* Avoid copy constructor
```
src/core/lib/gpr/log_linux.cc:78:33: error: copying variable of type 'grpc_core::PthreadTlsImpl<long>' invokes deleted constructor
static GPR_THREAD_LOCAL(long) tid = 0;
^ ~
./src/core/lib/gpr/tls.h:64:3: note: 'PthreadTlsImpl' has been explicitly marked deleted here
PthreadTlsImpl(const PthreadTlsImpl&) = delete;
^
1 error generated.
```
* Generalize pthread TLS to any trivial type
Use multiple pthread keys for types larger than a machine word.
Implement generic timer TLS optimization on all platforms.
See code commentary for an explanation.
Add an additional constructor to allow `log_linux.cc` to compile with
GPR_PTHREAD_TLS. Without it:
```
../../third_party/grpc/src/core/lib/gpr/log_linux.cc:78:33: error: no viable conversion from 'int' to 'grpc_core::PthreadTlsImpl<long>'
static GPR_THREAD_LOCAL(long) tid = 0;
^ ~
../../third_party/grpc/src/core/lib/gpr/tls.h:64:3: note: candidate constructor not viable: no known conversion from 'int' to 'const grpc_core::PthreadTlsImpl<long> &' for 1st argument
PthreadTlsImpl(const PthreadTlsImpl&) = delete;
^
1 error generated.
```
* Implement type safety for TLS
This is mostly free when compiler support is available, but requires
careful templating when implemented using pthread.
Significantly slimmed the tls.h interface; it now only defines the "TLS
keyword" for each supported compiler, delegating enforcement of correct
usage (i.e. must be static) to the compiler itself.
Implemented implicit conversion for the pthread wrapper so it can be
used (mostly) the same as native support. Notable exception to this is
that static_cast<void*> is needed when printing a pointer stored in TLS
as %p.
* Use GPR_THREAD_LOCAL macros consistently
More generic configuration system is introduced in order to i) unify the
way how modules access the configurations instead of using low-level
get/setenv functions and ii) enable the customization for where configuration
is stored. This could be extended to support flag, file, etc.
Default configuration system uses environment variables as before so
basically this is expected to work just as it did. This behavior can
change by redefining GPR_GLOBAL_CONFIG_DEFINE_*type* macros.
* Migrated configuration
GRPC_CLIENT_CHANNEL_BACKUP_POLL_INTERVAL_MS
GRPC_EXPERIMENTAL_DISABLE_FLOW_CONTROL
GRPC_ABORT_ON_LEAKS
GRPC_NOT_USE_SYSTEM_SSL_ROOTS
alignment options (for cache-alignment).
We shrink by:
1) Removing an unnecessary zone pointer.
2) Replacing gpr_mu (40 bytes when using pthread_mutex_t) with
std::atomic_flag.
We also header-inline the fastpath alloc (ie. when not doing a zone
alloc) and move the malloc() for a zone alloc outside of the mutex
critical zone, which allows us to replace the mutex with a spinlock.
We also cache-align created arenas.