/**
* Perform optimizations for an inline guard.
*
* <p>Precondition: InlineGuard.conforms(cb)
*
* @param ir the governing IR
* @param cb the instruction to optimize
* @param bb the basic block holding if
* @return {@code true} iff made a transformation
*/
private boolean processInlineGuard(IR ir, Instruction cb, BasicBlock bb) {
BasicBlock targetBlock = cb.getBranchTarget();
Instruction targetLabel = targetBlock.firstInstruction();
// get the first real instruction at the branch target
// NOTE: this instruction is not necessarily in targetBlock,
// iff targetBlock has no real instructions
Instruction targetInst = firstRealInstructionFollowing(targetLabel);
if (targetInst == null || targetInst == cb) {
return false;
}
boolean endsBlock = cb.nextInstructionInCodeOrder().operator() == BBEND;
if (endsBlock) {
Instruction nextLabel = firstLabelFollowing(cb);
if (targetLabel == nextLabel) {
// found a conditional branch to the next instruction. just remove it.
cb.remove();
return true;
}
Instruction nextI = firstRealInstructionFollowing(nextLabel);
if (nextI != null && Goto.conforms(nextI)) {
// replicate Goto
cb.insertAfter(nextI.copyWithoutLinks());
bb.recomputeNormalOut(ir); // fix the CFG
return true;
}
}
// do we fall through to a block that has only a goto?
BasicBlock fallThrough = bb.getFallThroughBlock();
if (fallThrough != null) {
Instruction fallThroughInstruction = fallThrough.firstRealInstruction();
if ((fallThroughInstruction != null) && Goto.conforms(fallThroughInstruction)) {
// copy goto to bb
bb.appendInstruction(fallThroughInstruction.copyWithoutLinks());
bb.recomputeNormalOut(ir);
}
}
if (Goto.conforms(targetInst)) {
// conditional branch to unconditional branch.
// change conditional branch target to latter's target
InlineGuard.setTarget(cb, (BranchOperand) Goto.getTarget(targetInst).copy());
bb.recomputeNormalOut(ir); // fix the CFG
return true;
}
if (targetBlock.isEmpty()) {
// branch to an empty block. Change target to the next block.
BasicBlock nextBlock = targetBlock.getFallThroughBlock();
InlineGuard.setTarget(cb, nextBlock.makeJumpTarget());
bb.recomputeNormalOut(ir); // fix the CFG
return true;
}
return false;
}
/**
* Update the value graph to account for a given InlineGuard instruction.
*
* <p><b>PRECONDITION:</b> <code> InlineGuard.conforms(s); </code>
*
* @param s the instruction in question
*/
private void processInlineGuard(Instruction s) {
ValueGraphVertex v = new ValueGraphVertex(s);
graph.addGraphNode(v);
nameMap.put(s, v);
if (s.operator() == IG_PATCH_POINT) {
// the 'goal' is irrelevant for patch_point guards.
v.setLabel(s.operator(), 1);
link(v, findOrCreateVertex(bypassMoves(InlineGuard.getValue(s))), 0);
} else {
v.setLabel(s.operator(), 2);
link(v, findOrCreateVertex(bypassMoves(InlineGuard.getValue(s))), 0);
link(v, findOrCreateVertex(InlineGuard.getGoal(s)), 1);
}
}
/**
* Update the value graph to account for a given instruction.
*
* @param s the instruction in question
*/
private void processInstruction(Instruction s) {
// TODO: support all necessary types of instructions
if (s.isDynamicLinkingPoint()) {
processCall(s);
} else if (Move.conforms(s)) {
processMove(s);
} else if (s.operator == PI) {
processPi(s);
} else if (New.conforms(s)) {
processNew(s);
} else if (NewArray.conforms(s)) {
processNewArray(s);
} else if (Unary.conforms(s)) {
processUnary(s);
} else if (GuardedUnary.conforms(s)) {
processGuardedUnary(s);
} else if (NullCheck.conforms(s)) {
processNullCheck(s);
} else if (ZeroCheck.conforms(s)) {
processZeroCheck(s);
} else if (Binary.conforms(s)) {
processBinary(s);
} else if (GuardedBinary.conforms(s)) {
processGuardedBinary(s);
} else if (InlineGuard.conforms(s)) {
processInlineGuard(s);
} else if (IfCmp.conforms(s)) {
processIfCmp(s);
} else if (Call.conforms(s)) {
processCall(s);
} else if (MonitorOp.conforms(s)) {
processCall(s);
} else if (Prepare.conforms(s)) {
processCall(s);
} else if (Attempt.conforms(s)) {
processCall(s);
} else if (CacheOp.conforms(s)) {
processCall(s);
} else if (ALoad.conforms(s)) {
processALoad(s);
} else if (PutField.conforms(s)) {
processPutField(s);
} else if (PutStatic.conforms(s)) {
processPutStatic(s);
} else if (AStore.conforms(s)) {
processAStore(s);
} else if (Phi.conforms(s)) {
processPhi(s);
} else if (s.operator() == IR_PROLOGUE) {
processPrologue(s);
}
}
/**
* This method actually does the work of attempting to peephole optimize a branch instruction. See
* Muchnick ~p.590
*
* @param ir the containing IR
* @param s the branch instruction to optimize
* @param bb the containing basic block
* @return {@code true} if an optimization was applied, {@code false} otherwise
*/
@Override
protected boolean optimizeBranchInstruction(IR ir, Instruction s, BasicBlock bb) {
if (Goto.conforms(s)) {
return processGoto(ir, s, bb);
} else if (IfCmp.conforms(s)) {
return processConditionalBranch(ir, s, bb);
} else if (InlineGuard.conforms(s)) {
return processInlineGuard(ir, s, bb);
} else if (IfCmp2.conforms(s)) {
return processTwoTargetConditionalBranch(ir, s, bb);
} else {
return false;
}
}
/**
* Transform to eliminate redundant branches passed on GVNs and dominator information.
*
* @param ir The IR on which to apply the phase
*/
public void perform(IR ir) {
// (1) Remove redundant conditional branches and locally fix the PHIs
GlobalValueNumberState gvns = ir.HIRInfo.valueNumbers;
DominatorTree dt = ir.HIRInfo.dominatorTree;
for (BasicBlockEnumeration bbs = ir.getBasicBlocks(); bbs.hasMoreElements(); ) {
BasicBlock candBB = bbs.next();
Instruction candTest = candBB.firstBranchInstruction();
if (candTest == null) continue;
if (!(IfCmp.conforms(candTest) || InlineGuard.conforms(candTest))) continue;
GVCongruenceClass cc = gvns.congruenceClass(candTest);
if (cc.size() > 1) {
for (ValueGraphVertex vertex : cc) {
Instruction poss = (Instruction) vertex.getName();
if (poss != candTest) {
BasicBlock notTaken = getNotTakenBlock(poss);
BasicBlock taken = poss.getBranchTarget();
if (taken == notTaken) continue; // both go to same block, so we don't know anything!
if (notTaken.hasOneIn() && dt.dominates(notTaken, candBB)) {
if (DEBUG)
VM.sysWrite(candTest + " is dominated by not-taken branch of " + poss + "\n");
removeCondBranch(candBB, candTest, ir, poss);
cc.removeVertex(gvns.valueGraph.getVertex(candTest));
break;
}
if (taken.hasOneIn() && dt.dominates(taken, candBB)) {
if (DEBUG)
VM.sysWrite(candTest + " is dominated by taken branch of " + poss + "\n");
takeCondBranch(candBB, candTest, ir);
cc.removeVertex(gvns.valueGraph.getVertex(candTest));
break;
}
}
}
}
}
// (2) perform a Depth-first search of the control flow graph,
// and remove any nodes we have made unreachable
removeUnreachableCode(ir);
}