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/* NSC -- new scala compiler
* Copyright 2005 LAMP/EPFL
* @author Iulian Dragos
*/
// $Id$
package scala.tools.nsc.backend.opt;
import scala.collection._
import scala.collection.immutable.{Map, HashMap, Set, HashSet};
import scala.tools.nsc.backend.icode.analysis.LubError;
import scala.tools.nsc.symtab._;
/**
*/
abstract class DeadCodeElimination extends SubComponent {
import global._;
import icodes._;
import icodes.opcodes._;
val phaseName = "dce";
/** Create a new phase */
override def newPhase(p: Phase) = new DeadCodeEliminationPhase(p);
/** Dead code elimination phase.
*/
class DeadCodeEliminationPhase(prev: Phase) extends ICodePhase(prev) {
def name = phaseName
val dce = new DeadCode();
override def apply(c: IClass): Unit =
if (settings.Xdce.value)
dce.analyzeClass(c)
}
/** Remove dead code.
*/
class DeadCode {
def analyzeClass(cls: IClass): Unit =
cls.methods.foreach { m =>
this.method = m
// analyzeMethod(m);
dieCodeDie(m)
}
val rdef = new reachingDefinitions.ReachingDefinitionsAnalysis;
/** Use-def chain: give the reaching definitions at the beginning of given instruction. */
var defs: Map[(BasicBlock, Int), Set[rdef.lattice.Definition]] = HashMap.empty
/** Useful instructions which have not been scanned yet. */
val worklist: mutable.Set[(BasicBlock, Int)] = new mutable.LinkedHashSet
/** what instructions have been marked as useful? */
val useful: mutable.Map[BasicBlock, mutable.BitSet] = new mutable.HashMap
/** the current method. */
var method: IMethod = _
def dieCodeDie(m: IMethod): Unit = if (m.code ne null) {
log("dead code elimination on " + m);
// (new DepthFirstLinerizer).linearize(m)
m.code.blocks.clear
m.code.blocks ++= linearizer.linearize(m)
collectRDef(m)
mark
sweep(m)
}
/** collect reaching definitions and initial useful instructions for this method. */
def collectRDef(m: IMethod): Unit = if (m.code ne null) {
defs = HashMap.empty; worklist.clear; useful.clear;
rdef.init(m);
rdef.run;
for (val bb <- m.code.blocks.toList) {
useful(bb) = new mutable.BitSet(bb.size)
var rd = rdef.in(bb);
for (val Pair(i, idx) <- bb.toList.zipWithIndex) {
i match {
case LOAD_LOCAL(l) =>
defs = defs + ((bb, idx)) -> rd._1
// Console.println(i + ": " + (bb, idx) + " rd: " + rd + " and having: " + defs)
case RETURN(_) | JUMP(_) | CJUMP(_, _, _, _) | CZJUMP(_, _, _, _) |
DROP(_) | THROW() | STORE_FIELD(_, _) | STORE_ARRAY_ITEM(_) |
LOAD_EXCEPTION() | SWITCH(_, _) | MONITOR_ENTER() | MONITOR_EXIT() => worklist += ((bb, idx))
case CALL_METHOD(m1, _) if isSideEffecting(m1) => worklist += ((bb, idx))
case CALL_METHOD(m1, SuperCall(_)) =>
worklist += ((bb, idx)) // super calls to constructor
case _ => ()
}
rd = rdef.interpret(bb, idx, rd);
}
}
}
/** Mark useful instructions. Instructions in the worklist are each inspected and their
* dependecies are marked useful too, and added to the worklist.
*/
def mark {
// log("Starting with worklist: " + worklist)
while (!worklist.isEmpty) {
val (bb, idx) = worklist.elements.next
worklist -= ((bb, idx))
if (settings.debug.value)
log("Marking instr: \tBB_" + bb + ": " + idx + " " + bb(idx))
val instr = bb(idx)
if (!useful(bb)(idx)) {
useful(bb) += idx
instr match {
case LOAD_LOCAL(l1) =>
for (val (l2, bb1, idx1) <- defs((bb, idx)); l1 == l2; !useful(bb1)(idx1))
worklist += ((bb1, idx1))
case nw @ NEW(_) =>
assert(nw.init ne null, "null new.init at: " + bb + ": " + idx + "(" + instr + ")")
worklist += findInstruction(bb, nw.init)
case LOAD_EXCEPTION() =>
()
case _ =>
for (val (bb1, idx1) <- findDefs(bb, idx, instr.consumed); !useful(bb1)(idx1))
worklist += ((bb1, idx1))
}
}
}
}
def sweep(m: IMethod) {
val compensations = computeCompensations(m)
for (val bb <- m.code.blocks.toList) {
// Console.println("** Sweeping block " + bb + " **")
val oldInstr = bb.toList
bb.open
bb.clear
for (val Pair(i, idx) <- oldInstr.zipWithIndex) {
if (useful(bb)(idx)) {
// log(" " + i + " is useful")
bb.emit(i, i.pos)
compensations.get(bb, idx) match {
case Some(is) => is foreach bb.emit
case None => ()
}
} else {
i match {
case NEW(REFERENCE(sym)) =>
log("skipped object creation: " + sym)
//Console.println("skipping class file altogether: " + sym.fullNameString)
icodes.classes -= sym
case _ => ()
}
log("Skipped: bb_" + bb + ": " + idx + "( " + i + ")")
}
}
if (bb.size > 0)
bb.close
else
log("empty block encountered")
}
}
private def computeCompensations(m: IMethod): mutable.Map[(BasicBlock, Int), List[Instruction]] = {
val compensations: mutable.Map[(BasicBlock, Int), List[Instruction]] = new mutable.HashMap
for (val bb <- m.code.blocks.toList) {
assert(bb.isClosed, "Open block in computeCompensations")
for (val (i, idx) <- bb.toList.zipWithIndex) {
if (!useful(bb)(idx)) {
val defs = findDefs(bb, idx, i.consumed)
for (val d <- defs) {
if (!compensations.isDefinedAt(d))
compensations(d) = i.consumedTypes map DROP
}
}
}
}
compensations
}
private def withClosed[a](bb: BasicBlock)(f: => a): a = {
if (bb.size > 0) bb.close
val res = f
if (bb.size > 0) bb.open
res
}
private def findInstruction(bb: BasicBlock, i: Instruction): (BasicBlock, Int) = {
def find(bb: BasicBlock): Option[(BasicBlock, Int)] = {
var xs = bb.toList
xs.zipWithIndex find { hd => hd._1 eq i } match {
case Some(_, idx) => Some(bb, idx)
case None => None
}
}
for (val b <- linearizer.linearizeAt(method, bb))
find(b) match {
case Some(p) => return p
case None => ()
}
abort("could not find init in: " + method)
}
def findDefs(bb: BasicBlock, idx: Int, m: Int): List[(BasicBlock, Int)] = if (idx > 0) {
assert(bb.isClosed)
var instrs = bb.getArray
var res: List[(BasicBlock, Int)] = Nil
var i = idx
var n = m
var d = 0
// "I look for who produced the 'n' elements below the 'd' topmost slots of the stack"
while (n > 0 && i > 0) {
i = i - 1
val prod = instrs(i).produced
if (prod > d) {
res = (bb, i) :: res
n = n - (prod - d)
if (bb(i) != LOAD_EXCEPTION)
d = instrs(i).consumed
} else {
d = d - prod
d = d + instrs(i).consumed;
}
}
if (n > 0) {
val stack = rdef.in(bb)._2
assert(stack.length >= n, "entry stack is too small, expected: " + n + " found: " + stack)
stack.drop(d).take(n) foreach { defs =>
res = defs.toList ::: res
}
}
res
} else {
val stack = rdef.in(bb)._2
assert(stack.length >= m, "entry stack is too small, expected: " + m + " found: " + stack)
stack.take(m) flatMap (.toList)
}
/** Is 'sym' a side-effecting method? TODO: proper analysis. */
private def isSideEffecting(sym: Symbol): Boolean = {
!(sym.isGetter // for testing only
|| (sym.isConstructor
&& sym.owner.owner == definitions.getModule("scala.runtime").moduleClass)
|| (sym.isConstructor && inliner.isClosureClass(sym.owner)))
}
} /* DeadCode */
}
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