[llvm] r211900 - Re-apply r211287: Remove support for LLVM runtime multi-threading.
Sean Silva
chisophugis at gmail.com
Tue Jan 27 11:41:34 PST 2015
On Tue, Jan 27, 2015 at 7:13 PM, Owen Anderson <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>
> 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
> > 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
> >
> ==============================================================================
> > --- 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
> >
> ==============================================================================
> > --- 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
> >
> ==============================================================================
> > --- 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
> >
> ==============================================================================
> > --- 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
> >
> ==============================================================================
> > --- 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
> >
> ==============================================================================
> > --- 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
> >
> ==============================================================================
> > --- 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
> >
> >
> >
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>
>
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