From 4af3ccbbea4f6d2cf2d90b9b004c3da59f33510c Mon Sep 17 00:00:00 2001
From: Chris Lattner <sabre@nondot.org>
Date: Fri, 6 Sep 2002 02:35:34 +0000
Subject: [PATCH] Check in the implementation of critical edge detection and
 splitting

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@3588 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/Utils/CriticalEdge.cpp | 84 +++++++++++++++++++++++++++
 1 file changed, 84 insertions(+)
 create mode 100644 lib/Transforms/Utils/CriticalEdge.cpp

diff --git a/lib/Transforms/Utils/CriticalEdge.cpp b/lib/Transforms/Utils/CriticalEdge.cpp
new file mode 100644
index 00000000000..07a3a542e55
--- /dev/null
+++ b/lib/Transforms/Utils/CriticalEdge.cpp
@@ -0,0 +1,84 @@
+//===-- Local.h - Functions to perform local transformations -----*- C++ -*--=//
+//
+// This family of functions perform various local transformations to the
+// program.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/iTerminators.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Support/CFG.h"
+
+// isCriticalEdge - Return true if the specified edge is a critical edge.
+// Critical edges are edges from a block with multiple successors to a block
+// with multiple predecessors.
+//
+bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
+  assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
+  if (TI->getNumSuccessors() <= 1) return false;
+
+  const BasicBlock *Dest = TI->getSuccessor(SuccNum);
+  pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
+
+  // If there is more than one predecessor, this is a critical edge...
+  assert(I != E && "No preds, but we have an edge to the block?");
+  ++I;        // Skip one edge due to the incoming arc from TI.
+  return I != E;
+}
+
+// SplitCriticalEdge - Insert a new node node to split the critical edge.  This
+// will update DominatorSet, ImmediateDominator and DominatorTree information if
+// it is available, thus calling this pass will not invalidate either of them.
+//
+void SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
+  assert(isCriticalEdge(TI, SuccNum) &&
+         "Cannot break a critical edge, if it isn't a critical edge");
+  BasicBlock *TIBB = TI->getParent();
+
+  // Create a new basic block, linking it into the CFG.
+  BasicBlock *NewBB = new BasicBlock(TIBB->getName()+"_crit_edge");
+
+  // Create our unconditional branch...
+  BranchInst *BI = new BranchInst(TI->getSuccessor(SuccNum));
+  NewBB->getInstList().push_back(BI);
+  
+  // Branch to the new block, breaking the edge...
+  TI->setSuccessor(SuccNum, NewBB);
+
+  // Insert the block into the function... right after the block TI lives in.
+  Function &F = *TIBB->getParent();
+  F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
+
+  // Now if we have a pass object, update analysis information.  Currently we
+  // update DominatorSet and DominatorTree information if it's available.
+  //
+  if (P) {
+    // Should we update DominatorSet information?
+    if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
+      // The blocks that dominate the new one are the blocks that dominate TIBB
+      // plus the new block itself.
+      DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
+      DomSet.insert(NewBB);  // A block always dominates itself.
+      DS->addBasicBlock(NewBB, DomSet);
+    }
+
+    // Should we update ImmdediateDominator information?
+    if (ImmediateDominators *ID =
+        P->getAnalysisToUpdate<ImmediateDominators>()) {
+      // TIBB is the new immediate dominator for NewBB.  NewBB doesn't dominate
+      // anything.
+      ID->addNewBlock(NewBB, TIBB);
+    }
+
+    // Should we update DominatorTree information?
+    if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
+      DominatorTree::Node *TINode = DT->getNode(TIBB);
+
+      // The new block is not the immediate dominator for any other nodes, but
+      // TINode is the immediate dominator for the new node.
+      //
+      DT->createNewNode(NewBB, TINode);
+    }
+  }
+}