llvm-6502/lib/Analysis/CFG.cpp
Chandler Carruth 56e1394c88 [cleanup] Move the Dominators.h and Verifier.h headers into the IR
directory. These passes are already defined in the IR library, and it
doesn't make any sense to have the headers in Analysis.

Long term, I think there is going to be a much better way to divide
these matters. The dominators code should be fully separated into the
abstract graph algorithm and have that put in Support where it becomes
obvious that evn Clang's CFGBlock's can use it. Then the verifier can
manually construct dominance information from the Support-driven
interface while the Analysis library can provide a pass which both
caches, reconstructs, and supports a nice update API.

But those are very long term, and so I don't want to leave the really
confusing structure until that day arrives.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199082 91177308-0d34-0410-b5e6-96231b3b80d8
2014-01-13 09:26:24 +00:00

245 lines
8.8 KiB
C++

//===-- CFG.cpp - BasicBlock analysis --------------------------------------==//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This family of functions performs analyses on basic blocks, and instructions
// contained within basic blocks.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/CFG.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/IR/Dominators.h"
using namespace llvm;
/// FindFunctionBackedges - 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 llvm::FindFunctionBackedges(const Function &F,
SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
const BasicBlock *BB = &F.getEntryBlock();
if (succ_begin(BB) == succ_end(BB))
return;
SmallPtrSet<const BasicBlock*, 8> Visited;
SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack;
SmallPtrSet<const BasicBlock*, 8> InStack;
Visited.insert(BB);
VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
InStack.insert(BB);
do {
std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back();
const BasicBlock *ParentBB = Top.first;
succ_const_iterator &I = Top.second;
bool FoundNew = false;
while (I != succ_end(ParentBB)) {
BB = *I++;
if (Visited.insert(BB)) {
FoundNew = true;
break;
}
// Successor is in VisitStack, it's a back edge.
if (InStack.count(BB))
Result.push_back(std::make_pair(ParentBB, BB));
}
if (FoundNew) {
// Go down one level if there is a unvisited successor.
InStack.insert(BB);
VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
} else {
// Go up one level.
InStack.erase(VisitStack.pop_back_val().first);
}
} while (!VisitStack.empty());
}
/// GetSuccessorNumber - 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 llvm::GetSuccessorNumber(BasicBlock *BB, BasicBlock *Succ) {
TerminatorInst *Term = BB->getTerminator();
#ifndef NDEBUG
unsigned e = Term->getNumSuccessors();
#endif
for (unsigned i = 0; ; ++i) {
assert(i != e && "Didn't find edge?");
if (Term->getSuccessor(i) == Succ)
return i;
}
}
/// isCriticalEdge - 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 llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
bool AllowIdenticalEdges) {
assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
if (TI->getNumSuccessors() == 1) return false;
const BasicBlock *Dest = TI->getSuccessor(SuccNum);
const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);
// If there is more than one predecessor, this is a critical edge...
assert(I != E && "No preds, but we have an edge to the block?");
const BasicBlock *FirstPred = *I;
++I; // Skip one edge due to the incoming arc from TI.
if (!AllowIdenticalEdges)
return I != E;
// If AllowIdenticalEdges is true, then we allow this edge to be considered
// non-critical iff all preds come from TI's block.
while (I != E) {
const BasicBlock *P = *I;
if (P != FirstPred)
return true;
// Note: leave this as is until no one ever compiles with either gcc 4.0.1
// or Xcode 2. This seems to work around the pred_iterator assert in PR 2207
E = pred_end(P);
++I;
}
return false;
}
// LoopInfo contains a mapping from basic block to the innermost loop. Find
// the outermost loop in the loop nest that contains BB.
static const Loop *getOutermostLoop(const LoopInfo *LI, const BasicBlock *BB) {
const Loop *L = LI->getLoopFor(BB);
if (L) {
while (const Loop *Parent = L->getParentLoop())
L = Parent;
}
return L;
}
// True if there is a loop which contains both BB1 and BB2.
static bool loopContainsBoth(const LoopInfo *LI,
const BasicBlock *BB1, const BasicBlock *BB2) {
const Loop *L1 = getOutermostLoop(LI, BB1);
const Loop *L2 = getOutermostLoop(LI, BB2);
return L1 != NULL && L1 == L2;
}
static bool isPotentiallyReachableInner(SmallVectorImpl<BasicBlock *> &Worklist,
BasicBlock *StopBB,
const DominatorTree *DT,
const LoopInfo *LI) {
// When the stop block is unreachable, it's dominated from everywhere,
// regardless of whether there's a path between the two blocks.
if (DT && !DT->isReachableFromEntry(StopBB))
DT = 0;
// Limit the number of blocks we visit. The goal is to avoid run-away compile
// times on large CFGs without hampering sensible code. Arbitrarily chosen.
unsigned Limit = 32;
SmallSet<const BasicBlock*, 64> Visited;
do {
BasicBlock *BB = Worklist.pop_back_val();
if (!Visited.insert(BB))
continue;
if (BB == StopBB)
return true;
if (DT && DT->dominates(BB, StopBB))
return true;
if (LI && loopContainsBoth(LI, BB, StopBB))
return true;
if (!--Limit) {
// We haven't been able to prove it one way or the other. Conservatively
// answer true -- that there is potentially a path.
return true;
}
if (const Loop *Outer = LI ? getOutermostLoop(LI, BB) : 0) {
// All blocks in a single loop are reachable from all other blocks. From
// any of these blocks, we can skip directly to the exits of the loop,
// ignoring any other blocks inside the loop body.
Outer->getExitBlocks(Worklist);
} else {
for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
Worklist.push_back(*I);
}
} while (!Worklist.empty());
// We have exhausted all possible paths and are certain that 'To' can not be
// reached from 'From'.
return false;
}
bool llvm::isPotentiallyReachable(const BasicBlock *A, const BasicBlock *B,
const DominatorTree *DT, const LoopInfo *LI) {
assert(A->getParent() == B->getParent() &&
"This analysis is function-local!");
SmallVector<BasicBlock*, 32> Worklist;
Worklist.push_back(const_cast<BasicBlock*>(A));
return isPotentiallyReachableInner(Worklist, const_cast<BasicBlock*>(B),
DT, LI);
}
bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B,
const DominatorTree *DT, const LoopInfo *LI) {
assert(A->getParent()->getParent() == B->getParent()->getParent() &&
"This analysis is function-local!");
SmallVector<BasicBlock*, 32> Worklist;
if (A->getParent() == B->getParent()) {
// The same block case is special because it's the only time we're looking
// within a single block to see which instruction comes first. Once we
// start looking at multiple blocks, the first instruction of the block is
// reachable, so we only need to determine reachability between whole
// blocks.
BasicBlock *BB = const_cast<BasicBlock *>(A->getParent());
// If the block is in a loop then we can reach any instruction in the block
// from any other instruction in the block by going around a backedge.
if (LI && LI->getLoopFor(BB) != 0)
return true;
// Linear scan, start at 'A', see whether we hit 'B' or the end first.
for (BasicBlock::const_iterator I = A, E = BB->end(); I != E; ++I) {
if (&*I == B)
return true;
}
// Can't be in a loop if it's the entry block -- the entry block may not
// have predecessors.
if (BB == &BB->getParent()->getEntryBlock())
return false;
// Otherwise, continue doing the normal per-BB CFG walk.
for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
Worklist.push_back(*I);
if (Worklist.empty()) {
// We've proven that there's no path!
return false;
}
} else {
Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
}
if (A->getParent() == &A->getParent()->getParent()->getEntryBlock())
return true;
if (B->getParent() == &A->getParent()->getParent()->getEntryBlock())
return false;
return isPotentiallyReachableInner(Worklist,
const_cast<BasicBlock*>(B->getParent()),
DT, LI);
}