Improve reporting.

Author: maleadt

lib/cl/state.jl

@@ -163,53 +163,62 @@ struct SVMBackend <: AbstractMemoryBackend end
 struct USMBackend <: AbstractMemoryBackend end
 struct BDABackend <: AbstractMemoryBackend end
 
-function default_memory_backend(dev::Device)
-    # determine if USM is supported
-    usm = if usm_supported(dev)
-        caps = usm_capabilities(dev)
-        caps.host.access && caps.device.access
-    else
-        false
+function supported_memory_backends(dev::Device)
+    backends = AbstractMemoryBackend[]
+
+    # unified shared memory is the first choice, as it gives us separate host and device
+    # memory spaces that can be directly referenced by raw pointers.
+    if usm_supported(dev)
+        usm_caps = usm_capabilities(dev)
+        if usm_caps.host.access && usm_caps.device.access
+            push!(backends, USMBackend())
+        end
     end
 
-    bda = bda_supported(dev)
+    # plain old device buffers are second choice, but require an extension to support being
+    # referenced by raw pointers.
+    if bda_supported(dev)
+        push!(backends, BDABackend())
+    end
 
-    # determine if SVM is available (if needed)
-    svm = let
-        caps = svm_capabilities(dev)
-        caps.coarse_grain_buffer
+    # shared virtual memory is last, because it comes at a performance cost.
+    svm_caps = svm_capabilities(dev)
+    if svm_caps.coarse_grain_buffer
+        push!(backends, SVMBackend())
     end
 
+    return backends
+end
+
+function default_memory_backend(dev::Device)
+    supported_backends = supported_memory_backends(dev)
+    isempty(supported_backends) && return nothing
+
     preferred_backend = load_preference(OpenCL, "memory_backend", "auto")
     if preferred_backend == "auto"
-        if usm
+        first(supported_backends)
+    else
+        backend = if preferred_backend == "usm"
             USMBackend()
+        elseif preferred_backend == "bda"
+            BDABackend()
+        elseif preferred_backend == "svm"
+            SVMBackend()
         else
-            if svm
-                SVMBackend()
-            elseif bda
-                BDABackend()
-            else
-                error("Device $dev does not support USM, coarse-grained SVM, or Buffer Device Address, one of which is required by OpenCL.jl")
-            end
+            error("Unknown memory backend '$preferred_backend' requested")
         end
-    elseif preferred_backend == "usm"
-        usm || error("Use of USM memory backend requested, which is not supported by device $dev")
-        USMBackend()
-    elseif preferred_backend == "bda"
-        bda || error("Use of Buffer Device Address memory backend requested, which is not supported by device $dev")
-        BDABackend()
-    elseif preferred_backend == "svm"
-        svm || error("Use of coarse-grained SVM memory backend requested, which is not supported by device $dev")
-        SVMBackend()
-    else
-        error("Unknown memory backend '$preferred_backend' requested")
+        in(backend, supported_backends) || return nothing
+        backend
     end
 end
 
 function memory_backend()
     return get!(task_local_storage(), :CLMemoryBackend) do
-        default_memory_backend(device())
+        backend = default_memory_backend(device())
+        if backend === nothing
+            error("Device $(device()) does not support any of the available memory backends")
+        end
+        backend
     end
 end
 

src/util.jl

@@ -81,13 +81,15 @@ function versioninfo(io::IO=stdout)
             # show a list of tags
             tags = []
             ## memory back-end
-            backend = cl.default_memory_backend(device)
-            if backend == cl.SVMBackend()
-                push!(tags, "svm")
-            elseif backend == cl.BDABackend()
-                push!(tags, "bda")
-            elseif backend == cl.USMBackend()
-                push!(tags, "usm")
+            for backend in cl.supported_memory_backends(device)
+                suffix = backend == cl.default_memory_backend(device) ? "*" : ""
+                if backend isa cl.SVMBackend
+                    push!(tags, "svm"*suffix)
+                elseif backend isa cl.BDABackend
+                    push!(tags, "bda"*suffix)
+                elseif backend isa cl.USMBackend
+                    push!(tags, "usm"*suffix)
+                end
             end
             ## relevant extensions
             if in("cl_khr_fp16", device.extensions)