[PATCH] D31768: [Core] Fix parallel_for for Linux

[James Henderson via Phabricator via llvm-commits]

jhenderson created this revision.

https://reviews.llvm.org/rL299635 exposed a latent bug in the Linux implementation of parallel_for, which resulted in it calling the function outside of the range requested, resulting later in a segmentation fault. This change fixes this issue and adds a unit test.


https://reviews.llvm.org/D31768

Files:
  include/lld/Core/Parallel.h
  unittests/CoreTests/ParallelTest.cpp


Index: unittests/CoreTests/ParallelTest.cpp
===================================================================
--- unittests/CoreTests/ParallelTest.cpp
+++ unittests/CoreTests/ParallelTest.cpp
@@ -29,3 +29,18 @@
   lld::parallel_sort(std::begin(array), std::end(array));
   ASSERT_TRUE(std::is_sorted(std::begin(array), std::end(array)));
 }
+
+TEST(Parallel, parallel_for) {
+  // We need to test the case with a TaskSize > 1. We are white-box testing
+  // here. The TaskSize is calculated as (End - Begin) / 1024 at the time of
+  // writing.
+  uint32_t range[2050];
+  std::fill(range, range + 2050, 1);
+  lld::parallel_for(0, 2049, [&range](size_t I) { ++range[I]; });
+
+  uint32_t expected[2049];
+  std::fill(expected, expected + 2049, 2);
+  ASSERT_TRUE(std::equal(range, range + 2049, expected));
+  // Check that we don't write past the end of the requested range.
+  ASSERT_EQ(range[2049], 1);
+}
Index: include/lld/Core/Parallel.h
===================================================================
--- include/lld/Core/Parallel.h
+++ include/lld/Core/Parallel.h
@@ -318,12 +318,11 @@
 
   TaskGroup Tg;
   IndexTy I = Begin;
-  for (; I < End; I += TaskSize) {
+  for (; I + TaskSize < End; I += TaskSize) {
     Tg.spawn([=, &Fn] {
       for (IndexTy J = I, E = I + TaskSize; J != E; ++J)
         Fn(J);
     });
-    Begin += TaskSize;
   }
   Tg.spawn([=, &Fn] {
     for (IndexTy J = I; J < End; ++J)


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