From 0764636b95f1b6f523202a291b9534eb826dd715 Mon Sep 17 00:00:00 2001
From: Alexey Lapshin <a.v.lapshin@mail.ru>
Date: Tue, 24 Jan 2023 14:05:43 +0100
Subject: [PATCH] [Support] Avoid using main thread for llvm::parallelFor().

The llvm::parallelFor() uses threads created by ThreadPoolExecutor as well as main thread.
The index for the main thread matches with the index for the first thread created by ThreadPoolExecutor.
It results in that getThreadIndex returns the same value for different threads.
To avoid thread index clashing - do not use main thread for llvm::parallelFor():

parallel::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)    <<<< executed by main thread.
  Fn(Begin);                     <<<<
return;                          <<<<

Differential Revision: https://reviews.llvm.org/D142317

(cherry picked from commit 10a796a0beb22ec604b3038b544cc9058facea79)
---
 llvm/lib/Support/Parallel.cpp | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Support/Parallel.cpp b/llvm/lib/Support/Parallel.cpp
index 23ed9d813548e..c256d256be4fa 100644
--- a/llvm/lib/Support/Parallel.cpp
+++ b/llvm/lib/Support/Parallel.cpp
@@ -214,8 +214,12 @@ void llvm::parallelFor(size_t Begin, size_t End,
           Fn(I);
       });
     }
-    for (; Begin != End; ++Begin)
-      Fn(Begin);
+    if (Begin != End) {
+      TG.spawn([=, &Fn] {
+        for (size_t I = Begin; I != End; ++I)
+          Fn(I);
+      });
+    }
     return;
   }
 #endif