MachineBasicBlock::updateTerminator now requires an explicit layout successor.

Previously, it tried to infer the correct destination block from the
successor list, but this is a rather tricky propspect, given the
existence of successors that occur mid-block, such as invoke, and
potentially in the future, callbr/INLINEASM_BR. (INLINEASM_BR, in
particular would be problematic, because its successor blocks are not
distinct from "normal" successors, as EHPads are.)

Instead, require the caller to pass in the expected fallthrough
successor explicitly. In most callers, the correct block is
immediately clear. But, in MachineBlockPlacement, we do need to record
the original ordering, before starting to reorder blocks.

Unfortunately, the goal of decoupling the behavior of end-of-block
jumps from the successor list has not been fully accomplished in this
patch, as there is currently no other way to determine whether a block
is intended to fall-through, or end as unreachable. Further work is
needed there.

Differential Revision: https://reviews.llvm.org/D79605

1 files changed, 40 insertions, 53 deletions

diff --git a/llvm/lib/CodeGen/MachineBasicBlock.cpp b/llvm/lib/CodeGen/MachineBasicBlock.cpp
index 9abd24e..e848741 100644
--- a/llvm/lib/CodeGen/MachineBasicBlock.cpp
+++ b/llvm/lib/CodeGen/MachineBasicBlock.cpp
@@ -559,7 +559,11 @@ void MachineBasicBlock::moveAfter(MachineBasicBlock *NewBefore) {
   getParent()->splice(++NewBefore->getIterator(), getIterator());
 }
 
-void MachineBasicBlock::updateTerminator() {
+void MachineBasicBlock::updateTerminator(
+    MachineBasicBlock *PreviousLayoutSuccessor) {
+  LLVM_DEBUG(dbgs() << "Updating terminators on " << printMBBReference(*this)
+                    << "\n");
+
   const TargetInstrInfo *TII = getParent()->getSubtarget().getInstrInfo();
   // A block with no successors has no concerns with fall-through edges.
   if (this->succ_empty())
@@ -578,25 +582,21 @@ void MachineBasicBlock::updateTerminator() {
       if (isLayoutSuccessor(TBB))
         TII->removeBranch(*this);
     } else {
-      // The block has an unconditional fallthrough. If its successor is not its
-      // layout successor, insert a branch. First we have to locate the only
-      // non-landing-pad successor, as that is the fallthrough block.
-      for (succ_iterator SI = succ_begin(), SE = succ_end(); SI != SE; ++SI) {
-        if ((*SI)->isEHPad())
-          continue;
-        assert(!TBB && "Found more than one non-landing-pad successor!");
-        TBB = *SI;
-      }
-
-      // If there is no non-landing-pad successor, the block has no fall-through
-      // edges to be concerned with.
-      if (!TBB)
+      // The block has an unconditional fallthrough, or the end of the block is
+      // unreachable.
+
+      // Unfortunately, whether the end of the block is unreachable is not
+      // immediately obvious; we must fall back to checking the successor list,
+      // and assuming that if the passed in block is in the succesor list and
+      // not an EHPad, it must be the intended target.
+      if (!PreviousLayoutSuccessor || !isSuccessor(PreviousLayoutSuccessor) ||
+          PreviousLayoutSuccessor->isEHPad())
         return;
 
-      // Finally update the unconditional successor to be reached via a branch
-      // if it would not be reached by fallthrough.
-      if (!isLayoutSuccessor(TBB))
-        TII->insertBranch(*this, TBB, nullptr, Cond, DL);
+      // If the unconditional successor block is not the current layout
+      // successor, insert a branch to jump to it.
+      if (!isLayoutSuccessor(PreviousLayoutSuccessor))
+        TII->insertBranch(*this, PreviousLayoutSuccessor, nullptr, Cond, DL);
     }
     return;
   }
@@ -617,38 +617,20 @@ void MachineBasicBlock::updateTerminator() {
     return;
   }
 
-  // Walk through the successors and find the successor which is not a landing
-  // pad and is not the conditional branch destination (in TBB) as the
-  // fallthrough successor.
-  MachineBasicBlock *FallthroughBB = nullptr;
-  for (succ_iterator SI = succ_begin(), SE = succ_end(); SI != SE; ++SI) {
-    if ((*SI)->isEHPad() || *SI == TBB)
-      continue;
-    assert(!FallthroughBB && "Found more than one fallthrough successor.");
-    FallthroughBB = *SI;
-  }
-
-  if (!FallthroughBB) {
-    if (canFallThrough()) {
-      // We fallthrough to the same basic block as the conditional jump targets.
-      // Remove the conditional jump, leaving unconditional fallthrough.
-      // FIXME: This does not seem like a reasonable pattern to support, but it
-      // has been seen in the wild coming out of degenerate ARM test cases.
-      TII->removeBranch(*this);
-
-      // Finally update the unconditional successor to be reached via a branch if
-      // it would not be reached by fallthrough.
-      if (!isLayoutSuccessor(TBB))
-        TII->insertBranch(*this, TBB, nullptr, Cond, DL);
-      return;
-    }
+  // We now know we're going to fallthrough to PreviousLayoutSuccessor.
+  assert(PreviousLayoutSuccessor);
+  assert(!PreviousLayoutSuccessor->isEHPad());
+  assert(isSuccessor(PreviousLayoutSuccessor));
 
-    // We enter here iff exactly one successor is TBB which cannot fallthrough
-    // and the rest successors if any are EHPads.  In this case, we need to
-    // change the conditional branch into unconditional branch.
+  if (PreviousLayoutSuccessor == TBB) {
+    // We had a fallthrough to the same basic block as the conditional jump
+    // targets.  Remove the conditional jump, leaving an unconditional
+    // fallthrough or an unconditional jump.
     TII->removeBranch(*this);
-    Cond.clear();
-    TII->insertBranch(*this, TBB, nullptr, Cond, DL);
+    if (!isLayoutSuccessor(TBB)) {
+      Cond.clear();
+      TII->insertBranch(*this, TBB, nullptr, Cond, DL);
+    }
     return;
   }
 
@@ -657,14 +639,14 @@ void MachineBasicBlock::updateTerminator() {
     if (TII->reverseBranchCondition(Cond)) {
       // We can't reverse the condition, add an unconditional branch.
       Cond.clear();
-      TII->insertBranch(*this, FallthroughBB, nullptr, Cond, DL);
+      TII->insertBranch(*this, PreviousLayoutSuccessor, nullptr, Cond, DL);
       return;
     }
     TII->removeBranch(*this);
-    TII->insertBranch(*this, FallthroughBB, nullptr, Cond, DL);
-  } else if (!isLayoutSuccessor(FallthroughBB)) {
+    TII->insertBranch(*this, PreviousLayoutSuccessor, nullptr, Cond, DL);
+  } else if (!isLayoutSuccessor(PreviousLayoutSuccessor)) {
     TII->removeBranch(*this);
-    TII->insertBranch(*this, TBB, FallthroughBB, Cond, DL);
+    TII->insertBranch(*this, TBB, PreviousLayoutSuccessor, Cond, DL);
   }
 }
 
@@ -908,6 +890,7 @@ MachineBasicBlock *MachineBasicBlock::SplitCriticalEdge(
     return nullptr;
 
   MachineFunction *MF = getParent();
+  MachineBasicBlock *PrevFallthrough = getNextNode();
   DebugLoc DL;  // FIXME: this is nowhere
 
   MachineBasicBlock *NMBB = MF->CreateMachineBasicBlock();
@@ -978,7 +961,11 @@ MachineBasicBlock *MachineBasicBlock::SplitCriticalEdge(
       Terminators.push_back(&*I);
   }
 
-  updateTerminator();
+  // Since we replaced all uses of Succ with NMBB, that should also be treated
+  // as the fallthrough successor
+  if (Succ == PrevFallthrough)
+    PrevFallthrough = NMBB;
+  updateTerminator(PrevFallthrough);
 
   if (Indexes) {
     SmallVector<MachineInstr*, 4> NewTerminators;