From bbd51cd0a108ea4d47eb6f957e1409507097dfc9 Mon Sep 17 00:00:00 2001
From: Bill Wendling <isanbard@gmail.com>
Date: Wed, 31 Mar 2010 22:54:38 +0000
Subject: [PATCH] Rewrite CorrectExtraCFGEdges() to make it more
 understandable.

* Set the "DestA" and "DestB" according to how they're understood by the
  method. I.e., if one or both of them should point to the "fall through" block,
  then point to the fall through block.

* Improve the loop that removes superfluous edges to be more understandable.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@100056 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/CodeGen/MachineBasicBlock.cpp | 58 ++++++++++++-------------------
 1 file changed, 23 insertions(+), 35 deletions(-)

diff --git a/lib/CodeGen/MachineBasicBlock.cpp b/lib/CodeGen/MachineBasicBlock.cpp
index fbe4fc2f46f..bd0ccb40f53 100644
--- a/lib/CodeGen/MachineBasicBlock.cpp
+++ b/lib/CodeGen/MachineBasicBlock.cpp
@@ -23,6 +23,7 @@
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/Assembly/Writer.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/LeakDetector.h"
 #include "llvm/Support/raw_ostream.h"
@@ -459,54 +460,41 @@ bool MachineBasicBlock::CorrectExtraCFGEdges(MachineBasicBlock *DestA,
   //    conditional branch followed by an unconditional branch. DestA is the
   //    'true' destination and DestB is the 'false' destination.
 
-  bool MadeChange = false;
-  bool AddedFallThrough = false;
+  bool Changed = false;
 
   MachineFunction::iterator FallThru =
     llvm::next(MachineFunction::iterator(this));
-  
-  if (isCond) {
-    // If this block ends with a conditional branch that falls through to its
-    // successor, set DestB as the successor.
-    if (DestB == 0 && FallThru != getParent()->end()) {
+
+  if (DestA == 0 && DestB == 0) {
+    // Block falls through to successor.
+    DestA = FallThru;
+    DestB = FallThru;
+  } else if (DestA != 0 && DestB == 0) {
+    if (isCond)
+      // Block ends in conditional jump that falls through to successor.
       DestB = FallThru;
-      AddedFallThrough = true;
-    }
   } else {
-    // If this is an unconditional branch with no explicit dest, it must just be
-    // a fallthrough into DestA.
-    if (DestA == 0 && FallThru != getParent()->end()) {
-      DestA = FallThru;
-      AddedFallThrough = true;
-    }
+    assert(DestA && DestB && isCond &&
+           "CFG in a bad state. Cannot correct CFG edges");
   }
-  
+
+  // Remove superfluous edges. I.e., those which aren't destinations of this
+  // basic block, duplicate edges, or landing pads.
+  SmallPtrSet<const MachineBasicBlock*, 8> SeenMBBs;
   MachineBasicBlock::succ_iterator SI = succ_begin();
-  MachineBasicBlock *OrigDestA = DestA, *OrigDestB = DestB;
   while (SI != succ_end()) {
     const MachineBasicBlock *MBB = *SI;
-    if (MBB == DestA) {
-      DestA = 0;
-      ++SI;
-    } else if (MBB == DestB) {
-      DestB = 0;
-      ++SI;
-    } else if (MBB->isLandingPad() && 
-               MBB != OrigDestA && MBB != OrigDestB) {
-      ++SI;
-    } else {
-      // Otherwise, this is a superfluous edge, remove it.
+    if (!SeenMBBs.insert(MBB) ||
+        (MBB != DestA && MBB != DestB && !MBB->isLandingPad())) {
+      // This is a superfluous edge, remove it.
       SI = removeSuccessor(SI);
-      MadeChange = true;
+      Changed = true;
+    } else {
+      ++SI;
     }
   }
 
-  if (!AddedFallThrough)
-    assert(DestA == 0 && DestB == 0 && "MachineCFG is missing edges!");
-  else if (isCond)
-    assert(DestA == 0 && "MachineCFG is missing edges!");
-
-  return MadeChange;
+  return Changed;
 }
 
 /// findDebugLoc - find the next valid DebugLoc starting at MBBI, skipping