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//===- PruneEH.cpp - Pass which deletes unused exception handlers ---------===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements a simple interprocedural pass which walks the
// call-graph, turning invoke instructions into calls, iff the callee cannot
// throw an exception. It implements this as a bottom-up traversal of the
// call-graph.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "prune-eh"
#include "llvm/Transforms/IPO.h"
#include "llvm/CallGraphSCCPass.h"
#include "llvm/Constants.h"
#include "llvm/Function.h"
#include "llvm/Intrinsics.h"
#include "llvm/Instructions.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Compiler.h"
#include <set>
#include <algorithm>
using namespace llvm;
STATISTIC(NumRemoved, "Number of invokes removed");
STATISTIC(NumUnreach, "Number of noreturn calls optimized");
namespace {
struct VISIBILITY_HIDDEN PruneEH : public CallGraphSCCPass {
static char ID; // Pass identifcation, replacement for typeid
PruneEH() : CallGraphSCCPass((intptr_t)&ID) {}
/// DoesNotUnwind - This set contains all of the functions which we have
/// determined cannot unwind.
std::set<CallGraphNode*> DoesNotUnwind;
/// DoesNotReturn - This set contains all of the functions which we have
/// determined cannot return normally (but might unwind).
std::set<CallGraphNode*> DoesNotReturn;
// runOnSCC - Analyze the SCC, performing the transformation if possible.
bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
bool SimplifyFunction(Function *F);
void DeleteBasicBlock(BasicBlock *BB);
};
char PruneEH::ID = 0;
RegisterPass<PruneEH> X("prune-eh", "Remove unused exception handling info");
}
Pass *llvm::createPruneEHPass() { return new PruneEH(); }
bool PruneEH::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
CallGraph &CG = getAnalysis<CallGraph>();
bool MadeChange = false;
// First pass, scan all of the functions in the SCC, simplifying them
// according to what we know.
for (unsigned i = 0, e = SCC.size(); i != e; ++i)
if (Function *F = SCC[i]->getFunction())
MadeChange |= SimplifyFunction(F);
// Next, check to see if any callees might throw or if there are any external
// functions in this SCC: if so, we cannot prune any functions in this SCC.
// If this SCC includes the unwind instruction, we KNOW it throws, so
// obviously the SCC might throw.
//
bool SCCMightUnwind = false, SCCMightReturn = false;
for (unsigned i = 0, e = SCC.size();
(!SCCMightUnwind || !SCCMightReturn) && i != e; ++i) {
Function *F = SCC[i]->getFunction();
if (F == 0 || (F->isDeclaration() && !F->getIntrinsicID())) {
SCCMightUnwind = true;
SCCMightReturn = true;
} else {
if (F->isDeclaration())
SCCMightReturn = true;
// Check to see if this function performs an unwind or calls an
// unwinding function.
for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
if (isa<UnwindInst>(BB->getTerminator())) { // Uses unwind!
SCCMightUnwind = true;
} else if (isa<ReturnInst>(BB->getTerminator())) {
SCCMightReturn = true;
}
// Invoke instructions don't allow unwinding to continue, so we are
// only interested in call instructions.
if (!SCCMightUnwind)
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
if (CallInst *CI = dyn_cast<CallInst>(I)) {
if (Function *Callee = CI->getCalledFunction()) {
CallGraphNode *CalleeNode = CG[Callee];
// If the callee is outside our current SCC, or if it is not
// known to throw, then we might throw also.
if (std::find(SCC.begin(), SCC.end(), CalleeNode) == SCC.end()&&
!DoesNotUnwind.count(CalleeNode)) {
SCCMightUnwind = true;
break;
}
} else {
// Indirect call, it might throw.
SCCMightUnwind = true;
break;
}
}
if (SCCMightUnwind && SCCMightReturn) break;
}
}
}
// If the SCC doesn't unwind or doesn't throw, note this fact.
if (!SCCMightUnwind)
for (unsigned i = 0, e = SCC.size(); i != e; ++i)
DoesNotUnwind.insert(SCC[i]);
if (!SCCMightReturn)
for (unsigned i = 0, e = SCC.size(); i != e; ++i)
DoesNotReturn.insert(SCC[i]);
for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
// Convert any invoke instructions to non-throwing functions in this node
// into call instructions with a branch. This makes the exception blocks
// dead.
if (Function *F = SCC[i]->getFunction())
MadeChange |= SimplifyFunction(F);
}
return MadeChange;
}
// SimplifyFunction - Given information about callees, simplify the specified
// function if we have invokes to non-unwinding functions or code after calls to
// no-return functions.
bool PruneEH::SimplifyFunction(Function *F) {
CallGraph &CG = getAnalysis<CallGraph>();
bool MadeChange = false;
for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
if (Function *F = II->getCalledFunction())
if (DoesNotUnwind.count(CG[F])) {
SmallVector<Value*, 8> Args(II->op_begin()+3, II->op_end());
// Insert a call instruction before the invoke.
CallInst *Call = new CallInst(II->getCalledValue(),
&Args[0], Args.size(), "", II);
Call->takeName(II);
Call->setCallingConv(II->getCallingConv());
// Anything that used the value produced by the invoke instruction
// now uses the value produced by the call instruction.
II->replaceAllUsesWith(Call);
BasicBlock *UnwindBlock = II->getUnwindDest();
UnwindBlock->removePredecessor(II->getParent());
// Insert a branch to the normal destination right before the
// invoke.
new BranchInst(II->getNormalDest(), II);
// Finally, delete the invoke instruction!
BB->getInstList().pop_back();
// If the unwind block is now dead, nuke it.
if (pred_begin(UnwindBlock) == pred_end(UnwindBlock))
DeleteBasicBlock(UnwindBlock); // Delete the new BB.
++NumRemoved;
MadeChange = true;
}
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
if (CallInst *CI = dyn_cast<CallInst>(I++))
if (Function *Callee = CI->getCalledFunction())
if (DoesNotReturn.count(CG[Callee]) && !isa<UnreachableInst>(I)) {
// This call calls a function that cannot return. Insert an
// unreachable instruction after it and simplify the code. Do this
// by splitting the BB, adding the unreachable, then deleting the
// new BB.
BasicBlock *New = BB->splitBasicBlock(I);
// Remove the uncond branch and add an unreachable.
BB->getInstList().pop_back();
new UnreachableInst(BB);
DeleteBasicBlock(New); // Delete the new BB.
MadeChange = true;
++NumUnreach;
break;
}
}
return MadeChange;
}
/// DeleteBasicBlock - remove the specified basic block from the program,
/// updating the callgraph to reflect any now-obsolete edges due to calls that
/// exist in the BB.
void PruneEH::DeleteBasicBlock(BasicBlock *BB) {
assert(pred_begin(BB) == pred_end(BB) && "BB is not dead!");
CallGraph &CG = getAnalysis<CallGraph>();
CallGraphNode *CGN = CG[BB->getParent()];
for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; ) {
--I;
if (CallInst *CI = dyn_cast<CallInst>(I)) {
if (Function *Callee = CI->getCalledFunction())
CGN->removeCallEdgeTo(CG[Callee]);
} else if (InvokeInst *II = dyn_cast<InvokeInst>(I)) {
if (Function *Callee = II->getCalledFunction())
CGN->removeCallEdgeTo(CG[Callee]);
}
if (!I->use_empty())
I->replaceAllUsesWith(UndefValue::get(I->getType()));
}
// Get the list of successors of this block.
std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
for (unsigned i = 0, e = Succs.size(); i != e; ++i)
Succs[i]->removePredecessor(BB);
BB->eraseFromParent();
}
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