[llvm] [AMDGPU] Fix SSID merge in SIMemoryLegalizer to use smallest inclusiv… (PR #208300)

[Wooseok Lee via llvm-commits]

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@@ -130,26 +130,58 @@ class AMDGPUMachineModuleInfo final : public MachineModuleInfoELF {
     return ClusterOneAddressSpaceSSID;
   }
 
-  /// In AMDGPU target synchronization scopes are inclusive, meaning a
-  /// larger synchronization scope is inclusive of a smaller synchronization
-  /// scope.
+  /// In AMDGPU, synchronization scopes are inclusive: a larger scope is
+  /// inclusive of a smaller one (e.g. agent includes workgroup).
+  ///
+  /// Returns the merged synchronization scope of \p A and \p B: the smallest
+  /// scope that is inclusive of both. Takes the larger inclusion level and,
+  /// if either scope is cross-address-space, the result is also
+  /// cross-address-space (since a one-AS scope cannot subsume a cross-AS
+  /// scope at the same level).
   ///
-  /// \returns True if synchronization scope \p A is larger than or equal to
-  /// synchronization scope \p B, false if synchronization scope \p A is smaller
-  /// than synchronization scope \p B, or "std::nullopt" if either
-  /// synchronization scope \p A or \p B is not supported by the AMDGPU target.
-  std::optional<bool> isSyncScopeInclusion(SyncScope::ID A,
-                                           SyncScope::ID B) const {
+  /// \returns The merged scope ID, or "std::nullopt" if either scope is not
+  /// supported by the AMDGPU target.
+  std::optional<SyncScope::ID> getMergedSyncScopeID(SyncScope::ID A,
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wooseoklee wrote:

Thanks for the feedback. This is a good point. I will update the code.

https://github.com/llvm/llvm-project/pull/208300