BranchRelaxation: Fix computing indirect branch block size

[arsenm]

Fix new assertion failing on AArch64 due to updateTerminator modifying block sizes

[arsenm]

Add test which would infinite loop before

[nhaehnle]

lib/CodeGen/BranchRelaxation.cpp

210–212

Is this comment still accurate?

234–244

I think just using OrigBB instead of MBB would help readability.

401–406

This doesn't seem to be affected by your change, but I'm a bit confused about not seeing anything to account for the size change in MBB due to the erased MI.

[arsenm]

Address comments

[arsenm]

lib/CodeGen/BranchRelaxation.cpp

401–406

This is part of the change. I discovered that added/removed instruction size here is insufficient, since updateTerminator can also sneakily change the branch instructions.

[nhaehnle]

LGTM

[arsenm]

r285828

lib/CodeGen/BranchRelaxation.cpp

class BranchRelaxation : public MachineFunctionPass {
   MachineFunction *MF;
   const TargetInstrInfo *TII;
 
   bool relaxBranchInstructions();
   void scanFunction();
 
   MachineBasicBlock *createNewBlockAfter(MachineBasicBlock &BB);
 
-  MachineBasicBlock *splitBlockBeforeInstr(MachineInstr &MI);
+  MachineBasicBlock *splitBlockBeforeInstr(MachineInstr &MI,
+                                           MachineBasicBlock *DestBB);
   void adjustBlockOffsets(MachineBasicBlock &MBB);
   bool isBlockInRange(const MachineInstr &MI, const MachineBasicBlock &BB) const;
 
   bool fixupConditionalBranch(MachineInstr &MI);
   bool fixupUnconditionalBranch(MachineInstr &MI);
   uint64_t computeBlockSize(const MachineBasicBlock &MBB) const;
   unsigned getInstrOffset(const MachineInstr &MI) const;
   void dumpBBs();

 void BranchRelaxation::verify() {
 #ifndef NDEBUG
   unsigned PrevNum = MF->begin()->getNumber();
   for (MachineBasicBlock &MBB : *MF) {
     unsigned Align = MBB.getAlignment();
     unsigned Num = MBB.getNumber();
     assert(BlockInfo[Num].Offset % (1u << Align) == 0);
     assert(!Num || BlockInfo[PrevNum].postOffset(MBB) <= BlockInfo[Num].Offset);
+    assert(BlockInfo[Num].Size == computeBlockSize(MBB));
     PrevNum = Num;
   }
 #endif
 }
 
 /// print block size and offset information - debugging
 void BranchRelaxation::dumpBBs() {
   for (auto &MBB : *MF) {
MachineBasicBlock *BranchRelaxation::createNewBlockAfter(MachineBasicBlock &BB) {
   BlockInfo.insert(BlockInfo.begin() + NewBB->getNumber(), BasicBlockInfo());
 
   return NewBB;
 }
 
 /// Split the basic block containing MI into two blocks, which are joined by
 /// an unconditional branch.  Update data structures and renumber blocks to
 /// account for this change and returns the newly created block.
 /// NOTE: Successor list of the original BB is out of date after this function,
 /// and must be updated by the caller! Other transforms follow using this
 /// utility function, so no point updating now rather than waiting.
-MachineBasicBlock *BranchRelaxation::splitBlockBeforeInstr(MachineInstr &MI) {
+MachineBasicBlock *BranchRelaxation::splitBlockBeforeInstr(MachineInstr &MI,
+                                                           MachineBasicBlock *DestBB) {
   MachineBasicBlock *OrigBB = MI.getParent();
 
   // Create a new MBB for the code after the OrigBB.
   MachineBasicBlock *NewBB =
       MF->CreateMachineBasicBlock(OrigBB->getBasicBlock());
   MF->insert(++OrigBB->getIterator(), NewBB);
 
   // Splice the instructions starting with MI over to NewBB.
   NewBB->splice(NewBB->end(), OrigBB, MI.getIterator(), OrigBB->end());
 
   // Add an unconditional branch from OrigBB to NewBB.
   // Note the new unconditional branch is not being recorded.
   // There doesn't seem to be meaningful DebugInfo available; this doesn't
   // correspond to anything in the source.
   TII->insertUnconditionalBranch(*OrigBB, NewBB, DebugLoc());
 
   // Insert an entry into BlockInfo to align it properly with the block numbers.
   BlockInfo.insert(BlockInfo.begin() + NewBB->getNumber(), BasicBlockInfo());
 
+  MachineBasicBlock &MBB = *OrigBB;
+
+  NewBB->transferSuccessors(&MBB);
+  MBB.addSuccessor(NewBB);
+  MBB.addSuccessor(DestBB);
+
+  // Cleanup potential unconditional branch to successor block.
+  // Note that updateTerminator may change the size of the blocks.
+  NewBB->updateTerminator();
+  MBB.updateTerminator();
+
   // Figure out how large the OrigBB is.  As the first half of the original
   // block, it cannot contain a tablejump.  The size includes
   // the new jump we added.  (It should be possible to do this without
   // recounting everything, but it's very confusing, and this is rarely
   // executed.)
   BlockInfo[OrigBB->getNumber()].Size = computeBlockSize(*OrigBB);
 
   // Figure out how large the NewMBB is. As the second half of the original
for (const MachineBasicBlock *Succ : MBB->successors()) {
         BranchBB->addLiveIn(LiveIn);
     }
 
     BranchBB->sortUniqueLiveIns();
     BranchBB->addSuccessor(DestBB);
     MBB->replaceSuccessor(DestBB, BranchBB);
   }
 
   DebugLoc DL = MI.getDebugLoc();
   MI.eraseFromParent();
-
-  // insertUnconditonalBranch may have inserted a new block.
-  BlockInfo[MBB->getNumber()].Size += TII->insertIndirectBranch(
+  BlockInfo[BranchBB->getNumber()].Size += TII->insertIndirectBranch(
     *BranchBB, *DestBB, DL, DestOffset - SrcOffset, RS.get());
 
-  computeBlockSize(*BranchBB);
   adjustBlockOffsets(*MBB);
   return true;
 }
 
 bool BranchRelaxation::relaxBranchInstructions() {
   bool Changed = false;
 
   // Relaxing branches involves creating new basic blocks, so re-eval
   // end() for termination.
for (MachineBasicBlock::iterator J = MBB.getFirstTerminator();
       if (MI.isConditionalBranch()) {
         MachineBasicBlock *DestBB = TII->getBranchDestBlock(MI);
         if (!isBlockInRange(MI, *DestBB)) {
           if (Next != MBB.end() && Next->isConditionalBranch()) {
             // If there are multiple conditional branches, this isn't an
             // analyzable block. Split later terminators into a new block so
             // each one will be analyzable.
 
-            MachineBasicBlock *NewBB = splitBlockBeforeInstr(*Next);
-            NewBB->transferSuccessors(&MBB);
-            MBB.addSuccessor(NewBB);
-            MBB.addSuccessor(DestBB);
-
-            // Cleanup potential unconditional branch to successor block.
-            NewBB->updateTerminator();
-            MBB.updateTerminator();
+            splitBlockBeforeInstr(*Next, DestBB);
           } else {
             fixupConditionalBranch(MI);
             ++NumConditionalRelaxed;
           }
 
           Changed = true;
 
           // This may have modified all of the terminators, so start over.