Add a trivially simple pass to delete unreachable blocks from the CFG. This

pass is required to paper over problems in the code generator (primarily
live variables and its clients) which doesn't really have any well defined
semantics for unreachable code.

The proper solution to this problem is to have instruction selectors not
select blocks that are unreachable.  Until we have a instruction selection
framework available for use, however, we can't expect all instruction
selector writers to do this.  Until then, this pass should be used.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@14563 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/UnreachableBlockElim.cpp

@@ -0,0 +1,68 @@
+//===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
+// 
+//                     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 pass is an extremely simple version of the SimplifyCFG pass.  Its sole
+// job is to delete LLVM basic blocks that are not reachable from the entry
+// node.  To do this, it performs a simple depth first traversal of the CFG,
+// then deletes any unvisited nodes.
+//
+// Note that this pass is really a hack.  In particular, the instruction
+// selectors for various targets should just not generate code for unreachable
+// blocks.  Until LLVM has a more systematic way of defining instruction
+// selectors, however, we cannot really expect them to handle additional
+// complexity.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/Function.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/CFG.h"
+#include "Support/DepthFirstIterator.h"
+using namespace llvm;
+
+namespace {
+  class UnreachableBlockElim : public FunctionPass {
+    virtual bool runOnFunction(Function &F);
+  };
+  RegisterOpt<UnreachableBlockElim>
+  X("unreachableblockelim", "Remove unreachable blocks from the CFG");
+}
+
+FunctionPass *llvm::createUnreachableBlockEliminationPass() {
+  return new UnreachableBlockElim();
+}
+
+bool UnreachableBlockElim::runOnFunction(Function &F) {
+  std::set<BasicBlock*> Reachable;
+
+  // Mark all reachable blocks.
+  for (df_ext_iterator<Function*> I = df_ext_begin(&F, Reachable),
+         E = df_ext_end(&F, Reachable); I != E; ++I)
+    /* Mark all reachable blocks */;
+
+  // Loop over all dead blocks, remembering them and deleting all instructions
+  // in them.
+  std::vector<BasicBlock*> DeadBlocks;
+  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
+    if (!Reachable.count(I)) {
+      DeadBlocks.push_back(I);
+      for (succ_iterator SI = succ_begin(&*I), E = succ_end(&*I); SI != E; ++SI)
+        (*SI)->removePredecessor(I);
+      I->dropAllReferences();
+    }
+
+  if (DeadBlocks.empty()) return false;
+
+  // Actually remove the blocks now.
+  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
+    F.getBasicBlockList().erase(DeadBlocks[i]);
+
+  return true;
+}