[Support] Simplify parallelForEach{,N}

[MaskRay]

Summary

• Merge parallel_for_each into parallelForEach (this removes 1 Fn(...) call)
• Change parallelForEach to use parallelForEachN
• Move parallelForEachN into Parallel.cpp

My x86-64 lld executable is 100KiB smaller.
No noticeable difference in performance.

[MaskRay]

llvm/include/llvm/Support/Parallel.h

246

Removing

for (; Begin != End; ++Begin)
  Fn(Begin);
return;

is still correct (my lld executable will be 9KiB smaller on top of the current decrease) but make the TaskGroup have less parallelism.

[lattner]

Cool, thank you for working to improve this. As mentioned on https://reviews.llvm.org/D101699, dropping the special case for 1 element will have catastrophic performance impacts for some workloads (e.g. in the MLIR/CIRCT world) because of the problems that Threading.h has with nested parallelism.

Have you tried detemplating this entirely? If something is interesting to parallelize, then the granule of work should not be tiny. I'd consider moving this to take a unique_function<void()>, which would allow moving the implementation details of all of this out of line to a .cpp file.

[MaskRay]

simplify

[lattner]

niiice, thank you!

[rnk]

llvm/lib/Support/Parallel.cpp

198 ↗

(On Diff #402244)

I believe the old code was templated in an effort to avoid having an indirect call in the inner loop. However, I don't think that actually matters. These constructs have so much overhead that they are only useful for coarse-grained parallelism, not vectorizable inner loops.

[lattner]

llvm/lib/Support/Parallel.cpp

198 ↗

(On Diff #402244)

Right, I completely agree with you

llvm/include/llvm/Support/Parallel.h

 }
 
 // TaskGroup has a relatively high overhead, so we want to reduce
 // the number of spawn() calls. We'll create up to 1024 tasks here.
 // (Note that 1024 is an arbitrary number. This code probably needs
 // improving to take the number of available cores into account.)
 enum { MaxTasksPerGroup = 1024 };
 
-template <class IterTy, class FuncTy>
-void parallel_for_each(IterTy Begin, IterTy End, FuncTy Fn) {
-  // If we have zero or one items, then do not incur the overhead of spinning up
-  // a task group.  They are surprisingly expensive, and because they do not
-  // support nested parallelism, a single entry task group can block parallel
-  // execution underneath them.
-  auto NumItems = std::distance(Begin, End);
-  if (NumItems <= 1) {
-    if (NumItems)
-      Fn(*Begin);
-    return;
-  }
-
-  // Limit the number of tasks to MaxTasksPerGroup to limit job scheduling
-  // overhead on large inputs.
-  ptrdiff_t TaskSize = NumItems / MaxTasksPerGroup;
-  if (TaskSize == 0)
-    TaskSize = 1;
-
-  TaskGroup TG;
-  while (TaskSize < std::distance(Begin, End)) {
-    TG.spawn([=, &Fn] { std::for_each(Begin, Begin + TaskSize, Fn); });
-    Begin += TaskSize;
-  }
-  std::for_each(Begin, End, Fn);
-}
-
-template <class IndexTy, class FuncTy>
-void parallel_for_each_n(IndexTy Begin, IndexTy End, FuncTy Fn) {
-  // If we have zero or one items, then do not incur the overhead of spinning up
-  // a task group.  They are surprisingly expensive, and because they do not
-  // support nested parallelism, a single entry task group can block parallel
-  // execution underneath them.
-  auto NumItems = End - Begin;
-  if (NumItems <= 1) {
-    if (NumItems)
-      Fn(Begin);
-    return;
-  }
-
-  // Limit the number of tasks to MaxTasksPerGroup to limit job scheduling
-  // overhead on large inputs.
-  ptrdiff_t TaskSize = NumItems / MaxTasksPerGroup;
-  if (TaskSize == 0)
-    TaskSize = 1;
-
-  TaskGroup TG;
-  IndexTy I = Begin;
-  for (; I + TaskSize < End; I += TaskSize) {
-    TG.spawn([=, &Fn] {
-      for (IndexTy J = I, E = I + TaskSize; J != E; ++J)
-        Fn(J);
-    });
-  }
-  for (IndexTy J = I; J < End; ++J)
-    Fn(J);
-}
-
 template <class IterTy, class ResultTy, class ReduceFuncTy,
           class TransformFuncTy>
 ResultTy parallel_transform_reduce(IterTy Begin, IterTy End, ResultTy Init,
                                    ReduceFuncTy Reduce,
                                    TransformFuncTy Transform) {
   // Limit the number of tasks to MaxTasksPerGroup to limit job scheduling
   // overhead on large inputs.
   size_t NumInputs = std::distance(Begin, End);
if (parallel::strategy.ThreadsRequested != 1) {
     return;
   }
 #endif
   llvm::sort(Start, End, Comp);
 }
 
 template <class IterTy, class FuncTy>
 void parallelForEach(IterTy Begin, IterTy End, FuncTy Fn) {
+  // If we have zero or one items, then do not incur the overhead of spinning up
+  // a task group.  They are surprisingly expensive, and because they do not
+  // support nested parallelism, a single entry task group can block parallel
+  // execution underneath them.
 #if LLVM_ENABLE_THREADS
-  if (parallel::strategy.ThreadsRequested != 1) {
-    parallel::detail::parallel_for_each(Begin, End, Fn);
+  auto NumItems = std::distance(Begin, End);
+  if (NumItems > 1 && parallel::strategy.ThreadsRequested != 1) {
+    // Limit the number of tasks to MaxTasksPerGroup to limit job scheduling
+    // overhead on large inputs.
+    auto TaskSize = NumItems / parallel::detail::MaxTasksPerGroup;
+    if (TaskSize == 0)
+      TaskSize = 1;
+
+    parallel::detail::TaskGroup TG;
+    while (TaskSize < std::distance(Begin, End)) {
+      TG.spawn([=, &Fn] { std::for_each(Begin, Begin + TaskSize, Fn); });
+      Begin += TaskSize;
+    }
+    std::for_each(Begin, End, Fn);
     return;
   }
 #endif
+
   std::for_each(Begin, End, Fn);
 }
 
 template <class FuncTy>
 void parallelForEachN(size_t Begin, size_t End, FuncTy Fn) {
+  // If we have zero or one items, then do not incur the overhead of spinning up
+  // a task group.  They are surprisingly expensive, and because they do not
+  // support nested parallelism, a single entry task group can block parallel
+  // execution underneath them.
 #if LLVM_ENABLE_THREADS
-  if (parallel::strategy.ThreadsRequested != 1) {
-    parallel::detail::parallel_for_each_n(Begin, End, Fn);
+  auto NumItems = End - Begin;
+  if (NumItems > 1 && parallel::strategy.ThreadsRequested != 1) {
+    // Limit the number of tasks to MaxTasksPerGroup to limit job scheduling
+    // overhead on large inputs.
+    auto TaskSize = NumItems / parallel::detail::MaxTasksPerGroup;
+    if (TaskSize == 0)
+      TaskSize = 1;
+
+    parallel::detail::TaskGroup TG;
+    for (; Begin + TaskSize < End; Begin += TaskSize) {
+      TG.spawn([=, &Fn] {
+        for (size_t I = Begin, E = Begin + TaskSize; I != E; ++I)
+          Fn(I);
+      });
+    }
+    for (; Begin != End; ++Begin)
+      Fn(Begin);
     return;
   }
 #endif
-  for (size_t I = Begin; I != End; ++I)
-    Fn(I);
+
+  for (; Begin != End; ++Begin)
+    Fn(Begin);
 }
 
 template <class IterTy, class ResultTy, class ReduceFuncTy,
           class TransformFuncTy>
 ResultTy parallelTransformReduce(IterTy Begin, IterTy End, ResultTy Init,
                                  ReduceFuncTy Reduce,
                                  TransformFuncTy Transform) {
 #if LLVM_ENABLE_THREADS