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+//===- PruneEH.cpp - Pass which deletes unused exception handlers ---------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file 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, and marking functions 'nounwind' if they cannot throw.
+// It implements this as a bottom-up traversal of the call-graph.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/IPO.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Analysis/CallGraph.h"
+#include "llvm/Analysis/CallGraphSCCPass.h"
+#include "llvm/Analysis/EHPersonalities.h"
+#include "llvm/IR/CFG.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/InlineAsm.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/LLVMContext.h"
+#include <algorithm>
+using namespace llvm;
+
+#define DEBUG_TYPE "prune-eh"
+
+STATISTIC(NumRemoved, "Number of invokes removed");
+STATISTIC(NumUnreach, "Number of noreturn calls optimized");
+
+namespace {
+ struct PruneEH : public CallGraphSCCPass {
+ static char ID; // Pass identification, replacement for typeid
+ PruneEH() : CallGraphSCCPass(ID) {
+ initializePruneEHPass(*PassRegistry::getPassRegistry());
+ }
+
+ // runOnSCC - Analyze the SCC, performing the transformation if possible.
+ bool runOnSCC(CallGraphSCC &SCC) override;
+
+ bool SimplifyFunction(Function *F);
+ void DeleteBasicBlock(BasicBlock *BB);
+ };
+}
+
+char PruneEH::ID = 0;
+INITIALIZE_PASS_BEGIN(PruneEH, "prune-eh",
+ "Remove unused exception handling info", false, false)
+INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
+INITIALIZE_PASS_END(PruneEH, "prune-eh",
+ "Remove unused exception handling info", false, false)
+
+Pass *llvm::createPruneEHPass() { return new PruneEH(); }
+
+
+bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
+ SmallPtrSet<CallGraphNode *, 8> SCCNodes;
+ CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
+ bool MadeChange = false;
+
+ // Fill SCCNodes with the elements of the SCC. Used for quickly
+ // looking up whether a given CallGraphNode is in this SCC.
+ for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
+ SCCNodes.insert(*I);
+
+ // First pass, scan all of the functions in the SCC, simplifying them
+ // according to what we know.
+ for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
+ if (Function *F = (*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.
+ // Definitions that are weak and not declared non-throwing might be
+ // overridden at linktime with something that throws, so assume that.
+ // If this SCC includes the unwind instruction, we KNOW it throws, so
+ // obviously the SCC might throw.
+ //
+ bool SCCMightUnwind = false, SCCMightReturn = false;
+ for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end();
+ (!SCCMightUnwind || !SCCMightReturn) && I != E; ++I) {
+ Function *F = (*I)->getFunction();
+ if (!F) {
+ SCCMightUnwind = true;
+ SCCMightReturn = true;
+ } else if (F->isDeclaration() || F->mayBeOverridden()) {
+ SCCMightUnwind |= !F->doesNotThrow();
+ SCCMightReturn |= !F->doesNotReturn();
+ } else {
+ bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
+ bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
+ // Determine if we should scan for InlineAsm in a naked function as it
+ // is the only way to return without a ReturnInst. Only do this for
+ // no-inline functions as functions which may be inlined cannot
+ // meaningfully return via assembly.
+ bool CheckReturnViaAsm = CheckReturn &&
+ F->hasFnAttribute(Attribute::Naked) &&
+ F->hasFnAttribute(Attribute::NoInline);
+
+ if (!CheckUnwind && !CheckReturn)
+ continue;
+
+ for (const BasicBlock &BB : *F) {
+ const TerminatorInst *TI = BB.getTerminator();
+ if (CheckUnwind && TI->mayThrow()) {
+ SCCMightUnwind = true;
+ } else if (CheckReturn && isa<ReturnInst>(TI)) {
+ SCCMightReturn = true;
+ }
+
+ for (const Instruction &I : BB) {
+ if ((!CheckUnwind || SCCMightUnwind) &&
+ (!CheckReturnViaAsm || SCCMightReturn))
+ break;
+
+ // Check to see if this function performs an unwind or calls an
+ // unwinding function.
+ if (CheckUnwind && !SCCMightUnwind && I.mayThrow()) {
+ bool InstMightUnwind = true;
+ if (const auto *CI = dyn_cast<CallInst>(&I)) {
+ if (Function *Callee = CI->getCalledFunction()) {
+ CallGraphNode *CalleeNode = CG[Callee];
+ // If the callee is outside our current SCC then we may throw
+ // because it might. If it is inside, do nothing.
+ if (SCCNodes.count(CalleeNode) > 0)
+ InstMightUnwind = false;
+ }
+ }
+ SCCMightUnwind |= InstMightUnwind;
+ }
+ if (CheckReturnViaAsm && !SCCMightReturn)
+ if (auto ICS = ImmutableCallSite(&I))
+ if (const auto *IA = dyn_cast<InlineAsm>(ICS.getCalledValue()))
+ if (IA->hasSideEffects())
+ SCCMightReturn = true;
+ }
+
+ if (SCCMightUnwind && SCCMightReturn)
+ break;
+ }
+ }
+ }
+
+ // If the SCC doesn't unwind or doesn't throw, note this fact.
+ if (!SCCMightUnwind || !SCCMightReturn)
+ for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
+ Function *F = (*I)->getFunction();
+
+ if (!SCCMightUnwind && !F->hasFnAttribute(Attribute::NoUnwind)) {
+ F->addFnAttr(Attribute::NoUnwind);
+ MadeChange = true;
+ }
+
+ if (!SCCMightReturn && !F->hasFnAttribute(Attribute::NoReturn)) {
+ F->addFnAttr(Attribute::NoReturn);
+ MadeChange = true;
+ }
+ }
+
+ for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); 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 = (*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) {
+ bool MadeChange = false;
+ for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
+ if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
+ if (II->doesNotThrow() && canSimplifyInvokeNoUnwind(F)) {
+ SmallVector<Value*, 8> Args(II->arg_begin(), II->arg_end());
+ SmallVector<OperandBundleDef, 1> OpBundles;
+ II->getOperandBundlesAsDefs(OpBundles);
+
+ // Insert a call instruction before the invoke.
+ CallInst *Call = CallInst::Create(II->getCalledValue(), Args, OpBundles,
+ "", II);
+ Call->takeName(II);
+ Call->setCallingConv(II->getCallingConv());
+ Call->setAttributes(II->getAttributes());
+ Call->setDebugLoc(II->getDebugLoc());
+
+ // Anything that used the value produced by the invoke instruction
+ // now uses the value produced by the call instruction. Note that we
+ // do this even for void functions and calls with no uses so that the
+ // callgraph edge is updated.
+ II->replaceAllUsesWith(Call);
+ BasicBlock *UnwindBlock = II->getUnwindDest();
+ UnwindBlock->removePredecessor(II->getParent());
+
+ // Insert a branch to the normal destination right before the
+ // invoke.
+ BranchInst::Create(II->getNormalDest(), II);
+
+ // Finally, delete the invoke instruction!
+ BB->getInstList().pop_back();
+
+ // If the unwind block is now dead, nuke it.
+ if (pred_empty(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 (CI->doesNotReturn() && !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->getContext(), &*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_empty(BB) && "BB is not dead!");
+ CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
+
+ 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 (!isa<IntrinsicInst>(I))
+ CGN->removeCallEdgeFor(CI);
+ } else if (InvokeInst *II = dyn_cast<InvokeInst>(I))
+ CGN->removeCallEdgeFor(II);
+ 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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