Add an early implementation of a partial inlining pass. The idea behind this

is that, for functions whose bodies are entirely guarded by an if-statement, it
can be profitable to pull the test out of the callee and into the caller.

This code has had some cursory testing, but still has a number of known issues
on the LLVM test suite.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73338 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 171 insertions, 0 deletions

diff --git a/lib/Transforms/IPO/PartialInlining.cpp b/lib/Transforms/IPO/PartialInlining.cpp
new file mode 100644
index 0000000000..b3a2554039
--- /dev/null
+++ b/lib/Transforms/IPO/PartialInlining.cpp
@@ -0,0 +1,171 @@
+//===- PartialInlining.cpp - Inline parts of functions --------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass performs partial inlining, typically by inlining an if statement
+// that surrounds the body of the function.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "partialinlining"
+#include "llvm/Transforms/IPO.h"
+#include "llvm/Instructions.h"
+#include "llvm/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Transforms/Utils/Cloning.h"
+#include "llvm/Transforms/Utils/FunctionUtils.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/CFG.h"
+using namespace llvm;
+
+namespace {
+  struct VISIBILITY_HIDDEN PartialInliner : public ModulePass {
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const { }
+    static char ID; // Pass identification, replacement for typeid
+    PartialInliner() : ModulePass(&ID) {}
+    
+    bool runOnModule(Module& M);
+    
+  private:
+    Function* unswitchFunction(Function* F);
+  };
+}
+
+char PartialInliner::ID = 0;
+static RegisterPass<PartialInliner> X("partial-inliner", "Partial Inliner");
+
+ModulePass* llvm::createPartialInliningPass() { return new PartialInliner(); }
+
+Function* PartialInliner::unswitchFunction(Function* F) {
+  // First, verify that this function is an unswitching candidate...
+  BasicBlock* entryBlock = F->begin();
+  if (!isa<BranchInst>(entryBlock->getTerminator()))
+    return 0;
+  
+  BasicBlock* returnBlock = 0;
+  BasicBlock* nonReturnBlock = 0;
+  unsigned returnCount = 0;
+  for (succ_iterator SI = succ_begin(entryBlock), SE = succ_end(entryBlock);
+       SI != SE; ++SI)
+    if (isa<ReturnInst>((*SI)->getTerminator())) {
+      returnBlock = *SI;
+      returnCount++;
+    } else
+      nonReturnBlock = *SI;
+  
+  if (returnCount != 1)
+    return 0;
+  
+  // Clone the function, so that we can hack away on it.
+  DenseMap<const Value*, Value*> ValueMap;
+  Function* duplicateFunction = CloneFunction(F, ValueMap);
+  duplicateFunction->setLinkage(GlobalValue::InternalLinkage);
+  F->getParent()->getFunctionList().push_back(duplicateFunction);
+  BasicBlock* newEntryBlock = cast<BasicBlock>(ValueMap[entryBlock]);
+  BasicBlock* newReturnBlock = cast<BasicBlock>(ValueMap[returnBlock]);
+  BasicBlock* newNonReturnBlock = cast<BasicBlock>(ValueMap[nonReturnBlock]);
+  
+  // Go ahead and update all uses to the duplicate, so that we can just
+  // use the inliner functionality when we're done hacking.
+  F->replaceAllUsesWith(duplicateFunction);
+  
+  // Special hackery is needed with PHI nodes that have inputs from more than
+  // one extracted block.  For simplicity, just split the PHIs into a two-level
+  // sequence of PHIs, some of which will go in the extracted region, and some
+  // of which will go outside.
+  BasicBlock* preReturn = newReturnBlock;
+  newReturnBlock = newReturnBlock->splitBasicBlock(
+                                              newReturnBlock->getFirstNonPHI());
+  BasicBlock::iterator I = preReturn->begin();
+  BasicBlock::iterator Ins = newReturnBlock->begin();
+  while (I != preReturn->end()) {
+    PHINode* OldPhi = dyn_cast<PHINode>(I);
+    if (!OldPhi) break;
+    
+    PHINode* retPhi = PHINode::Create(OldPhi->getType(), "", Ins);
+    OldPhi->replaceAllUsesWith(retPhi);
+    Ins = newReturnBlock->getFirstNonPHI();
+    
+    retPhi->addIncoming(I, preReturn);
+    retPhi->addIncoming(OldPhi->getIncomingValueForBlock(newEntryBlock),
+                        newEntryBlock);
+    OldPhi->removeIncomingValue(newEntryBlock);
+    
+    ++I;
+  }
+  newEntryBlock->getTerminator()->replaceUsesOfWith(preReturn, newReturnBlock);
+  
+  // Gather up the blocks that we're going to extract.
+  std::vector<BasicBlock*> toExtract;
+  toExtract.push_back(newNonReturnBlock);
+  for (Function::iterator FI = duplicateFunction->begin(),
+       FE = duplicateFunction->end(); FI != FE; ++FI)
+    if (&*FI != newEntryBlock && &*FI != newReturnBlock &&
+        &*FI != newNonReturnBlock)
+      toExtract.push_back(FI);
+      
+  // The CodeExtractor needs a dominator tree.
+  DominatorTree DT;
+  DT.runOnFunction(*duplicateFunction);
+  
+  // Extract the body of the the if.
+  Function* extractedFunction = ExtractCodeRegion(DT, toExtract);
+  
+  // Inline the top-level if test into all callers.
+  std::vector<User*> Users(duplicateFunction->use_begin(), 
+                           duplicateFunction->use_end());
+  for (std::vector<User*>::iterator UI = Users.begin(), UE = Users.end();
+       UI != UE; ++UI)
+    if (CallInst* CI = dyn_cast<CallInst>(*UI))
+      InlineFunction(CI);
+    else if (InvokeInst* II = dyn_cast<InvokeInst>(*UI))
+      InlineFunction(II);
+  
+  // Ditch the duplicate, since we're done with it, and rewrite all remaining
+  // users (function pointers, etc.) back to the original function.
+  duplicateFunction->replaceAllUsesWith(F);
+  duplicateFunction->eraseFromParent();
+  
+  return extractedFunction;
+}
+
+bool PartialInliner::runOnModule(Module& M) {
+  std::vector<Function*> worklist;
+  worklist.reserve(M.size());
+  for (Module::iterator FI = M.begin(), FE = M.end(); FI != FE; ++FI)
+    if (!FI->use_empty() && !FI->isDeclaration())
+    worklist.push_back(&*FI);
+    
+  bool changed = false;
+  while (!worklist.empty()) {
+    Function* currFunc = worklist.back();
+    worklist.pop_back();
+  
+    if (currFunc->use_empty()) continue;
+    
+    bool recursive = false;
+    for (Function::use_iterator UI = currFunc->use_begin(),
+         UE = currFunc->use_end(); UI != UE; ++UI)
+      if (Instruction* I = dyn_cast<Instruction>(UI))
+        if (I->getParent()->getParent() == currFunc) {
+          recursive = true;
+          break;
+        }
+    if (recursive) continue;
+          
+    
+    if (Function* newFunc = unswitchFunction(currFunc)) {
+      worklist.push_back(newFunc);
+      changed = true;
+    }
+    
+  }
+  
+  return changed;
+}
\ No newline at end of file