https://bugs.llvm.org/show_bug.cgi?id=39172
Bug ID: 39172
Summary: [NVPTX] Wrong results or hang on GPU when using
lastprivate() on SPMD construct with full runtime
Product: OpenMP
Version: unspecified
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Clang Compiler Support
Assignee: unassignedclangb...@nondot.org
Reporter: hah...@hahnjo.de
CC: llvm-bugs@lists.llvm.org
The following program uses an SPMD construct with lastprivate() clause and the
runtime has to be initialized because of schedule(dynamic):
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[]) {
int last;
#pragma omp target teams distribute parallel for map(from: last)
lastprivate(last) schedule(dynamic)
for (int i = 0; i < 10000; i++) {
last = i;
}
printf("last = %d\n", last);
return EXIT_SUCCESS;
}
Compiled with current Clang trunk the application delivers wrong results (last
= 0) with -O0 and -O1 and hangs with -O2 and -O3. I think this is due to the
same problem: The generated code calls __kmpc_data_sharing_push_stack() to get
memory for storing "int last;" to implement lastprivate(). This works if the
runtime is uninitialized because there is a special case in libomptarget-nvptx
which returns the *same memory location for all threads* (see
https://reviews.llvm.org/D51875).
However the original contract of __kmpc_data_sharing_push_stack() was to return
*unique memory for each thread* which explains the wrong result. For higher
optimization levels I'd guess that LLVM exploits undefined behaviour somewhere
which makes the application hang?
The only solution I can think of is to introduce new entry points in
libomptarget-nvptx with the required contract: Return the same memory location
for all calling threads in the same thread block. Opinions?
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