/**
* Attempt to coalesce register r2 into register r1. If this is legal,
*
* <ul>
* <li>rewrite all defs and uses of r2 as defs and uses of r1
* <li>update the liveness information
* <li>update the def-use chains
* </ul>
*
* <strong>PRECONDITION </strong> def-use chains must be computed and valid. <strong>PRECONDITION
* </strong> instructions are numbered, with numbers stored in OPT_Instruction.scratch
*
* @param ir the governing IR
* @param live liveness information for the IR
* @param r1
* @param r2
* @return true if the transformation succeeded, false otherwise.
*/
public static boolean attempt(
OPT_IR ir, OPT_LiveAnalysis live, OPT_Register r1, OPT_Register r2) {
// make sure r1 and r2 are not simultaneously live
if (isLiveAtDef(r2, r1, live)) return false;
if (isLiveAtDef(r1, r2, live)) return false;
// Liveness is OK. Check for SPLIT operations
if (split(r1, r2)) return false;
// Don't merge a register with itself
if (r1 == r2) return false;
// Update liveness information to reflect the merge.
live.merge(r1, r2);
// Merge the defs.
for (OPT_RegisterOperandEnumeration e = OPT_DefUse.defs(r2); e.hasMoreElements(); ) {
OPT_RegisterOperand def = (OPT_RegisterOperand) e.nextElement();
OPT_DefUse.removeDef(def);
def.register = r1;
OPT_DefUse.recordDef(def);
}
// Merge the uses.
for (OPT_RegisterOperandEnumeration e = OPT_DefUse.uses(r2); e.hasMoreElements(); ) {
OPT_RegisterOperand use = (OPT_RegisterOperand) e.nextElement();
OPT_DefUse.removeUse(use);
use.register = r1;
OPT_DefUse.recordUse(use);
}
return true;
}
/** Is there an instruction r1 = split r2 or r2 = split r1?? */
private static boolean split(OPT_Register r1, OPT_Register r2) {
for (OPT_RegisterOperandEnumeration e = OPT_DefUse.defs(r1); e.hasMoreElements(); ) {
OPT_RegisterOperand def = (OPT_RegisterOperand) e.nextElement();
OPT_Instruction s = def.instruction;
if (s.operator == SPLIT) {
OPT_Operand rhs = Unary.getVal(s);
if (rhs.similar(def)) return true;
}
}
for (OPT_RegisterOperandEnumeration e = OPT_DefUse.defs(r2); e.hasMoreElements(); ) {
OPT_RegisterOperand def = (OPT_RegisterOperand) e.nextElement();
OPT_Instruction s = def.instruction;
if (s.operator == SPLIT) {
OPT_Operand rhs = Unary.getVal(s);
if (rhs.similar(def)) return true;
}
}
return false;
}
/**
* Is register r1 live at any def of register r2?
*
* <p><strong>PRECONDITION </strong> def-use chains must be computed and valid.
* <strong>PRECONDITION </strong> instructions are numbered, with numbers stored in
* OPT_Instruction.scratch
*
* <p>Note: this implementation is not efficient. The liveness data structures must be re-designed
* to support this efficiently.
*/
private static boolean isLiveAtDef(OPT_Register r1, OPT_Register r2, OPT_LiveAnalysis live) {
for (Iterator e = live.iterateLiveIntervals(r1); e.hasNext(); ) {
OPT_LiveIntervalElement elem = (OPT_LiveIntervalElement) e.next();
OPT_BasicBlock bb = elem.getBasicBlock();
OPT_Instruction begin = (elem.getBegin() == null) ? bb.firstInstruction() : elem.getBegin();
OPT_Instruction end = (elem.getEnd() == null) ? bb.lastInstruction() : elem.getEnd();
int low = begin.scratch;
int high = end.scratch;
for (Enumeration defs = OPT_DefUse.defs(r2); defs.hasMoreElements(); ) {
OPT_Operand def = (OPT_Operand) defs.nextElement();
int n = def.instruction.scratch;
if (n >= low && n < high) {
return true;
}
}
}
// no conflict was found.
return false;
}