[llvm] [OpenMP] Fix num_iters in __kmpc_*_loop DeviceRTL functions (PR #133435)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-offload

Author: Sergio Afonso (skatrak)

<details>
<summary>Changes</summary>

This patch removes the addition of 1 to the number of iterations when calling the following DeviceRTL functions:
- `__kmpc_distribute_for_static_loop*`
- `__kmpc_distribute_static_loop*`
- `__kmpc_for_static_loop*`

Calls to these functions are currently only produced by the OMPIRBuilder from flang, which already passes the correct number of iterations to these functions. By adding 1 to the received `num_iters` variable, worksharing can produce incorrect results. This impacts flang OpenMP offloading of `do`, `distribute` and `distribute parallel do` constructs.

Expecting the application to pass `tripcount - 1` as the argument seems unexpected as well, so rather than updating flang I think it makes more sense to update the runtime.

---
Full diff: https://github.com/llvm/llvm-project/pull/133435.diff


1 Files Affected:

- (modified) offload/DeviceRTL/src/Workshare.cpp (+3-3) 


``````````diff
diff --git a/offload/DeviceRTL/src/Workshare.cpp b/offload/DeviceRTL/src/Workshare.cpp
index 861b9ca371ccd..a8759307b42bd 100644
--- a/offload/DeviceRTL/src/Workshare.cpp
+++ b/offload/DeviceRTL/src/Workshare.cpp
@@ -911,19 +911,19 @@ template <typename Ty> class StaticLoopChunker {
           IdentTy *loc, void (*fn)(TY, void *), void *arg, TY num_iters,       \
           TY num_threads, TY block_chunk, TY thread_chunk) {                   \
     ompx::StaticLoopChunker<TY>::DistributeFor(                                \
-        loc, fn, arg, num_iters + 1, num_threads, block_chunk, thread_chunk);  \
+        loc, fn, arg, num_iters, num_threads, block_chunk, thread_chunk);      \
   }                                                                            \
   [[gnu::flatten, clang::always_inline]] void                                  \
       __kmpc_distribute_static_loop##BW(IdentTy *loc, void (*fn)(TY, void *),  \
                                         void *arg, TY num_iters,               \
                                         TY block_chunk) {                      \
-    ompx::StaticLoopChunker<TY>::Distribute(loc, fn, arg, num_iters + 1,       \
+    ompx::StaticLoopChunker<TY>::Distribute(loc, fn, arg, num_iters,           \
                                             block_chunk);                      \
   }                                                                            \
   [[gnu::flatten, clang::always_inline]] void __kmpc_for_static_loop##BW(      \
       IdentTy *loc, void (*fn)(TY, void *), void *arg, TY num_iters,           \
       TY num_threads, TY thread_chunk) {                                       \
-    ompx::StaticLoopChunker<TY>::For(loc, fn, arg, num_iters + 1, num_threads, \
+    ompx::StaticLoopChunker<TY>::For(loc, fn, arg, num_iters, num_threads,     \
                                      thread_chunk);                            \
   }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/133435