¶llvm::BasicBlock* CreateControlFlowHub(
    llvm::DomTreeUpdater* DTU,
    SmallVectorImpl<llvm::BasicBlock*>&
        GuardBlocks,
    const SetVector<llvm::BasicBlock*>&
        Predecessors,
    const SetVector<llvm::BasicBlock*>&
        Successors,
    const llvm::StringRef Prefix)

Description

Given a set of incoming and outgoing blocks, create a "hub" such that every edge from an incoming block InBB to an outgoing block OutBB is now split into two edges, one from InBB to the hub and another from the hub to OutBB. The hub consists of a series of guard blocks, one for each outgoing block. Each guard block conditionally branches to the corresponding outgoing block, or the next guard block in the chain. These guard blocks are returned in the argument vector. Since the control flow edges from InBB to OutBB have now been replaced, the function also updates any PHINodes in OutBB. For each such PHINode, the operands corresponding to incoming blocks are moved to a new PHINode in the hub, and the hub is made an operand of the original PHINode. Input CFG: ---------- Def | v In1 In2 | | | | v v Foo ---> Out1 Out2 | v Use Create hub: Incoming = {In1, In2}, Outgoing = {Out1, Out2} ---------------------------------------------------------- Def | v In1 In2 Foo | Hub | | | + - - | - - + | | ' v ' V +------> Guard1 -----> Out1 ' | ' ' v ' ' Guard2 -----> Out2 ' ' | + - - - - - + | v Use Limitations: ----------- 1. This assumes that all terminators in the CFG are direct branches (the "br" instruction). The presence of any other control flow such as indirectbr, switch or callbr will cause an assert. 2. The updates to the PHINodes are not sufficient to restore SSA form. Consider a definition Def, its use Use, incoming block In2 and outgoing block Out2, such that: a. In2 is reachable from D or contains D. b. U is reachable from Out2 or is contained in Out2. c. U is not a PHINode if U is contained in Out2. Clearly, Def dominates Out2 since the program is valid SSA. But when the hub is introduced, there is a new path through the hub along which Use is reachable from entry without passing through Def, and SSA is no longer valid. To fix this, we need to look at all the blocks post-dominated by the hub on the one hand, and dominated by Out2 on the other. This is left for the caller to accomplish, since each specific use of this function may have additional information which simplifies this fixup. For example, see restoreSSA() in the UnifyLoopExits pass.

Declared at: llvm/include/llvm/Transforms/Utils/BasicBlockUtils.h:577

Parameters

llvm::DomTreeUpdater* DTU

SmallVectorImpl<llvm::BasicBlock*>& GuardBlocks

const SetVector<llvm::BasicBlock*>& Predecessors

const SetVector<llvm::BasicBlock*>& Successors

const llvm::StringRef Prefix