Initial checkin of the -prune-eh pass, a very simple exception handling removal pass

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@8250 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/IPO/PruneEH.cpp

@@ -0,0 +1,93 @@
+//===- PruneEH.cpp - Pass which deletes unused exception handlers ---------===//
+//
+// 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.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CallGraphSCCPass.h"
+#include "llvm/Function.h"
+#include "llvm/Intrinsics.h"
+#include "llvm/iTerminators.h"
+#include "llvm/iOther.h"
+#include "llvm/Analysis/CallGraph.h"
+#include "Support/Statistic.h"
+#include <set>
+
+namespace {
+  Statistic<> NumRemoved("prune-eh", "Number of invokes removed");
+
+  struct PruneEH : public CallGraphSCCPass {
+    /// DoesNotThrow - This set contains all of the functions which we have
+    /// determined cannot throw exceptions.
+    std::set<CallGraphNode*> DoesNotThrow;
+
+    // runOnSCC - Analyze the SCC, performing the transformation if possible.
+    bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
+  };
+  RegisterOpt<PruneEH> X("prune-eh", "Remove unused exception handling info");
+}
+
+
+bool PruneEH::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
+  CallGraph &CG = getAnalysis<CallGraph>();
+
+  // First, 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 llvm.unwind intrinsic, we KNOW it throws, so
+  // obviously the SCC might throw.
+  //
+  bool SCCMightThrow = false;
+  for (unsigned i = 0, e = SCC.size(); i != e; ++i)
+    if (!DoesNotThrow.count(SCC[i]) &&          // Calls maybe throwing fn
+        // Make sure this is not one of the fn's in the SCC.
+        std::find(SCC.begin(), SCC.end(), SCC[i]) == SCC.end()) {
+      SCCMightThrow = true; break;
+    } else if (Function *F = SCC[i]->getFunction())
+      if (F->isExternal() ||                             // Is external function
+          F->getIntrinsicID() == LLVMIntrinsic::unwind) {// Is unwind function!
+        SCCMightThrow = true; break;
+      }
+
+  bool MadeChange = false;
+
+  for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
+    // If the SCC can't throw, remember this for callers...
+    if (!SCCMightThrow)
+      DoesNotThrow.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)
+        if (InvokeInst *II = dyn_cast<InvokeInst>(I->getTerminator()))
+          if (Function *F = II->getCalledFunction())
+            if (DoesNotThrow.count(CG[F])) {
+              // Insert a call instruction before the invoke...
+              std::string Name = II->getName();  II->setName("");
+              Value *Call = new CallInst(II->getCalledValue(),
+                                         std::vector<Value*>(II->op_begin()+3,
+                                                             II->op_end()),
+                                         Name, II);
+              
+              // Anything that used the value produced by the invoke instruction
+              // now uses the value produced by the call instruction.
+              II->replaceAllUsesWith(Call);
+          
+              // 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();
+              
+              ++NumRemoved;
+              MadeChange = true;
+            }
+  }
+
+  return MadeChange; 
+}