<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Jan 27, 2015 at 7:13 PM, Owen Anderson <span dir="ltr"><<a href="mailto:resistor@mac.com" target="_blank">resistor@mac.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi Chandler,<br>
<br>
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.<br></blockquote><div><br></div><div>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.</div><div><br></div><div>-- Sean Silva</div><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<span class="HOEnZb"><font color="#888888"><br>
—Owen<br>
</font></span><div class="HOEnZb"><div class="h5"><br>
<br>
> On Jun 27, 2014, at 8:13 AM, Chandler Carruth <<a href="mailto:chandlerc@gmail.com">chandlerc@gmail.com</a>> wrote:<br>
><br>
> Author: chandlerc<br>
> Date: Fri Jun 27 10:13:01 2014<br>
> New Revision: 211900<br>
><br>
> URL: <a href="http://llvm.org/viewvc/llvm-project?rev=211900&view=rev" target="_blank">http://llvm.org/viewvc/llvm-project?rev=211900&view=rev</a><br>
> Log:<br>
> Re-apply r211287: Remove support for LLVM runtime multi-threading.<br>
><br>
> I'll fix the problems in libclang and other projects in ways that don't<br>
> require <mutex> until we sort out the cygwin situation.<br>
><br>
> Modified:<br>
> llvm/trunk/docs/ProgrammersManual.rst<br>
> llvm/trunk/include/llvm-c/Core.h<br>
> llvm/trunk/include/llvm/Support/ManagedStatic.h<br>
> llvm/trunk/include/llvm/Support/Threading.h<br>
> llvm/trunk/lib/IR/Core.cpp<br>
> llvm/trunk/lib/Support/Threading.cpp<br>
> llvm/trunk/unittests/Support/ManagedStatic.cpp<br>
><br>
> Modified: llvm/trunk/docs/ProgrammersManual.rst<br>
> URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/ProgrammersManual.rst?rev=211900&r1=211899&r2=211900&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/ProgrammersManual.rst?rev=211900&r1=211899&r2=211900&view=diff</a><br>
> ==============================================================================<br>
> --- llvm/trunk/docs/ProgrammersManual.rst (original)<br>
> +++ llvm/trunk/docs/ProgrammersManual.rst Fri Jun 27 10:13:01 2014<br>
> @@ -2170,46 +2170,13 @@ compiler, consider compiling LLVM and LL<br>
> using the resultant compiler to build a copy of LLVM with multithreading<br>
> support.<br>
><br>
> -.. _startmultithreaded:<br>
> -<br>
> -Entering and Exiting Multithreaded Mode<br>
> ----------------------------------------<br>
> -<br>
> -In order to properly protect its internal data structures while avoiding<br>
> -excessive locking overhead in the single-threaded case, the LLVM must intialize<br>
> -certain data structures necessary to provide guards around its internals. To do<br>
> -so, the client program must invoke ``llvm_start_multithreaded()`` before making<br>
> -any concurrent LLVM API calls. To subsequently tear down these structures, use<br>
> -the ``llvm_stop_multithreaded()`` call. You can also use the<br>
> -``llvm_is_multithreaded()`` call to check the status of multithreaded mode.<br>
> -<br>
> -Note that both of these calls must be made *in isolation*. That is to say that<br>
> -no other LLVM API calls may be executing at any time during the execution of<br>
> -``llvm_start_multithreaded()`` or ``llvm_stop_multithreaded``. It is the<br>
> -client's responsibility to enforce this isolation.<br>
> -<br>
> -The return value of ``llvm_start_multithreaded()`` indicates the success or<br>
> -failure of the initialization. Failure typically indicates that your copy of<br>
> -LLVM was built without multithreading support, typically because GCC atomic<br>
> -intrinsics were not found in your system compiler. In this case, the LLVM API<br>
> -will not be safe for concurrent calls. However, it *will* be safe for hosting<br>
> -threaded applications in the JIT, though :ref:`care must be taken<br>
> -<jitthreading>` to ensure that side exits and the like do not accidentally<br>
> -result in concurrent LLVM API calls.<br>
> -<br>
> .. _shutdown:<br>
><br>
> Ending Execution with ``llvm_shutdown()``<br>
> -----------------------------------------<br>
><br>
> When you are done using the LLVM APIs, you should call ``llvm_shutdown()`` to<br>
> -deallocate memory used for internal structures. This will also invoke<br>
> -``llvm_stop_multithreaded()`` if LLVM is operating in multithreaded mode. As<br>
> -such, ``llvm_shutdown()`` requires the same isolation guarantees as<br>
> -``llvm_stop_multithreaded()``.<br>
> -<br>
> -Note that, if you use scope-based shutdown, you can use the<br>
> -``llvm_shutdown_obj`` class, which calls ``llvm_shutdown()`` in its destructor.<br>
> +deallocate memory used for internal structures.<br>
><br>
> .. _managedstatic:<br>
><br>
> @@ -2217,15 +2184,11 @@ Lazy Initialization with ``ManagedStatic<br>
> ------------------------------------------<br>
><br>
> ``ManagedStatic`` is a utility class in LLVM used to implement static<br>
> -initialization of static resources, such as the global type tables. Before the<br>
> -invocation of ``llvm_shutdown()``, it implements a simple lazy initialization<br>
> -scheme. Once ``llvm_start_multithreaded()`` returns, however, it uses<br>
> +initialization of static resources, such as the global type tables. In a<br>
> +single-threaded environment, it implements a simple lazy initialization scheme.<br>
> +When LLVM is compiled with support for multi-threading, however, it uses<br>
> double-checked locking to implement thread-safe lazy initialization.<br>
><br>
> -Note that, because no other threads are allowed to issue LLVM API calls before<br>
> -``llvm_start_multithreaded()`` returns, it is possible to have<br>
> -``ManagedStatic``\ s of ``llvm::sys::Mutex``\ s.<br>
> -<br>
> .. _llvmcontext:<br>
><br>
> Achieving Isolation with ``LLVMContext``<br>
><br>
> Modified: llvm/trunk/include/llvm-c/Core.h<br>
> URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm-c/Core.h?rev=211900&r1=211899&r2=211900&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm-c/Core.h?rev=211900&r1=211899&r2=211900&view=diff</a><br>
> ==============================================================================<br>
> --- llvm/trunk/include/llvm-c/Core.h (original)<br>
> +++ llvm/trunk/include/llvm-c/Core.h Fri Jun 27 10:13:01 2014<br>
> @@ -2848,16 +2848,13 @@ void LLVMDisposePassManager(LLVMPassMana<br>
> * @{<br>
> */<br>
><br>
> -/** Allocate and initialize structures needed to make LLVM safe for<br>
> - multithreading. The return value indicates whether multithreaded<br>
> - initialization succeeded. Must be executed in isolation from all<br>
> - other LLVM api calls.<br>
> - @see llvm::llvm_start_multithreaded */<br>
> +/** Deprecated: Multi-threading can only be enabled/disabled with the compile<br>
> + time define LLVM_ENABLE_THREADS. This function always returns<br>
> + LLVMIsMultithreaded(). */<br>
> LLVMBool LLVMStartMultithreaded(void);<br>
><br>
> -/** Deallocate structures necessary to make LLVM safe for multithreading.<br>
> - Must be executed in isolation from all other LLVM api calls.<br>
> - @see llvm::llvm_stop_multithreaded */<br>
> +/** Deprecated: Multi-threading can only be enabled/disabled with the compile<br>
> + time define LLVM_ENABLE_THREADS. */<br>
> void LLVMStopMultithreaded(void);<br>
><br>
> /** Check whether LLVM is executing in thread-safe mode or not.<br>
><br>
> Modified: llvm/trunk/include/llvm/Support/ManagedStatic.h<br>
> URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/ManagedStatic.h?rev=211900&r1=211899&r2=211900&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/ManagedStatic.h?rev=211900&r1=211899&r2=211900&view=diff</a><br>
> ==============================================================================<br>
> --- llvm/trunk/include/llvm/Support/ManagedStatic.h (original)<br>
> +++ llvm/trunk/include/llvm/Support/ManagedStatic.h Fri Jun 27 10:13:01 2014<br>
> @@ -103,9 +103,6 @@ void llvm_shutdown();<br>
> /// llvm_shutdown() when it is destroyed.<br>
> struct llvm_shutdown_obj {<br>
> llvm_shutdown_obj() { }<br>
> - explicit llvm_shutdown_obj(bool multithreaded) {<br>
> - if (multithreaded) llvm_start_multithreaded();<br>
> - }<br>
> ~llvm_shutdown_obj() { llvm_shutdown(); }<br>
> };<br>
><br>
><br>
> Modified: llvm/trunk/include/llvm/Support/Threading.h<br>
> URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/Threading.h?rev=211900&r1=211899&r2=211900&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/Threading.h?rev=211900&r1=211899&r2=211900&view=diff</a><br>
> ==============================================================================<br>
> --- llvm/trunk/include/llvm/Support/Threading.h (original)<br>
> +++ llvm/trunk/include/llvm/Support/Threading.h Fri Jun 27 10:13:01 2014<br>
> @@ -7,7 +7,8 @@<br>
> //<br>
> //===----------------------------------------------------------------------===//<br>
> //<br>
> -// TThis file defines llvm_start_multithreaded() and friends.<br>
> +// This file declares helper functions for running LLVM in a multi-threaded<br>
> +// environment.<br>
> //<br>
> //===----------------------------------------------------------------------===//<br>
><br>
> @@ -15,22 +16,8 @@<br>
> #define LLVM_SUPPORT_THREADING_H<br>
><br>
> namespace llvm {<br>
> - /// llvm_start_multithreaded - Allocate and initialize structures needed to<br>
> - /// make LLVM safe for multithreading. The return value indicates whether<br>
> - /// multithreaded initialization succeeded. LLVM will still be operational<br>
> - /// on "failed" return, and will still be safe for hosting threading<br>
> - /// applications in the JIT, but will not be safe for concurrent calls to the<br>
> - /// LLVM APIs.<br>
> - /// THIS MUST EXECUTE IN ISOLATION FROM ALL OTHER LLVM API CALLS.<br>
> - bool llvm_start_multithreaded();<br>
> -<br>
> - /// llvm_stop_multithreaded - Deallocate structures necessary to make LLVM<br>
> - /// safe for multithreading.<br>
> - /// THIS MUST EXECUTE IN ISOLATION FROM ALL OTHER LLVM API CALLS.<br>
> - void llvm_stop_multithreaded();<br>
> -<br>
> - /// llvm_is_multithreaded - Check whether LLVM is executing in thread-safe<br>
> - /// mode or not.<br>
> + /// Returns true if LLVM is compiled with support for multi-threading, and<br>
> + /// false otherwise.<br>
> bool llvm_is_multithreaded();<br>
><br>
> /// llvm_execute_on_thread - Execute the given \p UserFn on a separate<br>
><br>
> Modified: llvm/trunk/lib/IR/Core.cpp<br>
> URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/Core.cpp?rev=211900&r1=211899&r2=211900&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/Core.cpp?rev=211900&r1=211899&r2=211900&view=diff</a><br>
> ==============================================================================<br>
> --- llvm/trunk/lib/IR/Core.cpp (original)<br>
> +++ llvm/trunk/lib/IR/Core.cpp Fri Jun 27 10:13:01 2014<br>
> @@ -2708,11 +2708,10 @@ void LLVMDisposePassManager(LLVMPassMana<br>
> /*===-- Threading ------------------------------------------------------===*/<br>
><br>
> LLVMBool LLVMStartMultithreaded() {<br>
> - return llvm_start_multithreaded();<br>
> + return LLVMIsMultithreaded();<br>
> }<br>
><br>
> void LLVMStopMultithreaded() {<br>
> - llvm_stop_multithreaded();<br>
> }<br>
><br>
> LLVMBool LLVMIsMultithreaded() {<br>
><br>
> Modified: llvm/trunk/lib/Support/Threading.cpp<br>
> URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/Threading.cpp?rev=211900&r1=211899&r2=211900&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/Threading.cpp?rev=211900&r1=211899&r2=211900&view=diff</a><br>
> ==============================================================================<br>
> --- llvm/trunk/lib/Support/Threading.cpp (original)<br>
> +++ llvm/trunk/lib/Support/Threading.cpp Fri Jun 27 10:13:01 2014<br>
> @@ -7,7 +7,8 @@<br>
> //<br>
> //===----------------------------------------------------------------------===//<br>
> //<br>
> -// This file implements llvm_start_multithreaded() and friends.<br>
> +// This file defines helper functions for running LLVM in a multi-threaded<br>
> +// environment.<br>
> //<br>
> //===----------------------------------------------------------------------===//<br>
><br>
> @@ -19,38 +20,14 @@<br>
><br>
> using namespace llvm;<br>
><br>
> -static bool multithreaded_mode = false;<br>
> -<br>
> -bool llvm::llvm_start_multithreaded() {<br>
> +bool llvm::llvm_is_multithreaded() {<br>
> #if LLVM_ENABLE_THREADS != 0<br>
> - assert(!multithreaded_mode && "Already multithreaded!");<br>
> - multithreaded_mode = true;<br>
> -<br>
> - // We fence here to ensure that all initialization is complete BEFORE we<br>
> - // return from llvm_start_multithreaded().<br>
> - sys::MemoryFence();<br>
> return true;<br>
> #else<br>
> return false;<br>
> #endif<br>
> }<br>
><br>
> -void llvm::llvm_stop_multithreaded() {<br>
> -#if LLVM_ENABLE_THREADS != 0<br>
> - assert(multithreaded_mode && "Not currently multithreaded!");<br>
> -<br>
> - // We fence here to insure that all threaded operations are complete BEFORE we<br>
> - // return from llvm_stop_multithreaded().<br>
> - sys::MemoryFence();<br>
> -<br>
> - multithreaded_mode = false;<br>
> -#endif<br>
> -}<br>
> -<br>
> -bool llvm::llvm_is_multithreaded() {<br>
> - return multithreaded_mode;<br>
> -}<br>
> -<br>
> #if LLVM_ENABLE_THREADS != 0 && defined(HAVE_PTHREAD_H)<br>
> #include <pthread.h><br>
><br>
><br>
> Modified: llvm/trunk/unittests/Support/ManagedStatic.cpp<br>
> URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/Support/ManagedStatic.cpp?rev=211900&r1=211899&r2=211900&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/Support/ManagedStatic.cpp?rev=211900&r1=211899&r2=211900&view=diff</a><br>
> ==============================================================================<br>
> --- llvm/trunk/unittests/Support/ManagedStatic.cpp (original)<br>
> +++ llvm/trunk/unittests/Support/ManagedStatic.cpp Fri Jun 27 10:13:01 2014<br>
> @@ -47,7 +47,6 @@ TEST(Initialize, MultipleThreads) {<br>
> void *p1 = test1::allocate_stack(a1);<br>
> void *p2 = test1::allocate_stack(a2);<br>
><br>
> - llvm_start_multithreaded();<br>
> pthread_t t1, t2;<br>
> pthread_create(&t1, &a1, test1::helper, nullptr);<br>
> pthread_create(&t2, &a2, test1::helper, nullptr);<br>
> @@ -55,7 +54,6 @@ TEST(Initialize, MultipleThreads) {<br>
> pthread_join(t2, nullptr);<br>
> free(p1);<br>
> free(p2);<br>
> - llvm_stop_multithreaded();<br>
> }<br>
> #endif<br>
><br>
><br>
><br>
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</div></div></blockquote></div><br></div></div>