Refactor code to create getFallThrough method in MachineBasicBlock.

[jsjodin]

Summary

This will be needed by the MachineCFGStructurizer.

[qcolombet]

Hi,

LGTM.

Please next time upload a patch with the full context (diff -U9999999).

Cheers,
-Quentin

include/llvm/CodeGen/MachineBasicBlock.h

448 ↗

(On Diff #79766)

double spaces (return__is)

llvm/trunk/include/llvm/CodeGen/MachineBasicBlock.h

#endif
 
   /// Return true if the specified MBB will be emitted immediately after this
   /// block, such that if this block exits by falling through, control will
   /// transfer to the specified MBB. Note that MBB need not be a successor at
   /// all, for example if this block ends with an unconditional branch to some
   /// other block.
   bool isLayoutSuccessor(const MachineBasicBlock *MBB) const;
 
-  /// Return true if the block can implicitly transfer control to the block
-  /// after it by falling off the end of it.  This should return false if it can
-  /// reach the block after it, but it uses an explicit branch to do so (e.g., a
-  /// table jump).  True is a conservative answer.
+
+  /// Return the fallthrough block if the block can implicitly
+  /// transfer control to the block after it by falling off the end of
+  /// it.  This should return null if it can reach the block after
+  /// it, but it uses an explicit branch to do so (e.g., a table
+  /// jump).  Non-null return  is a conservative answer.
+  MachineBasicBlock *getFallThrough();
+
+  /// Return true if the block can implicitly transfer control to the
+  /// block after it by falling off the end of it.  This should return
+  /// false if it can reach the block after it, but it uses an
+  /// explicit branch to do so (e.g., a table jump).  True is a
+  /// conservative answer.
   bool canFallThrough();
 
   /// Returns a pointer to the first instruction in this block that is not a
   /// PHINode instruction. When adding instructions to the beginning of the
   /// basic block, they should be added before the returned value, not before
   /// the first instruction, which might be PHI.
   /// Returns end() is there's no non-PHI instruction.
   iterator getFirstNonPHI();

llvm/trunk/lib/CodeGen/MachineBasicBlock.cpp

bool MachineBasicBlock::isSuccessor(const MachineBasicBlock *MBB) const {
   return is_contained(successors(), MBB);
 }
 
 bool MachineBasicBlock::isLayoutSuccessor(const MachineBasicBlock *MBB) const {
   MachineFunction::const_iterator I(this);
   return std::next(I) == MachineFunction::const_iterator(MBB);
 }
 
-bool MachineBasicBlock::canFallThrough() {
+MachineBasicBlock *MachineBasicBlock::getFallThrough() {
   MachineFunction::iterator Fallthrough = getIterator();
   ++Fallthrough;
   // If FallthroughBlock is off the end of the function, it can't fall through.
   if (Fallthrough == getParent()->end())
-    return false;
+    return nullptr;
 
   // If FallthroughBlock isn't a successor, no fallthrough is possible.
   if (!isSuccessor(&*Fallthrough))
-    return false;
+    return nullptr;
 
   // Analyze the branches, if any, at the end of the block.
   MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
   SmallVector<MachineOperand, 4> Cond;
   const TargetInstrInfo *TII = getParent()->getSubtarget().getInstrInfo();
   if (TII->analyzeBranch(*this, TBB, FBB, Cond)) {
     // If we couldn't analyze the branch, examine the last instruction.
     // If the block doesn't end in a known control barrier, assume fallthrough
     // is possible. The isPredicated check is needed because this code can be
     // called during IfConversion, where an instruction which is normally a
     // Barrier is predicated and thus no longer an actual control barrier.
-    return empty() || !back().isBarrier() || TII->isPredicated(back());
+    return (empty() || !back().isBarrier() || TII->isPredicated(back()))
+               ? &*Fallthrough
+               : nullptr;
   }
 
   // If there is no branch, control always falls through.
-  if (!TBB) return true;
+  if (!TBB) return &*Fallthrough;
 
   // If there is some explicit branch to the fallthrough block, it can obviously
   // reach, even though the branch should get folded to fall through implicitly.
   if (MachineFunction::iterator(TBB) == Fallthrough ||
       MachineFunction::iterator(FBB) == Fallthrough)
-    return true;
+    return &*Fallthrough;
 
   // If it's an unconditional branch to some block not the fall through, it
   // doesn't fall through.
-  if (Cond.empty()) return false;
+  if (Cond.empty()) return nullptr;
 
   // Otherwise, if it is conditional and has no explicit false block, it falls
   // through.
-  return FBB == nullptr;
+  return (FBB == nullptr) ? &*Fallthrough : nullptr;
+}
+
+bool MachineBasicBlock::canFallThrough() {
+  return getFallThrough() != nullptr;
 }
 
 MachineBasicBlock *MachineBasicBlock::SplitCriticalEdge(MachineBasicBlock *Succ,
                                                         Pass &P) {
   if (!canSplitCriticalEdge(Succ))
     return nullptr;
 
   MachineFunction *MF = getParent();