From fc3c82a804c9b5eda24d96c21f7e6ff66bc2c529 Mon Sep 17 00:00:00 2001
From: Chris Lattner <sabre@nondot.org>
Date: Fri, 2 Jul 2004 05:46:10 +0000
Subject: [PATCH] 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 | 68 ++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)
 create mode 100644 lib/CodeGen/UnreachableBlockElim.cpp

diff --git a/lib/CodeGen/UnreachableBlockElim.cpp b/lib/CodeGen/UnreachableBlockElim.cpp
new file mode 100644
index 00000000000..bcc124538d3
--- /dev/null
+++ b/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;
+}
-- 
2.11.0