[Support] heavyweight_hardware_concurrency uses affinity when counting cores fails, and never returns 0

[sammccall]

Summary

[Support] heavyweight_hardware_concurrency uses affinity when counting cores fails, and never returns 0

Previously it would fall back to std::thread::hardware_concurrency(),
which ignores affinity. llvm::hardware_concurrency() is better, but was
not available at the time.

Also, the case where std::thread::hardware_concurrency() returns 0 was
never handled. llvm::hardware_concurrency() never does this, so that's fixed.

[puremourning]

LGTM :)

[jfb]

The name of this patch is wrong?
Or rather, it's 2 in 1?

[sammccall]

ugh, I accidentally reused a branch and squashed two patches together. Let me try again...

[sammccall]

Rebase

[• serranolupe091]

hardware_concurrency

llvm/include/llvm/Support/Threading.h

#else
     }
     TsanHappensAfter(&flag.status);
 #endif
   }
 
   /// Get the amount of currency to use for tasks requiring significant
   /// memory or other resources. Currently based on physical cores, if
   /// available for the host system, otherwise falls back to
-  /// thread::hardware_concurrency().
+  /// llvm::hardware_concurrency().
   /// Returns 1 when LLVM is configured with LLVM_ENABLE_THREADS=OFF
   unsigned heavyweight_hardware_concurrency();
 
   /// Get the number of threads that the current program can execute
   /// concurrently. On some systems std::thread::hardware_concurrency() returns
   /// the total number of cores, without taking affinity into consideration.
   /// Returns 1 when LLVM is configured with LLVM_ENABLE_THREADS=OFF.
   /// Fallback to std::thread::hardware_concurrency() if sched_getaffinity is

llvm/lib/Support/Threading.cpp

unsigned llvm::heavyweight_hardware_concurrency() {
   // Since we can't get here unless LLVM_ENABLE_THREADS == 1, it is safe to use
   // `std::thread` directly instead of `llvm::thread` (and indeed, doing so
   // allows us to not define `thread` in the llvm namespace, which conflicts
   // with some platforms such as FreeBSD whose headers also define a struct
   // called `thread` in the global namespace which can cause ambiguity due to
   // ADL.
   int NumPhysical = sys::getHostNumPhysicalCores();
   if (NumPhysical == -1)
-    return std::thread::hardware_concurrency();
+    return llvm::hardware_concurrency();
   return NumPhysical;
 }
 
 unsigned llvm::hardware_concurrency() {
 #if defined(HAVE_SCHED_GETAFFINITY) && defined(HAVE_CPU_COUNT)
   cpu_set_t Set;
   if (sched_getaffinity(0, sizeof(Set), &Set))
     return CPU_COUNT(&Set);