[AVR] Insert unconditional branch when inserting MBBs between blocks with fallthrough

This updates the AVR Select8/Select16 expansion code so that, when
inserting the two basic blocks for true and false conditions, any
existing fallthrough on the previous block is preserved.

Prior to this patch, if the block before the Select pseudo fell through
to the subsequent block, two new basic blocks would be inserted at the
prior fallthrough point, changing the fallthrough destination.

The predecessor or successor lists were not updated, causing the
BranchFolding pass at -O1 and above the rearrange basic blocks, causing
an infinite loop. Not to mention the unconditional fallthrough to the
true block is incorrect in of itself.

This patch modifies the Select8/16 expansion so that, if inserting true
and false basic blocks at a fallthrough point, the implicit branch is
preserved by means of an explicit, unconditional branch to the previous
fallthrough destination.

Thanks to Carl Peto for reporting this bug.

This fixes avr-rust bug https://github.com/avr-rust/rust/issues/123.

llvm-svn: 351718

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/Target/AVR/AVRISelLowering.cpp b/llvm/lib/Target/AVR/AVRISelLowering.cpp
index 3116eb2a503..e95a41472bf 100644
--- a/llvm/lib/Target/AVR/AVRISelLowering.cpp
+++ b/llvm/lib/Target/AVR/AVRISelLowering.cpp
@@ -1634,6 +1634,15 @@ AVRTargetLowering::EmitInstrWithCustomInserter(MachineInstr &MI,
 
   MachineFunction *MF = MBB->getParent();
   const BasicBlock *LLVM_BB = MBB->getBasicBlock();
+  MachineBasicBlock *FallThrough = MBB->getFallThrough();
+
+  // If the current basic block falls through to another basic block,
+  // we must insert an unconditional branch to the fallthrough destination
+  // if we are to insert basic blocks at the prior fallthrough point.
+  if (FallThrough != nullptr) {
+    BuildMI(MBB, dl, TII.get(AVR::RJMPk)).addMBB(FallThrough);
+  }
+
   MachineBasicBlock *trueMBB = MF->CreateMachineBasicBlock(LLVM_BB);
   MachineBasicBlock *falseMBB = MF->CreateMachineBasicBlock(LLVM_BB);