Add interface for querying physical hardware concurrency

[tejohnson]

Summary

This will be used by ThinLTO to set the amount of backend
parallelism, which performs better when restricted to the number
of physical cores (on X86 at least, where getHostNumPhysicalCores is
currently defined). If not available this falls back to
thread::hardware_concurrency.

Note I didn't add to the thread class since that is a typedef to
std::thread where available.

[mehdi_amini]

LGTM.

I suggested a name change, but if you disagree with the rational, you can commit your version.

include/llvm/Support/Threading.h

121 ↗

(On Diff #74597)

I think we may want to name it hardware_coarse_concurrency. Because:

• This looks like expressing better what we're looking after.
• Hyperthreading is in some sense "physical concurrency", but sharing some resources.
• Other platforms may have something in between.

[tejohnson]

include/llvm/Support/Threading.h

121 ↗

(On Diff #74597)

I thought about that name after you mentioned it on the prior review thread. But I felt that saying "physical concurrency" is a better expression for what it is actually trying to give you. I think of the hyperthreading concurrency as "logical concurrency", vs physical concurrency due to physical cores.

[thakis]

The new test fails if I cmake with -DLLVM_ENABLE_THREADS=OFF. I guess this just needs a #if LLVM_ENABLE_THREADS?

[tejohnson]

In D25585#570756, @thakis wrote:

The new test fails if I cmake with -DLLVM_ENABLE_THREADS=OFF. I guess this just needs a #if LLVM_ENABLE_THREADS?

Yes that's right. With LLVM_ENABLE_THREADS=OFF hardware_concurrency will return 1, so the test will fail. I took the day off today, if you want to submit a fix then LGTM. I will do so tonight if you don't have a chance before then.

Thanks

llvm/trunk/include/llvm/Support/Threading.h

if (old_val == Uninitialized) {
       while (tmp != Done) {
         tmp = flag;
         sys::MemoryFence();
       }
     }
     TsanHappensAfter(&flag);
 #endif
   }
+
+  /// Get the amount of currency based on physical cores, if available for the
+  /// host system, otherwise falls back to thread::hardware_concurrency().
+  unsigned hardware_physical_concurrency();
 }
 
 #endif

llvm/trunk/lib/Support/Threading.cpp

 // This file defines helper functions for running LLVM in a multi-threaded
 // environment.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/Threading.h"
 #include "llvm/Config/config.h"
 #include "llvm/Support/Atomic.h"
+#include "llvm/Support/Host.h"
 #include "llvm/Support/Mutex.h"
 #include "llvm/Support/thread.h"
 #include <cassert>
 
 using namespace llvm;
 
 bool llvm::llvm_is_multithreaded() {
 #if LLVM_ENABLE_THREADS != 0

 // Support for non-Win32, non-pthread implementation.
 void llvm::llvm_execute_on_thread(void (*Fn)(void*), void *UserData,
                                   unsigned RequestedStackSize) {
   (void) RequestedStackSize;
   Fn(UserData);
 }
 
 #endif
+
+unsigned llvm::hardware_physical_concurrency() {
+  int NumPhysical = sys::getHostNumPhysicalCores();
+  if (NumPhysical == -1)
+    return thread::hardware_concurrency();
+  return NumPhysical;
+}

llvm/trunk/unittests/Support/CMakeLists.txt

add_llvm_unittest(SupportTests
   ReplaceFileTest.cpp
   ScaledNumberTest.cpp
   SourceMgrTest.cpp
   SpecialCaseListTest.cpp
   StreamingMemoryObjectTest.cpp
   StringPool.cpp
   SwapByteOrderTest.cpp
   TargetParserTest.cpp
+  Threading.cpp
   ThreadLocalTest.cpp
   ThreadPool.cpp
   TimerTest.cpp
   TimeValueTest.cpp
   TypeNameTest.cpp
   TrailingObjectsTest.cpp
   UnicodeTest.cpp
   YAMLIOTest.cpp

llvm/trunk/unittests/Support/Threading.cpp

+//===- unittests/Threading.cpp - Thread tests -----------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/Threading.h"
+#include "llvm/Support/thread.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+namespace {
+
+TEST(Threading, PhysicalConcurrency) {
+  auto Num = hardware_physical_concurrency();
+  // Since Num is unsigned this will also catch us trying to
+  // return -1.
+  ASSERT_LE(Num, thread::hardware_concurrency());
+}
+
+} // end anon namespace