[llvm] [Offload]: Skip copying of unused kernel-mapped data (PR #124723)

[Johannes Doerfert via llvm-commits]

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@@ -1197,6 +1197,82 @@ class PrivateArgumentManagerTy {
   }
 };
 
+/// Try to find redundant mappings associated with a kernel launch,
+/// and provide a masked version of the kernel argument types that
+/// avoid redundant to data transfers between the host and device.
+static std::unique_ptr<int64_t[]>
+maskRedundantTransfers(DeviceTy &Device, int32_t ArgNum, int64_t *ArgTypes,
+                       int64_t *ArgSizes, map_var_info_t *ArgNames,
+                       void **ArgPtrs, void **ArgMappers) {
+  std::unique_ptr<int64_t[]> ArgTypesOverride =
+      std::make_unique<int64_t[]>(ArgNum);
+
+  MappingInfoTy &MappingInfo = Device.getMappingInfo();
+  MappingInfoTy::HDTTMapAccessorTy HDTTMap =
+      MappingInfo.HostDataToTargetMap.getExclusiveAccessor();
+
+  int64_t UnusedArgs = 0;
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jdoerfert wrote:

This is not a good way to check for a "LastWasUnused". Make it explicit. Also, do we have a test where the object is used but the pointee is not? 

https://github.com/llvm/llvm-project/pull/124723