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https://github.com/c64scene-ar/llvm-6502.git
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279 91177308-0d34-0410-b5e6-96231b3b80d8
332 lines
12 KiB
C++
332 lines
12 KiB
C++
//===- DCE.cpp - Code to perform dead code elimination --------------------===//
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//
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// This file implements dead code elimination and basic block merging.
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//
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// Specifically, this:
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// * removes definitions with no uses (including unused constants)
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// * removes basic blocks with no predecessors
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// * merges a basic block into its predecessor if there is only one and the
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// predecessor only has one successor.
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// * Eliminates PHI nodes for basic blocks with a single predecessor
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// * Eliminates a basic block that only contains an unconditional branch
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//
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// TODO: This should REALLY be worklist driven instead of iterative. Right now,
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// we scan linearly through values, removing unused ones as we go. The problem
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// is that this may cause other earlier values to become unused. To make sure
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// that we get them all, we iterate until things stop changing. Instead, when
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// removing a value, recheck all of its operands to see if they are now unused.
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// Piece of cake, and more efficient as well.
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//
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// Note, this is not trivial, because we have to worry about invalidating
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// iterators. :(
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Optimizations/DCE.h"
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#include "llvm/Support/STLExtras.h"
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#include "llvm/Module.h"
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#include "llvm/Method.h"
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#include "llvm/BasicBlock.h"
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#include "llvm/iTerminators.h"
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#include "llvm/iOther.h"
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#include "llvm/Assembly/Writer.h"
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#include "llvm/CFG.h"
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#include <algorithm>
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using namespace cfg;
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struct ConstPoolDCE {
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enum { EndOffs = 0 };
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static bool isDCEable(const ConstPoolVal *CPV) {
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// TODO: The bytecode writer requires that all used types are in the
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// constant pool for the current method. This is messy and is really
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// irritating. FIXME
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return CPV->getType() != Type::TypeTy; // Don't DCE Type plane constants!
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}
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};
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struct BasicBlockDCE {
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enum { EndOffs = 1 };
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static bool isDCEable(const Instruction *I) {
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return !I->hasSideEffects();
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}
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};
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template<class Container, class DCEController>
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static bool RemoveUnusedDefs(Container &Vals, DCEController DCEControl) {
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bool Changed = false;
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int Offset = DCEController::EndOffs;
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for (typename Container::iterator DI = Vals.begin();
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DI != Vals.end()-Offset; ) {
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// Look for un"used" definitions...
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if ((*DI)->use_empty() && DCEController::isDCEable(*DI)) {
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// Bye bye
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//cerr << "Removing: " << *DI;
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delete Vals.remove(DI);
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Changed = true;
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} else {
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++DI;
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}
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}
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return Changed;
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}
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// RemoveSingularPHIs - This removes PHI nodes from basic blocks that have only
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// a single predecessor. This means that the PHI node must only have a single
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// RHS value and can be eliminated.
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//
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// This routine is very simple because we know that PHI nodes must be the first
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// things in a basic block, if they are present.
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//
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static bool RemoveSingularPHIs(BasicBlock *BB) {
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pred_iterator PI(pred_begin(BB));
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if (PI == pred_end(BB) || ++PI != pred_end(BB))
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return false; // More than one predecessor...
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Instruction *I = BB->front();
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if (!I->isPHINode()) return false; // No PHI nodes
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//cerr << "Killing PHIs from " << BB;
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//cerr << "Pred #0 = " << *pred_begin(BB);
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//cerr << "Method == " << BB->getParent();
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do {
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PHINode *PN = (PHINode*)I;
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assert(PN->getNumOperands() == 2 && "PHI node should only have one value!");
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Value *V = PN->getOperand(0);
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PN->replaceAllUsesWith(V); // Replace PHI node with its single value.
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delete BB->getInstList().remove(BB->begin());
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I = BB->front();
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} while (I->isPHINode());
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return true; // Yes, we nuked at least one phi node
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}
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bool opt::DoRemoveUnusedConstants(SymTabValue *S) {
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bool Changed = false;
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ConstantPool &CP = S->getConstantPool();
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for (ConstantPool::plane_iterator PI = CP.begin(); PI != CP.end(); ++PI)
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Changed |= RemoveUnusedDefs(**PI, ConstPoolDCE());
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return Changed;
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}
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static void ReplaceUsesWithConstant(Instruction *I) {
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// Get the method level constant pool
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ConstantPool &CP = I->getParent()->getParent()->getConstantPool();
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ConstPoolVal *CPV = 0;
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ConstantPool::PlaneType *P;
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if (!CP.getPlane(I->getType(), P)) { // Does plane exist?
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// Yes, is it empty?
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if (!P->empty()) CPV = P->front();
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}
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if (CPV == 0) { // We don't have an existing constant to reuse. Just add one.
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CPV = ConstPoolVal::getNullConstant(I->getType()); // Create a new constant
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// Add the new value to the constant pool...
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CP.insert(CPV);
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}
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// Make all users of this instruction reference the constant instead
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I->replaceAllUsesWith(CPV);
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}
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// PropogatePredecessors - This gets "Succ" ready to have the predecessors from
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// "BB". This is a little tricky because "Succ" has PHI nodes, which need to
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// have extra slots added to them to hold the merge edges from BB's
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// predecessors.
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//
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// Assumption: BB is the single predecessor of Succ.
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//
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static void PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
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assert(Succ->front()->isPHINode() && "Only works on PHId BBs!");
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// If there is more than one predecessor, and there are PHI nodes in
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// the successor, then we need to add incoming edges for the PHI nodes
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//
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const vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
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BasicBlock::iterator I = Succ->begin();
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do { // Loop over all of the PHI nodes in the successor BB
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PHINode *PN = (PHINode*)*I;
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Value *OldVal = PN->removeIncomingValue(BB);
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assert(OldVal && "No entry in PHI for Pred BB!");
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for (vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
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End = BBPreds.end(); PredI != End; ++PredI) {
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// Add an incoming value for each of the new incoming values...
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PN->addIncoming(OldVal, *PredI);
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}
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++I;
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} while ((*I)->isPHINode());
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}
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// SimplifyCFG - This function is used to do simplification of a CFG. For
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// example, it adjusts branches to branches to eliminate the extra hop, it
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// eliminates unreachable basic blocks, and does other "peephole" optimization
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// of the CFG. It returns true if a modification was made, and returns an
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// iterator that designates the first element remaining after the block that
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// was deleted.
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//
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// WARNING: The entry node of a method may not be simplified.
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//
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bool opt::SimplifyCFG(Method::iterator &BBIt) {
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assert(*BBIt && (*BBIt)->getParent() && "Block not embedded in method!");
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BasicBlock *BB = *BBIt;
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Method *M = BB->getParent();
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assert(BB->getTerminator() && "Degenerate basic block encountered!");
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assert(BB->getParent()->front() != BB && "Can't Simplify entry block!");
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// Remove basic blocks that have no predecessors... which are unreachable.
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if (pred_begin(BB) == pred_end(BB) &&
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!BB->hasConstantPoolReferences()) {
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//cerr << "Removing BB: \n" << BB;
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// Loop through all of our successors and make sure they know that one
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// of their predecessors is going away.
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for_each(succ_begin(BB), succ_end(BB),
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std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB));
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while (!BB->empty()) {
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Instruction *I = BB->back();
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// If this instruction is used, replace uses with an arbitrary
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// constant value. Because control flow can't get here, we don't care
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// what we replace the value with. Note that since this block is
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// unreachable, and all values contained within it must dominate their
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// uses, that all uses will eventually be removed.
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if (!I->use_empty()) ReplaceUsesWithConstant(I);
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// Remove the instruction from the basic block
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delete BB->getInstList().pop_back();
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}
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delete M->getBasicBlocks().remove(BBIt);
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return true;
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}
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// Check to see if this block has no instructions and only a single
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// successor. If so, replace block references with successor.
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succ_iterator SI(succ_begin(BB));
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if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
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Instruction *I = BB->front();
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if (I->isTerminator()) { // Terminator is the only instruction!
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BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
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//cerr << "Killing Trivial BB: \n" << BB;
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if (Succ != BB) { // Arg, don't hurt infinite loops!
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if (Succ->front()->isPHINode()) {
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// If our successor has PHI nodes, then we need to update them to
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// include entries for BB's predecessors, not for BB itself.
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//
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PropogatePredecessorsForPHIs(BB, Succ);
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}
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BB->replaceAllUsesWith(Succ);
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BB = M->getBasicBlocks().remove(BBIt);
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if (BB->hasName() && !Succ->hasName()) // Transfer name if we can
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Succ->setName(BB->getName());
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delete BB; // Delete basic block
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//cerr << "Method after removal: \n" << M;
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return true;
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}
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}
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}
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// Merge basic blocks into their predecessor if there is only one pred,
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// and if there is only one successor of the predecessor.
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pred_iterator PI(pred_begin(BB));
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if (PI != pred_end(BB) && *PI != BB && // Not empty? Not same BB?
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++PI == pred_end(BB) && !BB->hasConstantPoolReferences()) {
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BasicBlock *Pred = *pred_begin(BB);
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TerminatorInst *Term = Pred->getTerminator();
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assert(Term != 0 && "malformed basic block without terminator!");
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// Does the predecessor block only have a single successor?
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succ_iterator SI(succ_begin(Pred));
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if (++SI == succ_end(Pred)) {
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//cerr << "Merging: " << BB << "into: " << Pred;
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// Delete the unconditianal branch from the predecessor...
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BasicBlock::iterator DI = Pred->end();
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assert(Pred->getTerminator() &&
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"Degenerate basic block encountered!"); // Empty bb???
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delete Pred->getInstList().remove(--DI); // Destroy uncond branch
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// Move all definitions in the succecessor to the predecessor...
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while (!BB->empty()) {
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DI = BB->begin();
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Instruction *Def = BB->getInstList().remove(DI); // Remove from front
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Pred->getInstList().push_back(Def); // Add to end...
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}
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// Remove basic block from the method... and advance iterator to the
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// next valid block...
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BB = M->getBasicBlocks().remove(BBIt);
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// Make all PHI nodes that refered to BB now refer to Pred as their
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// source...
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BB->replaceAllUsesWith(Pred);
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// Inherit predecessors name if it exists...
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if (BB->hasName() && !Pred->hasName()) Pred->setName(BB->getName());
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delete BB; // You ARE the weakest link... goodbye
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return true;
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}
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}
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return false;
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}
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static bool DoDCEPass(Method *M) {
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Method::iterator BBIt, BBEnd = M->end();
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if (M->begin() == BBEnd) return false; // Nothing to do
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bool Changed = false;
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// Loop through now and remove instructions that have no uses...
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for (BBIt = M->begin(); BBIt != BBEnd; ++BBIt) {
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Changed |= RemoveUnusedDefs((*BBIt)->getInstList(), BasicBlockDCE());
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Changed |= RemoveSingularPHIs(*BBIt);
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}
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// Loop over all of the basic blocks (except the first one) and remove them
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// if they are unneeded...
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//
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for (BBIt = M->begin(), ++BBIt; BBIt != M->end(); ) {
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if (opt::SimplifyCFG(BBIt)) {
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Changed = true;
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} else {
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++BBIt;
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}
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}
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// Remove unused constants
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return Changed | opt::DoRemoveUnusedConstants(M);
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}
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// It is possible that we may require multiple passes over the code to fully
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// eliminate dead code. Iterate until we are done.
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//
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bool opt::DoDeadCodeElimination(Method *M) {
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bool Changed = false;
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while (DoDCEPass(M)) Changed = true;
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return Changed;
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}
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bool opt::DoDeadCodeElimination(Module *C) {
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bool Val = C->reduceApply(DoDeadCodeElimination);
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while (DoRemoveUnusedConstants(C)) Val = true;
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return Val;
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}
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