[Support] move llvm::llvm_is_multithread to header, NFC

[junaire]

Summary

This allows optimization without LTO.

[junaire]

Fix the build

[junaire]

Clang-format

[nikic]

I don't think we should use if constexpr if we don't actually require it's semantics. A normal if will get optimized just fine. The only actual change here is that the llvm_is_multithreaded() definition gets moved into the header, which allows this to optimize without LTO.

[barannikov88]

llvm/include/llvm/Support/Mutex.h

48–49

Wrong indentation here and in lock.

llvm/include/llvm/Support/Threading.h

54–56

[barannikov88]

llvm/include/llvm/Support/Mutex.h

28

There is also SmartRWMutex in RWMutex.h.

[thakis]

In D131313#3704232, @nikic wrote:

I don't think we should use if constexpr if we don't actually require it's semantics. A normal if will get optimized just fine. The only actual change here is that the llvm_is_multithreaded() definition gets moved into the header, which allows this to optimize without LTO.

I agree with this, FWIW.

[junaire]

I'm not if this patch is worth to continue...

[thakis]

Removing the else-after-ifs seems fine to me.

llvm/include/llvm/Support/Mutex.h

36–37

run git clang-format main

[junaire]

llvm/include/llvm/Support/Mutex.h

36–37

run git clang-format main

I have already formatted the patch via git clang-format HEAD~1. What's wrong with it?
clang-format version: Ubuntu clang-format version 14.0.0-1ubuntu1

[junaire]

git clang-format HEAD~1 can't format code that not been touched, format by hand.

[barannikov88]

llvm/include/llvm/Support/Mutex.h

48–49

Still holds

[barannikov88]

llvm/include/llvm/Support/Mutex.h

48–49

Never mind, compared wrong revisions

[junaire]

Thanks!

llvm/include/llvm/Support/Mutex.h

 namespace llvm
 {
   namespace sys
   {
     /// SmartMutex - A mutex with a compile time constant parameter that
     /// indicates whether this mutex should become a no-op when we're not
     /// running in multithreaded mode.
     template<bool mt_only>
     class SmartMutex {
       std::recursive_mutex impl;
       unsigned acquired = 0;
 
     public:
       bool lock() {
         if (!mt_only || llvm_is_multithreaded()) {
           impl.lock();
           return true;
-        } else {
-          // Single-threaded debugging code.  This would be racy in
-          // multithreaded mode, but provides not basic checks in single
-          // threaded mode.
-          ++acquired;
-          return true;
-        }
-      }
+        }
 
       bool unlock() {
         if (!mt_only || llvm_is_multithreaded()) {
           impl.unlock();
           return true;
-        } else {
-          // Single-threaded debugging code.  This would be racy in
-          // multithreaded mode, but provides not basic checks in single
-          // threaded mode.
-          assert(acquired && "Lock not acquired before release!");
-          --acquired;
-          return true;
-        }
-      }
+        }
 
       bool try_lock() {
         if (!mt_only || llvm_is_multithreaded())
           return impl.try_lock();
-        else return true;
+        return true;
       }
     };
 
     /// Mutex - A standard, always enforced mutex.
     typedef SmartMutex<false> Mutex;
 
     template <bool mt_only>
     using SmartScopedLock = std::lock_guard<SmartMutex<mt_only>>;
 
     typedef SmartScopedLock<false> ScopedLock;
   }
 }
 
 #endif

llvm/include/llvm/Support/Threading.h

 #include "llvm/Support/Atomic.h"
 #endif
 
 namespace llvm {
 class Twine;
 
 /// Returns true if LLVM is compiled with support for multi-threading, and
 /// false otherwise.
-bool llvm_is_multithreaded();
+constexpr bool llvm_is_multithreaded() { return LLVM_ENABLE_THREADS; }
 
 #if LLVM_THREADING_USE_STD_CALL_ONCE
 
   typedef std::once_flag once_flag;
 
 #else
 
   enum InitStatus { Uninitialized = 0, Wait = 1, Done = 2 };
 
   /// The llvm::once_flag structure
   ///

llvm/lib/Support/Threading.cpp

 
 using namespace llvm;
 
 //===----------------------------------------------------------------------===//
 //=== WARNING: Implementation here must contain only TRULY operating system
 //===          independent code.
 //===----------------------------------------------------------------------===//
 
-bool llvm::llvm_is_multithreaded() {
-#if LLVM_ENABLE_THREADS != 0
-  return true;
-#else
-  return false;
-#endif
-}
-
 #if LLVM_ENABLE_THREADS == 0 ||                                                \
     (!defined(_WIN32) && !defined(HAVE_PTHREAD_H))
 uint64_t llvm::get_threadid() { return 0; }
 
 uint32_t llvm::get_max_thread_name_length() { return 0; }
 
 void llvm::set_thread_name(const Twine &Name) {}