Change default storage mode to SharedStorage on UMA devices

- Add 'auto' mode (new default) that detects unified memory architecture
- Apple Silicon (UMA): defaults to SharedStorage (zero-copy CPU access)
- Intel discrete GPU: defaults to PrivateStorage
- User preference override still works via LocalPreferences.toml
- Add tests for default storage mode selection

Author: KaanKesginLW

src/array.jl

@@ -190,14 +190,18 @@ See also `VecOrMat`(@ref) for examples.
 """
 const MtlVecOrMat{T,S} = Union{MtlVector{T,S},MtlMatrix{T,S}}
 
-# default to private memory
-const DefaultStorageMode = let str = @load_preference("default_storage", "private")
+# default storage mode: "auto" selects based on unified memory architecture
+# - UMA devices (Apple Silicon): SharedStorage (zero-copy CPU access)
+# - Non-UMA devices (Intel discrete GPU): PrivateStorage
+const DefaultStorageMode = let str = @load_preference("default_storage", "auto")
     if str == "private"
         PrivateStorage
     elseif str == "shared"
         SharedStorage
+    elseif str == "auto"
+        MTLDevice(1).hasUnifiedMemory ? SharedStorage : PrivateStorage
     else
-        error("unknown default storage mode: $default_storage")
+        error("unknown default storage mode: $str")
     end
 end
 

test/array.jl

@@ -2,6 +2,25 @@ STORAGEMODES = [Metal.PrivateStorage, Metal.SharedStorage]
 
 @testset "array" begin
 
+@testset "default storage mode" begin
+    # Test that default storage mode respects UMA detection
+    if Metal.device().hasUnifiedMemory
+        @test Metal.DefaultStorageMode == Metal.SharedStorage
+    else
+        @test Metal.DefaultStorageMode == Metal.PrivateStorage
+    end
+
+    # Test that arrays created without explicit storage use the default
+    arr = MtlArray{Float32}(undef, 100)
+    @test Metal.storagemode(arr) == Metal.DefaultStorageMode
+
+    # Test that explicit storage mode overrides the default
+    arr_private = MtlArray{Float32, 1, Metal.PrivateStorage}(undef, 100)
+    arr_shared = MtlArray{Float32, 1, Metal.SharedStorage}(undef, 100)
+    @test Metal.storagemode(arr_private) == Metal.PrivateStorage
+    @test Metal.storagemode(arr_shared) == Metal.SharedStorage
+end
+
 let arr = MtlVector{Int}(undef, 1)
     @test sizeof(arr) == 8
     @test length(arr) == 1