[llvm] r211900 - Re-apply r211287: Remove support for LLVM runtime multi-threading.

Owen Anderson resistor at mac.com
Tue Jan 27 14:08:37 PST 2015 

So we’re all on the same page, this is a release build with assertions disabled on Darwin x86_64.

—Owen

> On Jan 27, 2015, at 11:59 AM, Owen Anderson <resistor at mac.com> wrote:
> 
> To illustrate, the top of a profile.  I’m running a single .ll file through LLC, with -time-compilations 10000.  I’ve focused the profiler only on the call tree beneath llvm::legacy::PassManagerImpl::run(), and charged a few system library functions to their callers in order to make it more readable.
> 
> <ThreadProfile.png>
> 
> Notice that we’re spending almost 13% of our time in reader/writer locks plus memory fences.  I assume the remaining few percent of regression come from optimizations inhibited by the presence of the locks.
> 
> —Owen
> 
> 
> 
>> On Jan 27, 2015, at 11:41 AM, Sean Silva <chisophugis at gmail.com <mailto:chisophugis at gmail.com>> wrote:
>> 
>> 
>> 
>> On Tue, Jan 27, 2015 at 7:13 PM, Owen Anderson <resistor at mac.com <mailto:resistor at mac.com>> wrote:
>> Hi Chandler,
>> 
>> I hate to revive an ancient thread, but our recent performance work has turned this up as a *massive* performance regression for compilation when the units of compilation are small.  I’m able to consistently measure slowdowns just within PassManager::run() on the order of 15-20%, with read-writer locks at the top of the profile.  Building without threading enabled is not an option, because some of our clients use LLVM in a multithreaded manner, while others do not.
>> 
>> Maybe I'm just out of the loop, but do you know what locks PassManager::run() would end up running up against? Off the top of my head I can't think of any.
>> 
>> -- Sean Silva
>> 
>>  
>> 
>> —Owen
>> 
>> 
>> > On Jun 27, 2014, at 8:13 AM, Chandler Carruth <chandlerc at gmail.com <mailto:chandlerc at gmail.com>> wrote:
>> >
>> > Author: chandlerc
>> > Date: Fri Jun 27 10:13:01 2014
>> > New Revision: 211900
>> >
>> > URL: http://llvm.org/viewvc/llvm-project?rev=211900&view=rev <http://llvm.org/viewvc/llvm-project?rev=211900&view=rev>
>> > Log:
>> > Re-apply r211287: Remove support for LLVM runtime multi-threading.
>> >
>> > I'll fix the problems in libclang and other projects in ways that don't
>> > require <mutex> until we sort out the cygwin situation.
>> >
>> > Modified:
>> >    llvm/trunk/docs/ProgrammersManual.rst
>> >    llvm/trunk/include/llvm-c/Core.h
>> >    llvm/trunk/include/llvm/Support/ManagedStatic.h
>> >    llvm/trunk/include/llvm/Support/Threading.h
>> >    llvm/trunk/lib/IR/Core.cpp
>> >    llvm/trunk/lib/Support/Threading.cpp
>> >    llvm/trunk/unittests/Support/ManagedStatic.cpp
>> >
>> > Modified: llvm/trunk/docs/ProgrammersManual.rst
>> > URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/ProgrammersManual.rst?rev=211900&r1=211899&r2=211900&view=diff <http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/ProgrammersManual.rst?rev=211900&r1=211899&r2=211900&view=diff>
>> > ==============================================================================
>> > --- llvm/trunk/docs/ProgrammersManual.rst (original)
>> > +++ llvm/trunk/docs/ProgrammersManual.rst Fri Jun 27 10:13:01 2014
>> > @@ -2170,46 +2170,13 @@ compiler, consider compiling LLVM and LL
>> > using the resultant compiler to build a copy of LLVM with multithreading
>> > support.
>> >
>> > -.. _startmultithreaded:
>> > -
>> > -Entering and Exiting Multithreaded Mode
>> > ----------------------------------------
>> > -
>> > -In order to properly protect its internal data structures while avoiding
>> > -excessive locking overhead in the single-threaded case, the LLVM must intialize
>> > -certain data structures necessary to provide guards around its internals.  To do
>> > -so, the client program must invoke ``llvm_start_multithreaded()`` before making
>> > -any concurrent LLVM API calls.  To subsequently tear down these structures, use
>> > -the ``llvm_stop_multithreaded()`` call.  You can also use the
>> > -``llvm_is_multithreaded()`` call to check the status of multithreaded mode.
>> > -
>> > -Note that both of these calls must be made *in isolation*.  That is to say that
>> > -no other LLVM API calls may be executing at any time during the execution of
>> > -``llvm_start_multithreaded()`` or ``llvm_stop_multithreaded``.  It is the
>> > -client's responsibility to enforce this isolation.
>> > -
>> > -The return value of ``llvm_start_multithreaded()`` indicates the success or
>> > -failure of the initialization.  Failure typically indicates that your copy of
>> > -LLVM was built without multithreading support, typically because GCC atomic
>> > -intrinsics were not found in your system compiler.  In this case, the LLVM API
>> > -will not be safe for concurrent calls.  However, it *will* be safe for hosting
>> > -threaded applications in the JIT, though :ref:`care must be taken
>> > -<jitthreading>` to ensure that side exits and the like do not accidentally
>> > -result in concurrent LLVM API calls.
>> > -
>> > .. _shutdown:
>> >
>> > Ending Execution with ``llvm_shutdown()``
>> > -----------------------------------------
>> >
>> > When you are done using the LLVM APIs, you should call ``llvm_shutdown()`` to
>> > -deallocate memory used for internal structures.  This will also invoke
>> > -``llvm_stop_multithreaded()`` if LLVM is operating in multithreaded mode.  As
>> > -such, ``llvm_shutdown()`` requires the same isolation guarantees as
>> > -``llvm_stop_multithreaded()``.
>> > -
>> > -Note that, if you use scope-based shutdown, you can use the
>> > -``llvm_shutdown_obj`` class, which calls ``llvm_shutdown()`` in its destructor.
>> > +deallocate memory used for internal structures.
>> >
>> > .. _managedstatic:
>> >
>> > @@ -2217,15 +2184,11 @@ Lazy Initialization with ``ManagedStatic
>> > ------------------------------------------
>> >
>> > ``ManagedStatic`` is a utility class in LLVM used to implement static
>> > -initialization of static resources, such as the global type tables.  Before the
>> > -invocation of ``llvm_shutdown()``, it implements a simple lazy initialization
>> > -scheme.  Once ``llvm_start_multithreaded()`` returns, however, it uses
>> > +initialization of static resources, such as the global type tables.  In a
>> > +single-threaded environment, it implements a simple lazy initialization scheme.
>> > +When LLVM is compiled with support for multi-threading, however, it uses
>> > double-checked locking to implement thread-safe lazy initialization.
>> >
>> > -Note that, because no other threads are allowed to issue LLVM API calls before
>> > -``llvm_start_multithreaded()`` returns, it is possible to have
>> > -``ManagedStatic``\ s of ``llvm::sys::Mutex``\ s.
>> > -
>> > .. _llvmcontext:
>> >
>> > Achieving Isolation with ``LLVMContext``
>> >
>> > Modified: llvm/trunk/include/llvm-c/Core.h
>> > URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm-c/Core.h?rev=211900&r1=211899&r2=211900&view=diff <http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm-c/Core.h?rev=211900&r1=211899&r2=211900&view=diff>
>> > ==============================================================================
>> > --- llvm/trunk/include/llvm-c/Core.h (original)
>> > +++ llvm/trunk/include/llvm-c/Core.h Fri Jun 27 10:13:01 2014
>> > @@ -2848,16 +2848,13 @@ void LLVMDisposePassManager(LLVMPassMana
>> >  * @{
>> >  */
>> >
>> > -/** Allocate and initialize structures needed to make LLVM safe for
>> > -    multithreading. The return value indicates whether multithreaded
>> > -    initialization succeeded. Must be executed in isolation from all
>> > -    other LLVM api calls.
>> > -    @see llvm::llvm_start_multithreaded */
>> > +/** Deprecated: Multi-threading can only be enabled/disabled with the compile
>> > +    time define LLVM_ENABLE_THREADS.  This function always returns
>> > +    LLVMIsMultithreaded(). */
>> > LLVMBool LLVMStartMultithreaded(void);
>> >
>> > -/** Deallocate structures necessary to make LLVM safe for multithreading.
>> > -    Must be executed in isolation from all other LLVM api calls.
>> > -    @see llvm::llvm_stop_multithreaded */
>> > +/** Deprecated: Multi-threading can only be enabled/disabled with the compile
>> > +    time define LLVM_ENABLE_THREADS. */
>> > void LLVMStopMultithreaded(void);
>> >
>> > /** Check whether LLVM is executing in thread-safe mode or not.
>> >
>> > Modified: llvm/trunk/include/llvm/Support/ManagedStatic.h
>> > URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/ManagedStatic.h?rev=211900&r1=211899&r2=211900&view=diff <http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/ManagedStatic.h?rev=211900&r1=211899&r2=211900&view=diff>
>> > ==============================================================================
>> > --- llvm/trunk/include/llvm/Support/ManagedStatic.h (original)
>> > +++ llvm/trunk/include/llvm/Support/ManagedStatic.h Fri Jun 27 10:13:01 2014
>> > @@ -103,9 +103,6 @@ void llvm_shutdown();
>> > /// llvm_shutdown() when it is destroyed.
>> > struct llvm_shutdown_obj {
>> >   llvm_shutdown_obj() { }
>> > -  explicit llvm_shutdown_obj(bool multithreaded) {
>> > -    if (multithreaded) llvm_start_multithreaded();
>> > -  }
>> >   ~llvm_shutdown_obj() { llvm_shutdown(); }
>> > };
>> >
>> >
>> > Modified: llvm/trunk/include/llvm/Support/Threading.h
>> > URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/Threading.h?rev=211900&r1=211899&r2=211900&view=diff <http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/Threading.h?rev=211900&r1=211899&r2=211900&view=diff>
>> > ==============================================================================
>> > --- llvm/trunk/include/llvm/Support/Threading.h (original)
>> > +++ llvm/trunk/include/llvm/Support/Threading.h Fri Jun 27 10:13:01 2014
>> > @@ -7,7 +7,8 @@
>> > //
>> > //===----------------------------------------------------------------------===//
>> > //
>> > -// TThis file defines llvm_start_multithreaded() and friends.
>> > +// This file declares helper functions for running LLVM in a multi-threaded
>> > +// environment.
>> > //
>> > //===----------------------------------------------------------------------===//
>> >
>> > @@ -15,22 +16,8 @@
>> > #define LLVM_SUPPORT_THREADING_H
>> >
>> > namespace llvm {
>> > -  /// llvm_start_multithreaded - Allocate and initialize structures needed to
>> > -  /// make LLVM safe for multithreading.  The return value indicates whether
>> > -  /// multithreaded initialization succeeded.  LLVM will still be operational
>> > -  /// on "failed" return, and will still be safe for hosting threading
>> > -  /// applications in the JIT, but will not be safe for concurrent calls to the
>> > -  /// LLVM APIs.
>> > -  /// THIS MUST EXECUTE IN ISOLATION FROM ALL OTHER LLVM API CALLS.
>> > -  bool llvm_start_multithreaded();
>> > -
>> > -  /// llvm_stop_multithreaded - Deallocate structures necessary to make LLVM
>> > -  /// safe for multithreading.
>> > -  /// THIS MUST EXECUTE IN ISOLATION FROM ALL OTHER LLVM API CALLS.
>> > -  void llvm_stop_multithreaded();
>> > -
>> > -  /// llvm_is_multithreaded - Check whether LLVM is executing in thread-safe
>> > -  /// mode or not.
>> > +  /// Returns true if LLVM is compiled with support for multi-threading, and
>> > +  /// false otherwise.
>> >   bool llvm_is_multithreaded();
>> >
>> >   /// llvm_execute_on_thread - Execute the given \p UserFn on a separate
>> >
>> > Modified: llvm/trunk/lib/IR/Core.cpp
>> > URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/Core.cpp?rev=211900&r1=211899&r2=211900&view=diff <http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/Core.cpp?rev=211900&r1=211899&r2=211900&view=diff>
>> > ==============================================================================
>> > --- llvm/trunk/lib/IR/Core.cpp (original)
>> > +++ llvm/trunk/lib/IR/Core.cpp Fri Jun 27 10:13:01 2014
>> > @@ -2708,11 +2708,10 @@ void LLVMDisposePassManager(LLVMPassMana
>> > /*===-- Threading ------------------------------------------------------===*/
>> >
>> > LLVMBool LLVMStartMultithreaded() {
>> > -  return llvm_start_multithreaded();
>> > +  return LLVMIsMultithreaded();
>> > }
>> >
>> > void LLVMStopMultithreaded() {
>> > -  llvm_stop_multithreaded();
>> > }
>> >
>> > LLVMBool LLVMIsMultithreaded() {
>> >
>> > Modified: llvm/trunk/lib/Support/Threading.cpp
>> > URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/Threading.cpp?rev=211900&r1=211899&r2=211900&view=diff <http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/Threading.cpp?rev=211900&r1=211899&r2=211900&view=diff>
>> > ==============================================================================
>> > --- llvm/trunk/lib/Support/Threading.cpp (original)
>> > +++ llvm/trunk/lib/Support/Threading.cpp Fri Jun 27 10:13:01 2014
>> > @@ -7,7 +7,8 @@
>> > //
>> > //===----------------------------------------------------------------------===//
>> > //
>> > -// This file implements llvm_start_multithreaded() and friends.
>> > +// This file defines helper functions for running LLVM in a multi-threaded
>> > +// environment.
>> > //
>> > //===----------------------------------------------------------------------===//
>> >
>> > @@ -19,38 +20,14 @@
>> >
>> > using namespace llvm;
>> >
>> > -static bool multithreaded_mode = false;
>> > -
>> > -bool llvm::llvm_start_multithreaded() {
>> > +bool llvm::llvm_is_multithreaded() {
>> > #if LLVM_ENABLE_THREADS != 0
>> > -  assert(!multithreaded_mode && "Already multithreaded!");
>> > -  multithreaded_mode = true;
>> > -
>> > -  // We fence here to ensure that all initialization is complete BEFORE we
>> > -  // return from llvm_start_multithreaded().
>> > -  sys::MemoryFence();
>> >   return true;
>> > #else
>> >   return false;
>> > #endif
>> > }
>> >
>> > -void llvm::llvm_stop_multithreaded() {
>> > -#if LLVM_ENABLE_THREADS != 0
>> > -  assert(multithreaded_mode && "Not currently multithreaded!");
>> > -
>> > -  // We fence here to insure that all threaded operations are complete BEFORE we
>> > -  // return from llvm_stop_multithreaded().
>> > -  sys::MemoryFence();
>> > -
>> > -  multithreaded_mode = false;
>> > -#endif
>> > -}
>> > -
>> > -bool llvm::llvm_is_multithreaded() {
>> > -  return multithreaded_mode;
>> > -}
>> > -
>> > #if LLVM_ENABLE_THREADS != 0 && defined(HAVE_PTHREAD_H)
>> > #include <pthread.h>
>> >
>> >
>> > Modified: llvm/trunk/unittests/Support/ManagedStatic.cpp
>> > URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/Support/ManagedStatic.cpp?rev=211900&r1=211899&r2=211900&view=diff <http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/Support/ManagedStatic.cpp?rev=211900&r1=211899&r2=211900&view=diff>
>> > ==============================================================================
>> > --- llvm/trunk/unittests/Support/ManagedStatic.cpp (original)
>> > +++ llvm/trunk/unittests/Support/ManagedStatic.cpp Fri Jun 27 10:13:01 2014
>> > @@ -47,7 +47,6 @@ TEST(Initialize, MultipleThreads) {
>> >   void *p1 = test1::allocate_stack(a1);
>> >   void *p2 = test1::allocate_stack(a2);
>> >
>> > -  llvm_start_multithreaded();
>> >   pthread_t t1, t2;
>> >   pthread_create(&t1, &a1, test1::helper, nullptr);
>> >   pthread_create(&t2, &a2, test1::helper, nullptr);
>> > @@ -55,7 +54,6 @@ TEST(Initialize, MultipleThreads) {
>> >   pthread_join(t2, nullptr);
>> >   free(p1);
>> >   free(p2);
>> > -  llvm_stop_multithreaded();
>> > }
>> > #endif
>> >
>> >
>> >
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