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[release/11.0] Use acquire reads for cast cache entries #133052

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[release/11.0] Use acquire reads for cast cache entries #133052

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16 changes: 4 additions & 12 deletions src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/CastCache.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -157,29 +157,21 @@ internal static CastResult TryGet(int[] table, nuint source, nuint target)
// we must read in this order: version -> [entry parts] -> version
// if version is odd or changes, the entry is inconsistent and thus ignored
uint version = Volatile.Read(ref pEntry._version);
nuint entrySource = pEntry._source;
nuint entrySource = Volatile.Read(ref pEntry._source);
// mask the lower version bit to make it even.
// This way we can check if version is odd or changing in just one compare.
version &= unchecked((uint)~1);

if (entrySource == source)
{
// we do ordinary reads of the entry parts and
// Volatile.ReadBarrier() before reading the version
nuint entryTargetAndResult = pEntry._targetAndResult;
// Acquire reads of both entry parts ensure that the second version
// read happens after the entry has been read.
nuint entryTargetAndResult = Volatile.Read(ref pEntry._targetAndResult);
// target never has its lower bit set.
// a matching entryTargetAndResult would the have same bits, except for the lowest one, which is the result.
entryTargetAndResult ^= target;
if (entryTargetAndResult <= 1)
{
// make sure the second read of 'version' happens after reading 'source' and 'targetAndResults'
//
// We can either:
// - use acquires for both _source and _targetAndResults or
// - issue a load barrier before reading _version
// benchmarks on available hardware (Jan 2020) show that use of a read barrier is cheaper.
Volatile.ReadBarrier();

if (version != pEntry._version)
{
// oh, so close, the entry is in inconsistent state.
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