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