detect functions that never return, and turn the instruction following a

call to them into an 'unreachable' instruction.

This triggers a bunch of times, particularly on gcc:

gzip: 36
gcc: 601
eon: 12
bzip: 38
llvm-svn: 21587

llvm/lib/Transforms/IPO/PruneEH.cpp

@@ -16,25 +16,35 @@
 
 #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/Statistic.h"
+#include "llvm/Support/CFG.h"
 #include <set>
 #include <algorithm>
 using namespace llvm;
 
 namespace {
   Statistic<> NumRemoved("prune-eh", "Number of invokes removed");
+  Statistic<> NumUnreach("prune-eh", "Number of noreturn calls optimized");
 
   struct PruneEH : public CallGraphSCCPass {
     /// DoesNotUnwind - This set contains all of the functions which we have
-    /// determined cannot throw exceptions.
+    /// 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);
   };
   RegisterOpt<PruneEH> X("prune-eh", "Remove unused exception handling info");
 }
@@ -44,28 +54,42 @@ ModulePass *llvm::createPruneEHPass() { return new PruneEH(); }
 
 bool PruneEH::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
   CallGraph &CG = getAnalysis<CallGraph>();
+  bool MadeChange = false;
 
-  // First, check to see if any callees might throw or if there are any external
+  // 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;
+  bool SCCMightUnwind = false, SCCMightReturn = false;
-  for (unsigned i = 0, e = SCC.size(); !SCCMightUnwind && i != e; ++i) {
+  for (unsigned i = 0, e = SCC.size();
+       (!SCCMightUnwind || !SCCMightReturn) && i != e; ++i) {
     Function *F = SCC[i]->getFunction();
     if (F == 0 || (F->isExternal() && !F->getIntrinsicID())) {
       SCCMightUnwind = true;
+      SCCMightReturn = true;
     } else {
+      if (F->isExternal())
+        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;
-          break;
+        } 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()) {
@@ -77,30 +101,45 @@ bool PruneEH::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
                   SCCMightUnwind = true;
                   break;
                 }
-
               } else {
                 // Indirect call, it might throw.
                 SCCMightUnwind = true;
                 break;
               }
             }
-        if (SCCMightUnwind) break;
+        if (SCCMightUnwind && SCCMightReturn) break;
       }
     }
   }
-  bool MadeChange = false;
+
+  // 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) {
-    // If the SCC can't throw, remember this for callers...
-    if (!SCCMightUnwind)
-      DoesNotUnwind.insert(SCC[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())
-      for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I)
+      MadeChange |= SimplifyFunction(F);
-        if (InvokeInst *II = dyn_cast<InvokeInst>(I->getTerminator()))
+  }
+
+  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])) {
           // Insert a call instruction before the invoke...
@@ -113,20 +152,74 @@ bool PruneEH::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
           // Anything that used the value produced by the invoke instruction
           // now uses the value produced by the call instruction.
           II->replaceAllUsesWith(Call);
-              II->getUnwindDest()->removePredecessor(II->getParent());
+          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!
-              I->getInstList().pop_back();
+          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; I)
+      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();
+}