//===-- Analysis/CFG.h - BasicBlock Analyses --------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This family of functions performs analyses on basic blocks, and instructions
// contained within basic blocks.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_ANALYSIS_CFG_H
#define LLVM_ANALYSIS_CFG_H
#include "llvm/ADT/GraphTraits.h"
#include "llvm/ADT/SmallPtrSet.h"
#include <utility>
namespace llvm {
class BasicBlock;
class DominatorTree;
class Function;
class Instruction;
class LoopInfo;
template <typename T> class SmallVectorImpl;
/// Analyze the specified function to find all of the loop backedges in the
/// function and return them. This is a relatively cheap (compared to
/// computing dominators and loop info) analysis.
///
/// The output is added to Result, as pairs of <from,to> edge info.
void FindFunctionBackedges(
const Function &F,
SmallVectorImpl<std::pair<const BasicBlock *, const BasicBlock *> > &
Result);
/// Search for the specified successor of basic block BB and return its position
/// in the terminator instruction's list of successors. It is an error to call
/// this with a block that is not a successor.
unsigned GetSuccessorNumber(const BasicBlock *BB, const BasicBlock *Succ);
/// Return true if the specified edge is a critical edge. Critical edges are
/// edges from a block with multiple successors to a block with multiple
/// predecessors.
///
bool isCriticalEdge(const Instruction *TI, unsigned SuccNum,
bool AllowIdenticalEdges = false);
bool isCriticalEdge(const Instruction *TI, const BasicBlock *Succ,
bool AllowIdenticalEdges = false);
/// Determine whether instruction 'To' is reachable from 'From', without passing
/// through any blocks in ExclusionSet, returning true if uncertain.
///
/// Determine whether there is a path from From to To within a single function.
/// Returns false only if we can prove that once 'From' has been executed then
/// 'To' can not be executed. Conservatively returns true.
///
/// This function is linear with respect to the number of blocks in the CFG,
/// walking down successors from From to reach To, with a fixed threshold.
/// Using DT or LI allows us to answer more quickly. LI reduces the cost of
/// an entire loop of any number of blocks to be the same as the cost of a
/// single block. DT reduces the cost by allowing the search to terminate when
/// we find a block that dominates the block containing 'To'. DT is most useful
/// on branchy code but not loops, and LI is most useful on code with loops but
/// does not help on branchy code outside loops.
bool isPotentiallyReachable(
const Instruction *From, const Instruction *To,
const SmallPtrSetImpl<BasicBlock *> *ExclusionSet = nullptr,
const DominatorTree *DT = nullptr, const LoopInfo *LI = nullptr);
/// Determine whether block 'To' is reachable from 'From', returning
/// true if uncertain.
///
/// Determine whether there is a path from From to To within a single function.
/// Returns false only if we can prove that once 'From' has been reached then
/// 'To' can not be executed. Conservatively returns true.
bool isPotentiallyReachable(
const BasicBlock *From, const BasicBlock *To,
const SmallPtrSetImpl<BasicBlock *> *ExclusionSet = nullptr,
const DominatorTree *DT = nullptr, const LoopInfo *LI = nullptr);
/// Determine whether there is at least one path from a block in
/// 'Worklist' to 'StopBB' without passing through any blocks in
/// 'ExclusionSet', returning true if uncertain.
///
/// Determine whether there is a path from at least one block in Worklist to
/// StopBB within a single function without passing through any of the blocks
/// in 'ExclusionSet'. Returns false only if we can prove that once any block
/// in 'Worklist' has been reached then 'StopBB' can not be executed.
/// Conservatively returns true.
bool isPotentiallyReachableFromMany(
SmallVectorImpl<BasicBlock *> &Worklist, const BasicBlock *StopBB,
const SmallPtrSetImpl<BasicBlock *> *ExclusionSet,
const DominatorTree *DT = nullptr, const LoopInfo *LI = nullptr);
/// Return true if the control flow in \p RPOTraversal is irreducible.
///
/// This is a generic implementation to detect CFG irreducibility based on loop
/// info analysis. It can be used for any kind of CFG (Loop, MachineLoop,
/// Function, MachineFunction, etc.) by providing an RPO traversal (\p
/// RPOTraversal) and the loop info analysis (\p LI) of the CFG. This utility
/// function is only recommended when loop info analysis is available. If loop
/// info analysis isn't available, please, don't compute it explicitly for this
/// purpose. There are more efficient ways to detect CFG irreducibility that
/// don't require recomputing loop info analysis (e.g., T1/T2 or Tarjan's
/// algorithm).
///
/// Requirements:
/// 1) GraphTraits must be implemented for NodeT type. It is used to access
/// NodeT successors.
// 2) \p RPOTraversal must be a valid reverse post-order traversal of the
/// target CFG with begin()/end() iterator interfaces.
/// 3) \p LI must be a valid LoopInfoBase that contains up-to-date loop
/// analysis information of the CFG.
///
/// This algorithm uses the information about reducible loop back-edges already
/// computed in \p LI. When a back-edge is found during the RPO traversal, the
/// algorithm checks whether the back-edge is one of the reducible back-edges in
/// loop info. If it isn't, the CFG is irreducible. For example, for the CFG
/// below (canonical irreducible graph) loop info won't contain any loop, so the
/// algorithm will return that the CFG is irreducible when checking the B <-
/// -> C back-edge.
///
/// (A->B, A->C, B->C, C->B, C->D)
/// A
/// / \
/// B<- ->C
/// |
/// D
///
template <class NodeT, class RPOTraversalT, class LoopInfoT,
class GT = GraphTraits<NodeT>>
bool containsIrreducibleCFG(RPOTraversalT &RPOTraversal, const LoopInfoT &LI) {
/// Check whether the edge (\p Src, \p Dst) is a reducible loop backedge
/// according to LI. I.e., check if there exists a loop that contains Src and
/// where Dst is the loop header.
auto isProperBackedge = [&](NodeT Src, NodeT Dst) {
for (const auto *Lp = LI.getLoopFor(Src); Lp; Lp = Lp->getParentLoop()) {
if (Lp->getHeader() == Dst)
return true;
}
return false;
};
SmallPtrSet<NodeT, 32> Visited;
for (NodeT Node : RPOTraversal) {
Visited.insert(Node);
for (NodeT Succ : make_range(GT::child_begin(Node), GT::child_end(Node))) {
// Succ hasn't been visited yet
if (!Visited.count(Succ))
continue;
// We already visited Succ, thus Node->Succ must be a backedge. Check that
// the head matches what we have in the loop information. Otherwise, we
// have an irreducible graph.
if (!isProperBackedge(Node, Succ))
return true;
}
}
return false;
}
} // End llvm namespace
#endif