[llvm-commits] [llvm] r100056 - /llvm/trunk/lib/CodeGen/MachineBasicBlock.cpp

[Bill Wendling]

Author: void
Date: Wed Mar 31 17:54:38 2010
New Revision: 100056

URL: http://llvm.org/viewvc/llvm-project?rev=100056&view=rev
Log:
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.

Modified:
    llvm/trunk/lib/CodeGen/MachineBasicBlock.cpp

Modified: llvm/trunk/lib/CodeGen/MachineBasicBlock.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/MachineBasicBlock.cpp?rev=100056&r1=100055&r2=100056&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/MachineBasicBlock.cpp (original)
+++ llvm/trunk/lib/CodeGen/MachineBasicBlock.cpp Wed Mar 31 17:54:38 2010
@@ -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 @@
   //    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