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

[Johannes Doerfert via llvm-commits]

================
@@ -1197,6 +1197,97 @@ class PrivateArgumentManagerTy {
   }
 };
 
+/// Try to determine if kernel argument is unused. This method
+/// takes a conservative approach, i.e. it may return false
+/// negatives but it should never return a false positive.
+static bool isArgUnused(tgt_map_type ArgType) {
+  bool IsArgUnused = ArgType == OMP_TGT_MAPTYPE_NONE ||
+                     ArgType == OMP_TGT_MAPTYPE_FROM ||
+                     ArgType == OMP_TGT_MAPTYPE_TO ||
+                     ArgType == (OMP_TGT_MAPTYPE_FROM | OMP_TGT_MAPTYPE_TO);
+  return IsArgUnused;
+}
+
+/// Try to find redundant mappings associated with a kernel launch,
+/// and provide a masked version of the kernel argument types that
+/// avoid redundant data transfers between the host and the 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);
+
+    bool AllArgsUnused = true;
+
+    for (int32_t I = 0; I < ArgNum; ++I) {
+      bool IsCustomMapped = ArgMappers && ArgMappers[I];
+
+      if (IsCustomMapped) {
+        ArgTypesOverride[I] = ArgTypes[I];
+        AllArgsUnused = false;
+        continue;
+      }
+
+        tgt_map_type ArgType = (tgt_map_type) ArgTypes[I];
+
+        bool IsArgUnused = false;
+
+        // Check for unused `map(buf[0:size])` mappings
+        IsArgUnused |= isArgUnused(ArgType);
+
+        bool IsArgMemberPtr = ArgType & OMP_TGT_MAPTYPE_MEMBER_OF &&
+                              ArgType & OMP_TGT_MAPTYPE_PTR_AND_OBJ;
+
+        tgt_map_type ArgTypeMemberPtrMasked =
+            (tgt_map_type)(ArgType & ~(OMP_TGT_MAPTYPE_MEMBER_OF |
+                                       OMP_TGT_MAPTYPE_PTR_AND_OBJ));
+
+        // Check for unused `map(wrapper.buf[0:size])` mappings
+        IsArgUnused |= AllArgsUnused && IsArgMemberPtr &&
+                       isArgUnused(ArgTypeMemberPtrMasked);
----------------
jdoerfert wrote:

Looking at AllArgsUnused here seems surprising. I expect you need to check the object this is a member of? I believe they are sorted, hence, it is always the last argument. Looking at all causes weird ordering interactions and prevents removal easily.

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