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Check if instanceDescription is an immediate value in loop #15713

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Aug 19, 2022
Merged

Check if instanceDescription is an immediate value in loop #15713

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13 changes: 11 additions & 2 deletions runtime/compiler/env/j9fieldsInfo.cpp
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
/*******************************************************************************
* Copyright (c) 2000, 2021 IBM Corp. and others
* Copyright (c) 2000, 2022 IBM Corp. and others
*
* This program and the accompanying materials are made available under
* the terms of the Eclipse Public License 2.0 which accompanies this
Expand Down Expand Up @@ -135,11 +135,14 @@ TR_VMFieldsInfo::TR_VMFieldsInfo(TR::Compilation * comp, J9Class *aClazz, int bu
UDATA *descriptorPtr = aClazz->instanceDescription;
UDATA descriptorWord=0;

bool useImmediate = false;
int32_t bitIndex = 0;
if ( ((UDATA) descriptorPtr) & BCT_J9DescriptionImmediate )
{
bitIndex++;
descriptorWord = ((UDATA) descriptorPtr) >> 1;

useImmediate = true;
}
else
{
Expand All @@ -158,7 +161,12 @@ TR_VMFieldsInfo::TR_VMFieldsInfo(TR::Compilation * comp, J9Class *aClazz, int bu
_gcDescriptor.push_back(countSlots);
}
countSlots++;
if (countSlots >= (slotsInHeader + numSlotsInObject))
// There is a case where the number of object slots are greater than the number of
// bits in a word and instanceDescription is used as an immediate value instead of
// a pointer, such as when the class doesn't have any references. We need to break
// the loop in this case before instanceDescription is dereferenced as a pointer.
if ((countSlots >= (slotsInHeader + numSlotsInObject)) ||
(useImmediate && bitIndex == (numBitsInWord - 1)))
{
break;
}
Expand All @@ -174,6 +182,7 @@ TR_VMFieldsInfo::TR_VMFieldsInfo(TR::Compilation * comp, J9Class *aClazz, int bu
bitIndex++;
}
}

// null terminated
_gcDescriptor.push_back(0);
}
Expand Down