 +2201,6 @@ bool SimplifyCFGOpt::run(BasicBlock *BB) {
 /// eliminates unreachable basic blocks, and does other "peephole" optimization
 /// of the CFG.  It returns true if a modification was made.
 ///
-/// WARNING:  The entry node of a function may not be simplified.
-///
 bool llvm::SimplifyCFG(BasicBlock *BB, const TargetData *TD) {
   return SimplifyCFGOpt(TD).run(BB);
 }
-- 
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