STM slightly conservative on write-only transactions

The STM appears to be slightly too conservative on write-only transactions.

It will store a copy of the current (that is, old) value on a writeTVar, even if it was never read from. This can cause a spurious retry.

E.g. atomically $ writeTVar tv 0 really has no reason to ever retry. Neither, in fact, does atomically $ mapM (\tv -> writeTVar tv 0) [tv1,tv2,tv3,tv4].

My suggestion is to instead indicate "no prior value" for that TVar, and then make no consistency checks on TVars marked as no prior value...

Jules

Trac metadata

Trac field	Value
Version	6.8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown