[ThinLTO] Allow usage of all hardware threads in the system

Before this patch, it wasn't possible to extend the ThinLTO threads to all SMT/CMT threads in the system. Only one thread per core was allowed, instructed by usage of llvm::heavyweight_hardware_concurrency() in the ThinLTO code. Any number passed to the LLD flag /opt:lldltojobs=..., or any other ThinLTO-specific flag, was previously interpreted in the context of llvm::heavyweight_hardware_concurrency(), which means SMT disabled.

One can now say in LLD:
/opt:lldltojobs=0 -- Use one std::thread / hardware core in the system (no SMT). Default value if flag not specified.
/opt:lldltojobs=N -- Limit usage to N threads, regardless of usage of heavyweight_hardware_concurrency().
/opt:lldltojobs=all -- Use all hardware threads in the system. Equivalent to /opt:lldltojobs=$(nproc) on Linux and /opt:lldltojobs=%NUMBER_OF_PROCESSORS% on Windows. When an affinity mask is set for the process, threads will be created only for the cores selected by the mask.

When N > number-of-hardware-threads-in-the-system, the threads in the thread pool will be dispatched equally on all CPU sockets (tested only on Windows).
When N <= number-of-hardware-threads-on-a-CPU-socket, the threads will remain on the CPU socket where the process started (only on Windows).

Differential Revision: https://reviews.llvm.org/D75153

1 files changed, 8 insertions, 8 deletions

diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
index f8affcb..f652bd6 100644
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -477,7 +477,8 @@ LTO::RegularLTOState::RegularLTOState(unsigned ParallelCodeGenParallelismLevel,
 LTO::ThinLTOState::ThinLTOState(ThinBackend Backend)
     : Backend(Backend), CombinedIndex(/*HaveGVs*/ false) {
   if (!Backend)
-    this->Backend = createInProcessThinBackend();
+    this->Backend =
+        createInProcessThinBackend(llvm::heavyweight_hardware_concurrency());
 }
 
 LTO::LTO(Config Conf, ThinBackend Backend,
@@ -1090,13 +1091,12 @@ class InProcessThinBackend : public ThinBackendProc {
 public:
   InProcessThinBackend(
       const Config &Conf, ModuleSummaryIndex &CombinedIndex,
-      unsigned ThinLTOParallelismLevel,
+      ThreadPoolStrategy ThinLTOParallelism,
       const StringMap<GVSummaryMapTy> &ModuleToDefinedGVSummaries,
       AddStreamFn AddStream, NativeObjectCache Cache)
       : ThinBackendProc(Conf, CombinedIndex, ModuleToDefinedGVSummaries),
-        BackendThreadPool(
-            heavyweight_hardware_concurrency(ThinLTOParallelismLevel)),
-        AddStream(std::move(AddStream)), Cache(std::move(Cache)) {
+        BackendThreadPool(ThinLTOParallelism), AddStream(std::move(AddStream)),
+        Cache(std::move(Cache)) {
     for (auto &Name : CombinedIndex.cfiFunctionDefs())
       CfiFunctionDefs.insert(
           GlobalValue::getGUID(GlobalValue::dropLLVMManglingEscape(Name)));
@@ -1192,13 +1192,13 @@ public:
 };
 } // end anonymous namespace
 
-ThinBackend lto::createInProcessThinBackend(unsigned ParallelismLevel) {
+ThinBackend lto::createInProcessThinBackend(ThreadPoolStrategy Parallelism) {
   return [=](const Config &Conf, ModuleSummaryIndex &CombinedIndex,
              const StringMap<GVSummaryMapTy> &ModuleToDefinedGVSummaries,
              AddStreamFn AddStream, NativeObjectCache Cache) {
     return std::make_unique<InProcessThinBackend>(
-        Conf, CombinedIndex, ParallelismLevel, ModuleToDefinedGVSummaries,
-        AddStream, Cache);
+        Conf, CombinedIndex, Parallelism, ModuleToDefinedGVSummaries, AddStream,
+        Cache);
   };
 }