[ThinLTO] Default backend threads to heavyweight_hardware_concurrency

Summary:
Changes default backend parallelism from thread::hardware_concurrency to
the new llvm::heavyweight_hardware_concurrency, which for X86 Linux
defaults to the number of physical cores (and will fall back to
thread::hardware_concurrency otherwise). This avoid oversubscribing
the physical cores using hyperthreading.

Reviewers: mehdi_amini, pcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25775

llvm-svn: 284618

llvm/lib/LTO/LTO.cpp

@@ -29,6 +29,7 @@
 #include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/TargetRegistry.h"
 #include "llvm/Support/ThreadPool.h"
+#include "llvm/Support/Threading.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/Target/TargetOptions.h"
@@ -227,7 +228,8 @@ LTO::RegularLTOState::RegularLTOState(unsigned ParallelCodeGenParallelismLevel,
 
 LTO::ThinLTOState::ThinLTOState(ThinBackend Backend) : Backend(Backend) {
   if (!Backend)
-    this->Backend = createInProcessThinBackend(thread::hardware_concurrency());
+    this->Backend =
+        createInProcessThinBackend(llvm::heavyweight_hardware_concurrency());
 }
 
 LTO::LTO(Config Conf, ThinBackend Backend,

llvm/lib/LTO/ThinLTOCodeGenerator.cpp

@@ -43,6 +43,7 @@
 #include "llvm/Support/SHA1.h"
 #include "llvm/Support/TargetRegistry.h"
 #include "llvm/Support/ThreadPool.h"
+#include "llvm/Support/Threading.h"
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/Transforms/IPO.h"
 #include "llvm/Transforms/IPO/FunctionImport.h"
@@ -64,8 +65,8 @@ extern cl::opt<bool> LTODiscardValueNames;
 
 namespace {
 
-static cl::opt<int> ThreadCount("threads",
-                                cl::init(std::thread::hardware_concurrency()));
+static cl::opt<int>
+    ThreadCount("threads", cl::init(llvm::heavyweight_hardware_concurrency()));
 
 static void diagnosticHandler(const DiagnosticInfo &DI) {
   DiagnosticPrinterRawOStream DP(errs());

llvm/tools/gold/gold-plugin.cpp

@@ -118,8 +118,8 @@ namespace options {
   static unsigned OptLevel = 2;
   // Default parallelism of 0 used to indicate that user did not specify.
   // Actual parallelism default value depends on implementation.
-  // Currently only affects ThinLTO, where the default is the
-  // hardware_concurrency.
+  // Currently only affects ThinLTO, where the default is
+  // llvm::heavyweight_hardware_concurrency.
   static unsigned Parallelism = 0;
   // Default regular LTO codegen parallelism (number of partitions).
   static unsigned ParallelCodeGenParallelismLevel = 1;

llvm/tools/llvm-lto2/llvm-lto2.cpp

@@ -20,6 +20,7 @@
 #include "llvm/LTO/LTO.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/TargetSelect.h"
+#include "llvm/Support/Threading.h"
 
 using namespace llvm;
 using namespace lto;
@@ -52,7 +53,7 @@ static cl::opt<bool>
                                        "distributed backend case"));
 
 static cl::opt<int> Threads("-thinlto-threads",
-                            cl::init(thread::hardware_concurrency()));
+                            cl::init(llvm::heavyweight_hardware_concurrency()));
 
 static cl::list<std::string> SymbolResolutions(
     "r",