[libomptarget] Allow calls to omp_target_memcpy with 0 size.

[grokos]

Summary

Currently, calling omp_target_memcpy with 0 size is treated as an error and the function returns OFFLOAD_FAIL. However, there is nothing in the standard prohibiting a 0-length memcpy. This patch changes the current behavior and makes omp_target_memcpy return immediately with OFFLOAD_SUCCESS if length is 0.

[JonChesterfield]

Semantic change is good.

Did you consider putting the equal zero test within the existing <= condition? Length zero should be infrequent (and when statically detected we should elide the call), so we can have one fewer conditional branch on the hot path. Probably in the noise though.

[grokos]

Slight optimization - removed condition from the hot path.

[jdoerfert]

LGTM

[JonChesterfield]

Thanks!

openmp/libomptarget/src/api.cpp

EXTERN int omp_target_memcpy(void *dst, void *src, size_t length,
     size_t dst_offset, size_t src_offset, int dst_device, int src_device) {
   TIMESCOPE();
   DP("Call to omp_target_memcpy, dst device %d, src device %d, "
       "dst addr " DPxMOD ", src addr " DPxMOD ", dst offset %zu, "
       "src offset %zu, length %zu\n", dst_device, src_device, DPxPTR(dst),
       DPxPTR(src), dst_offset, src_offset, length);
 
   if (!dst || !src || length <= 0) {
+    if (length == 0) {
+      DP("Call to omp_target_memcpy with zero length, nothing to do\n");
+      return OFFLOAD_SUCCESS;
+    }
+
     REPORT("Call to omp_target_memcpy with invalid arguments\n");
     return OFFLOAD_FAIL;
   }
 
   if (src_device != omp_get_initial_device() && !device_is_ready(src_device)) {
     REPORT("omp_target_memcpy returns OFFLOAD_FAIL\n");
     return OFFLOAD_FAIL;
   }