Fix s2i improperly clobbering its child's register

Previously, the Power codegen's implementation of the s2i opcode would
attempt to reuse the child node's register if its reference count was 1.
While this works most of the time, this causes issues if the child node
itself reused a register from its child. In this case, the clobbered
register may belong to a node whose reference count is not 1, which can
result in incorrect behaviour.

In general, there is no reason to try and reuse the register of the
child node in s2i, since an extsh instruction is always required. Doing
so does not avoid any register shuffles or additional instructions.
Accordingly, the s2i evaluator will now always allocate its own
register.

Signed-off-by: Ben Thomas <ben@benthomas.ca>

compiler/p/codegen/UnaryEvaluator.cpp

@@ -275,53 +275,18 @@ TR::Register *OMR::Power::TreeEvaluator::b2aEvaluator(TR::Node *node, TR::CodeGe
 TR::Register *OMR::Power::TreeEvaluator::s2iEvaluator(TR::Node *node, TR::CodeGenerator *cg)
    {
    TR::Node *child  = node->getFirstChild();
-   TR::Register *trgReg = 0;
-
-   TR_ASSERT(node->getOpCodeValue() == TR::s2i, "s2iEvaluator called for a %s node",
-             node->getOpCode().getName());
+   TR::Register *trgReg = cg->allocateRegister();
 
-   if (node->getOpCodeValue() == TR::s2i)// s2i sign extend
+   if (child->getOpCode().isLoad() && !child->getRegister() && child->getReferenceCount() == 1)
       {
-      trgReg = child->getRegister();
-      if (!trgReg && child->getOpCode().isMemoryReference() && child->getReferenceCount() == 1)
-         {
-         trgReg = cg->allocateRegister();
-         TR::MemoryReference *tempMR = TR::MemoryReference::createWithRootLoadOrStore(cg, child, 2);
-         generateTrg1MemInstruction(cg, TR::InstOpCode::lha, node, trgReg, tempMR);
-         tempMR->decNodeReferenceCounts(cg);
-         }
-      else
-         {
-         if(!trgReg)
-           {
-           // child reg is null. need to evaluate child
-           trgReg = cg->evaluate(child);
-           if(child->getReferenceCount() == 1)
-             {
-             generateTrg1Src1Instruction(cg, TR::InstOpCode::extsh, node, trgReg, trgReg);
-             }
-           else
-             {
-             TR::Register * trgRegOld = trgReg;
-             trgReg = cg->allocateRegister();
-             generateTrg1Src1Instruction(cg, TR::InstOpCode::extsh, node, trgReg, trgRegOld);
-             }
-           }
-         else if(child->getReferenceCount() > 1)
-           {
-           // child has been evaluated, but will be used later
-           TR::Register * trgRegOld = trgReg;
-           trgReg = cg->allocateRegister();
-           generateTrg1Src1Instruction(cg, TR::InstOpCode::extsh, node, trgReg, trgRegOld);
-           }
-         else
-           {
-           generateTrg1Src1Instruction(cg, TR::InstOpCode::extsh, node, trgReg, trgReg);
-           }
-         }
+      TR::MemoryReference *tempMR = TR::MemoryReference::createWithRootLoadOrStore(cg, child, 2);
+      generateTrg1MemInstruction(cg, TR::InstOpCode::lha, node, trgReg, tempMR);
+      tempMR->decNodeReferenceCounts(cg);
       }
    else
-      trgReg = cg->gprClobberEvaluate(child);
+      {
+      generateTrg1Src1Instruction(cg, TR::InstOpCode::extsh, node, trgReg, cg->evaluate(child));
+      }
 
    node->setRegister(trgReg);
    cg->decReferenceCount(child);