Instead of explicitly disabling a feature by declaring the variable and
set it to false, this change supports the optional flags. I.e., you can
skip certain flags if you are not using it.
This optional feature supports both forms,
1. Value: A parameter for a feature. E.g., EnableRandomOffset
2. Type: A C++ type implementing a feature. E.g., ConditionVariableT
On the other hand, to access the flags will be through one of the
wrappers, BaseConfig/PrimaryConfig/SecondaryConfig/CacheConfig
(CacheConfig is embedded in SecondaryConfig). These wrappers have the
getters to access the value and the type. When adding a new feature, we
need to add it to `allocator_config.def` and mark the new variable with
either *_REQUIRED_* or *_OPTIONAL_* macro so that the accessor will be
generated properly.
In addition, also remove the need of `UseConditionVariable` to flip
on/off of condition variable. Now we only need to define the type of
condition variable.
To enable the condition variable, you have to define both
UseConditionVariable and the ConditionVariableT. Otherwise, it'll be
disabled. However, you may want to disable the condition variable by
setting UseConditionVariable=false, for example, while measuring the
performance and you want to turn it off temporarily. Instead of
requiring the removal of the variable, examining its value makes more
sense.
This may improve the waiting of `Region->MMLock` while trying to refill
the freelist. Instead of always waiting on the completion of
`populateFreeListAndPopBatch()` or `releaseToOSMaybe()`, `pushBlocks()`
also refills the freelist. This increases the chance of earlier return
from `popBatches()`.
The support of condition variable hasn't been done for all platforms.
Therefore, add another `popBatchWithCV()` and it can be configured in
the allocator configuration by setting `Primary::UseConditionVariable`
and the desired `ConditionVariableT`.
Reviewed By: cferris
Differential Revision: https://reviews.llvm.org/D156146
