/*
* This file is part of the Jikes RVM project (http://jikesrvm.org).
*
* This file is licensed to You under the Eclipse Public License (EPL);
* You may not use this file except in compliance with the License. You
* may obtain a copy of the License at
*
* http://www.opensource.org/licenses/eclipse-1.0.php
*
* See the COPYRIGHT.txt file distributed with this work for information
* regarding copyright ownership.
*/
package org.jikesrvm.compilers.opt;
import org.jikesrvm.VM;
import org.jikesrvm.compilers.opt.driver.CompilerPhase;
import org.jikesrvm.compilers.opt.ir.IfCmp;
import org.jikesrvm.compilers.opt.ir.Move;
import org.jikesrvm.compilers.opt.ir.NullCheck;
import org.jikesrvm.compilers.opt.ir.BasicBlock;
import org.jikesrvm.compilers.opt.ir.BasicBlockEnumeration;
import org.jikesrvm.compilers.opt.ir.IR;
import org.jikesrvm.compilers.opt.ir.IRTools;
import org.jikesrvm.compilers.opt.ir.Instruction;
import org.jikesrvm.compilers.opt.ir.InstructionEnumeration;
import static org.jikesrvm.compilers.opt.ir.Operators.BBEND;
import static org.jikesrvm.compilers.opt.ir.Operators.CHECKCAST;
import static org.jikesrvm.compilers.opt.ir.Operators.CHECKCAST_NOTNULL;
import static org.jikesrvm.compilers.opt.ir.Operators.GOTO;
import static org.jikesrvm.compilers.opt.ir.Operators.NULL_CHECK;
import static org.jikesrvm.compilers.opt.ir.Operators.REF_IFCMP;
import static org.jikesrvm.compilers.opt.ir.Operators.REF_MOVE;
import org.jikesrvm.compilers.opt.ir.Register;
import org.jikesrvm.compilers.opt.ir.TypeCheck;
import org.jikesrvm.compilers.opt.ir.operand.NullConstantOperand;
import org.jikesrvm.compilers.opt.ir.operand.RegisterOperand;
/**
* Perform simple peephole optimizations to reduce the overhead of
* checking casts. This code was inspired by some special cases in
* handling checkcast in HIR2LIR, but the actual code is all different.
*
* <p> There are currently the following optimizations:
* <ul>
* <li> 1. If a checkcast is just before a nullcheck, invert them and
* convert the checkcast into a checkcast_not_null
* <li> 2. If a checkcast is followed by a branch based on a null test of
* the same variable, then push the cast below the conditional on
* the path where the obejct is known not to be null. And convert
* it to a checkcast_not_null
* </ul>
*/
public final class LocalCastOptimization extends CompilerPhase {
public String getName() {
return "Local Cast Optimizations";
}
public void reportAdditionalStats() {
VM.sysWrite(" ");
VM.sysWrite(container.counter1 / container.counter2 * 100, 2);
VM.sysWrite("% Infrequent BBs");
}
/**
* Return this instance of this phase. This phase contains no
* per-compilation instance fields.
* @param ir not used
* @return this
*/
public CompilerPhase newExecution(IR ir) {
return this;
}
/**
* Main routine: perform the transformation.
* @param ir the IR to transform
*/
public void perform(IR ir) {
// loop over all basic blocks ...
for (BasicBlockEnumeration e = ir.getBasicBlocks(); e.hasMoreElements();) {
BasicBlock bb = e.next();
if (bb.isEmpty()) continue;
container.counter2++;
if (bb.getInfrequent()) {
container.counter1++;
if (ir.options.FREQ_FOCUS_EFFORT) continue;
}
// visit each instruction in the basic block
for (InstructionEnumeration ie = bb.forwardInstrEnumerator(); ie.hasMoreElements();) {
Instruction s = ie.next();
if (TypeCheck.conforms(s) && (invertNullAndTypeChecks(s) || pushTypeCheckBelowIf(s, ir))) {
// hack: we may have modified the instructions; start over
ie = bb.forwardInstrEnumerator();
}
}
}
}
/**
* If there's a checkcast followed by a null check, move the checkcast
* after the null check, since the null pointer exception must be thrown
* anyway.
* @param s the potential checkcast instruction
* @return true iff the transformation happened
*/
private boolean invertNullAndTypeChecks(Instruction s) {
if (s.operator() == CHECKCAST) {
Register r = TypeCheck.getRef(s).asRegister().getRegister();
Instruction n = s.nextInstructionInCodeOrder();
while (n.operator() == REF_MOVE &&
Move.getVal(n) instanceof RegisterOperand &&
Move.getVal(n).asRegister().getRegister() == r) {
r = Move.getResult(n).asRegister().getRegister();
n = n.nextInstructionInCodeOrder();
}
if (n.operator() == NULL_CHECK &&
TypeCheck.getRef(s).asRegister().getRegister() == NullCheck.getRef(n).asRegister().getRegister()) {
s.remove();
TypeCheck.mutate(s,
CHECKCAST_NOTNULL,
TypeCheck.getClearResult(s),
TypeCheck.getClearRef(s),
TypeCheck.getClearType(s),
NullCheck.getGuardResult(n).copy());
n.insertAfter(s);
return true;
}
}
return false;
}
/**
* Where legal, move a type check below an if instruction.
* @param s the potential typecheck instruction
* @param ir the governing IR
*/
private boolean pushTypeCheckBelowIf(Instruction s, IR ir) {
if (s.operator() == CHECKCAST) {
Register r = TypeCheck.getRef(s).asRegister().getRegister();
Instruction n = s.nextInstructionInCodeOrder();
/* find moves of the checked value, so that we can also
optimize cases where the checked value is moved before
it is used
*/
while (n.operator() == REF_MOVE &&
Move.getVal(n) instanceof RegisterOperand &&
Move.getVal(n).asRegister().getRegister() == r) {
r = Move.getResult(n).asRegister().getRegister();
n = n.nextInstructionInCodeOrder();
}
if (n.operator() == REF_IFCMP &&
IfCmp.getVal2(n) instanceof NullConstantOperand &&
IfCmp.getVal1(n) instanceof RegisterOperand &&
r == IfCmp.getVal1(n).asRegister().getRegister()) {
BasicBlock newBlock, patchBlock;
BasicBlock myBlock = n.getBasicBlock();
Instruction after = n.nextInstructionInCodeOrder();
if (IfCmp.getCond(n).isEQUAL())
/* We fall through on non-NULL values, so the
checkcast must be on the not-taken path
from the branch. There are 3 cases:
1. n is the last instruction in its basic block,
in which case control falls through to the next
block in code order. This case is if the
instruction after n is a BBEND
*/ {
if (after.operator() == BBEND) {
patchBlock = myBlock.nextBasicBlockInCodeOrder();
} else if (after.operator() == GOTO) {
/* 2. n is followed by an unconditional goto. In
this case control jumps to the target of the
goto.
*/
patchBlock = after.getBranchTarget();
} else if (after.operator() == REF_IFCMP) {
/* 3. n is followed by another conditional branch. In
this case, we will split the basic block to make
n the last instruction in the block, and then
we have the fall through case again.
*/
patchBlock = myBlock.splitNodeAt(n, ir);
myBlock.insertOut(patchBlock);
ir.cfg.linkInCodeOrder(myBlock, patchBlock);
} else {
/* this is a bad thing */
return false;
}
} else
/* We branch on not-NULL values, so the checkcast
must be spliced in before the branch target
*/ {
patchBlock = n.getBranchTarget();
}
/* add block between branch and appropriate successor */
newBlock = IRTools.makeBlockOnEdge(myBlock, patchBlock, ir);
/* put check in new block */
s.remove();
TypeCheck.mutate(s,
CHECKCAST_NOTNULL,
TypeCheck.getClearResult(s),
TypeCheck.getClearRef(s),
TypeCheck.getClearType(s),
IfCmp.getGuardResult(n).copyRO());
newBlock.prependInstruction(s);
return true;
}
}
return false;
}
}