/**
* Sorts move instructions added via {@code addMoveToEnd} during phi removal so that results don't
* overwrite sources that are used. For use after all phis have been removed and all calls to
* addMoveToEnd() have been made.
*
* <p>This is necessary because copy-propogation may have left us in a state where the same basic
* block has the same register as a phi operand and a result. In this case, the register in the
* phi operand always refers value before any other phis have executed.
*/
public void scheduleMovesFromPhis() {
if (movesFromPhisAtBeginning > 1) {
List<SsaInsn> toSchedule;
toSchedule = insns.subList(0, movesFromPhisAtBeginning);
scheduleUseBeforeAssigned(toSchedule);
SsaInsn firstNonPhiMoveInsn = insns.get(movesFromPhisAtBeginning);
/*
* TODO: It's actually possible that this case never happens,
* because a move-exception block, having only one predecessor in
* SSA form, perhaps is never on a dominance frontier.
*/
if (firstNonPhiMoveInsn.isMoveException()) {
if (true) {
/*
* We've yet to observe this case, and if it can occur the
* code written to handle it probably does not work.
*/
throw new RuntimeException("Unexpected: moves from " + "phis before move-exception");
} else {
/*
* A move-exception insn must be placed first in this block
* We need to move it there, and deal with possible
* interference.
*/
boolean moveExceptionInterferes = false;
int moveExceptionResult = firstNonPhiMoveInsn.getResult().getReg();
/*
* Does the move-exception result reg interfere with the phi
* moves?
*/
for (SsaInsn insn : toSchedule) {
if (insn.isResultReg(moveExceptionResult) || insn.isRegASource(moveExceptionResult)) {
moveExceptionInterferes = true;
break;
}
}
if (!moveExceptionInterferes) {
// This is the easy case.
insns.remove(movesFromPhisAtBeginning);
insns.add(0, firstNonPhiMoveInsn);
} else {
/*
* We need to move the result to a spare reg and move it
* back.
*/
RegisterSpec originalResultSpec = firstNonPhiMoveInsn.getResult();
int spareRegister = parent.borrowSpareRegister(originalResultSpec.getCategory());
// We now move it to a spare register.
firstNonPhiMoveInsn.changeResultReg(spareRegister);
RegisterSpec tempSpec = firstNonPhiMoveInsn.getResult();
insns.add(0, firstNonPhiMoveInsn);
// And here we move it back.
NormalSsaInsn toAdd =
new NormalSsaInsn(
new PlainInsn(
Rops.opMove(tempSpec.getType()),
SourcePosition.NO_INFO,
originalResultSpec,
RegisterSpecList.make(tempSpec)),
this);
/*
* Place it immediately after the phi-moves, overwriting
* the move-exception that was there.
*/
insns.set(movesFromPhisAtBeginning + 1, toAdd);
}
}
}
}
if (movesFromPhisAtEnd > 1) {
scheduleUseBeforeAssigned(
insns.subList(insns.size() - movesFromPhisAtEnd - 1, insns.size() - 1));
}
// Return registers borrowed here and in scheduleUseBeforeAssigned().
parent.returnSpareRegisters();
}
/**
* Checks to see if the register is used in a bitset, taking into account its category/width.
*
* @param regsUsed set, indexed by register number
* @param rs register to mark as used
* @return true if register is fully or partially (for the case of wide registers) used.
*/
private static boolean checkRegUsed(BitSet regsUsed, RegisterSpec rs) {
int reg = rs.getReg();
int category = rs.getCategory();
return regsUsed.get(reg) || (category == 2 ? regsUsed.get(reg + 1) : false);
}
/**
* Ensures that all move operations in this block occur such that reads of any register happen
* before writes to that register. NOTE: caller is expected to returnSpareRegisters()! TODO: See
* Briggs, et al "Practical Improvements to the Construction and Destruction of Static Single
* Assignment Form" section 5. a) This can be done in three passes.
*
* @param toSchedule List of instructions. Must consist only of moves.
*/
private void scheduleUseBeforeAssigned(List<SsaInsn> toSchedule) {
BitSet regsUsedAsSources = new BitSet(parent.getRegCount());
// TODO: Get rid of this.
BitSet regsUsedAsResults = new BitSet(parent.getRegCount());
int sz = toSchedule.size();
int insertPlace = 0;
while (insertPlace < sz) {
int oldInsertPlace = insertPlace;
// Record all registers used as sources in this block.
for (int i = insertPlace; i < sz; i++) {
setRegsUsed(regsUsedAsSources, toSchedule.get(i).getSources().get(0));
setRegsUsed(regsUsedAsResults, toSchedule.get(i).getResult());
}
/*
* If there are no circular dependencies, then there exists n
* instructions where n > 1 whose result is not used as a source.
*/
for (int i = insertPlace; i < sz; i++) {
SsaInsn insn = toSchedule.get(i);
/*
* Move these n registers to the front, since they overwrite
* nothing.
*/
if (!checkRegUsed(regsUsedAsSources, insn.getResult())) {
Collections.swap(toSchedule, i, insertPlace++);
}
}
/*
* If we've made no progress in this iteration, there's a circular
* dependency. Split it using the temp reg.
*/
if (oldInsertPlace == insertPlace) {
SsaInsn insnToSplit = null;
// Find an insn whose result is used as a source.
for (int i = insertPlace; i < sz; i++) {
SsaInsn insn = toSchedule.get(i);
if (checkRegUsed(regsUsedAsSources, insn.getResult())
&& checkRegUsed(regsUsedAsResults, insn.getSources().get(0))) {
insnToSplit = insn;
/*
* We're going to split this insn; move it to the front.
*/
Collections.swap(toSchedule, insertPlace, i);
break;
}
}
// At least one insn will be set above.
RegisterSpec result = insnToSplit.getResult();
RegisterSpec tempSpec = result.withReg(parent.borrowSpareRegister(result.getCategory()));
NormalSsaInsn toAdd =
new NormalSsaInsn(
new PlainInsn(
Rops.opMove(result.getType()),
SourcePosition.NO_INFO,
tempSpec,
insnToSplit.getSources()),
this);
toSchedule.add(insertPlace++, toAdd);
RegisterSpecList newSources = RegisterSpecList.make(tempSpec);
NormalSsaInsn toReplace =
new NormalSsaInsn(
new PlainInsn(
Rops.opMove(result.getType()), SourcePosition.NO_INFO, result, newSources),
this);
toSchedule.set(insertPlace, toReplace);
// The size changed.
sz = toSchedule.size();
}
regsUsedAsSources.clear();
regsUsedAsResults.clear();
}
}
/**
* Sets the register as used in a bitset, taking into account its category/width.
*
* @param regsUsed set, indexed by register number
* @param rs register to mark as used
*/
private static void setRegsUsed(BitSet regsUsed, RegisterSpec rs) {
regsUsed.set(rs.getReg());
if (rs.getCategory() > 1) {
regsUsed.set(rs.getReg() + 1);
}
}
