[OpenMP] Support for global variables when in auto zero-copy. (#80876)

When building without unified_shared_memory, global variables are
declared in the device binary and allocated upon loading onto GPU
memory. However, when running in zero-copy mode (same as with
unified_shared_memory) D2H and H2D copies for mapped local and global
variables are turned off. This patch turns back on H2D and D2H copies
when they refer to global variables, enabling an application built
without unified_shared_memory to work correctly with global variables
when run under automatic zero-copy.

Co-authored-by: Doru Bercea <doru.bercea@amd.com>
Co-authored-by: Jan-Patrick Lehr <janpatrick.lehr@amd.com>

2 files changed, 87 insertions, 1 deletions

diff --git a/openmp/libomptarget/src/omptarget.cpp b/openmp/libomptarget/src/omptarget.cpp
index f97cacf..821669d 100644
--- a/openmp/libomptarget/src/omptarget.cpp
+++ b/openmp/libomptarget/src/omptarget.cpp
@@ -188,7 +188,8 @@ static int initLibrary(DeviceTy &Device) {
           // If unified memory is active, the corresponding global is a device
           // reference to the host global. We need to initialize the pointer on
           // the deive to point to the memory on the host.
-          if (PM->getRequirements() & OMP_REQ_UNIFIED_SHARED_MEMORY) {
+          if ((PM->getRequirements() & OMP_REQ_UNIFIED_SHARED_MEMORY) ||
+              (PM->getRequirements() & OMPX_REQ_AUTO_ZERO_COPY)) {
             if (Device.RTL->data_submit(DeviceId, DeviceEntry.addr, Entry.addr,
                                         Entry.size) != OFFLOAD_SUCCESS)
               REPORT("Failed to write symbol for USM %s\n", Entry.name);
diff --git a/openmp/libomptarget/test/mapping/auto_zero_copy_globals.cpp b/openmp/libomptarget/test/mapping/auto_zero_copy_globals.cpp
new file mode 100644
index 0000000..4a13d27
--- /dev/null
+++ b/openmp/libomptarget/test/mapping/auto_zero_copy_globals.cpp
@@ -0,0 +1,85 @@
+// clang-format off
+// RUN: %libomptarget-compilexx-generic
+// RUN: env OMPX_APU_MAPS=1 HSA_XNACK=1 LIBOMPTARGET_INFO=60 %libomptarget-run-generic 2>&1 \
+// RUN: | %fcheck-generic -check-prefix=CHECK
+
+// UNSUPPORTED: aarch64-unknown-linux-gnu
+// UNSUPPORTED: aarch64-unknown-linux-gnu-LTO
+// UNSUPPORTED: nvptx64-nvidia-cuda
+// UNSUPPORTED: nvptx64-nvidia-cuda-LTO
+// UNSUPPORTED: x86_64-pc-linux-gnu
+// UNSUPPORTED: x86_64-pc-linux-gnu-LTO
+
+// REQUIRES: unified_shared_memory
+
+// clang-format on
+
+#include <cstdint>
+#include <cstdio>
+
+/// Test for globals under automatic zero-copy.
+/// Because we are building without unified_shared_memory
+/// requirement pragma, all globals are allocated in the device
+/// memory of all used GPUs. To ensure those globals contain the intended
+/// values, we need to execute H2D and D2H memory copies even if we are running
+/// in automatic zero-copy. This only applies to globals. Local variables (their
+/// host pointers) are passed to the kernels by-value, according to the
+/// automatic zero-copy behavior.
+
+#pragma omp begin declare target
+int32_t x;     // 4 bytes
+int32_t z[10]; // 40 bytes
+int32_t *k;    // 20 bytes
+#pragma omp end declare target
+
+int main() {
+  int32_t *dev_k = nullptr;
+  x = 3;
+  int32_t y = -1;
+  for (size_t t = 0; t < 10; t++)
+    z[t] = t;
+  k = new int32_t[5];
+
+  printf("Host pointer for k = %p\n", k);
+  for (size_t t = 0; t < 5; t++)
+    k[t] = -t;
+
+/// target update to forces a copy between host and device global, which we must
+/// execute to keep the two global copies consistent. CHECK: Copying data from
+/// host to device, HstPtr={{.*}}, TgtPtr={{.*}}, Size=40, Name=z
+#pragma omp target update to(z[ : 10])
+
+/// target map with always modifier (for x) forces a copy between host and
+/// device global, which we must execute to keep the two global copies
+/// consistent. k's content (host address) is passed by-value to the kernel
+/// (Size=20 case). y, being a local variable, is also passed by-value to the
+/// kernel (Size=4 case) CHECK: Return HstPtrBegin {{.*}} Size=4 for unified
+/// shared memory CHECK: Return HstPtrBegin {{.*}} Size=20 for unified shared
+/// memory CHECK: Copying data from host to device, HstPtr={{.*}},
+/// TgtPtr={{.*}}, Size=4, Name=x
+#pragma omp target map(to : k[ : 5]) map(always, tofrom : x) map(tofrom : y)   \
+    map(from : dev_k)
+  {
+    x++;
+    y++;
+    for (size_t t = 0; t < 10; t++)
+      z[t]++;
+    dev_k = k;
+  }
+/// CHECK-NOT: Copying data from device to host, TgtPtr={{.*}}, HstPtr={{.*}},
+/// Size=20, Name=k
+
+/// CHECK: Copying data from device to host, TgtPtr={{.*}}, HstPtr={{.*}},
+/// Size=4, Name=x
+
+/// CHECK: Copying data from device to host, TgtPtr={{.*}}, HstPtr={{.*}},
+/// Size=40, Name=z
+#pragma omp target update from(z[ : 10])
+
+  /// CHECK-NOT: k pointer not correctly passed to kernel
+  if (dev_k != k)
+    printf("k pointer not correctly passed to kernel\n");
+
+  delete[] k;
+  return 0;
+}