ggml: No-alloc mode

Callers can set a backend buffer type to be no-alloc, meaning that
it does not allocate memory for tensors or operations. This can
be used for calculating memory requirements. Tensors and graphs
must be recreated with no-alloc set to false before loading data.

Defaults to false for newly created backend buffer types.

ml/backend/ggml/ggml/include/ggml-backend.h

@@ -35,6 +35,7 @@ extern "C" {
     //
 
     GGML_API const char *          ggml_backend_buft_name          (ggml_backend_buffer_type_t buft);
+    GGML_API void                  ggml_backend_buft_set_alloc     (ggml_backend_buffer_type_t buft, bool alloc);
     GGML_API ggml_backend_buffer_t ggml_backend_buft_alloc_buffer  (ggml_backend_buffer_type_t buft, size_t size);
     GGML_API size_t                ggml_backend_buft_get_alignment (ggml_backend_buffer_type_t buft);
     GGML_API size_t                ggml_backend_buft_get_max_size  (ggml_backend_buffer_type_t buft);

ml/backend/ggml/ggml/src/ggml-backend-impl.h

@@ -32,6 +32,7 @@ extern "C" {
         struct ggml_backend_buffer_type_i  iface;
         ggml_backend_dev_t device;
         void * context;
+        bool no_alloc;
     };
 
     //
@@ -63,6 +64,7 @@ extern "C" {
         void * context;
         size_t size;
         enum ggml_backend_buffer_usage usage;
+        bool no_alloc;
     };
 
     GGML_API ggml_backend_buffer_t ggml_backend_buffer_init(

ml/backend/ggml/ggml/src/ggml-backend.cpp

@@ -35,12 +35,22 @@ const char * ggml_backend_buft_name(ggml_backend_buffer_type_t buft) {
     return buft->iface.get_name(buft);
 }
 
+void ggml_backend_buft_set_alloc(ggml_backend_buffer_type_t buft, bool alloc) {
+    buft->no_alloc = !alloc;
+}
+
 ggml_backend_buffer_t ggml_backend_buft_alloc_buffer(ggml_backend_buffer_type_t buft, size_t size) {
     if (size == 0) {
         // return a dummy buffer for zero-sized allocations
         return ggml_backend_buffer_init(buft, {}, NULL, 0);
     }
 
+    if (buft->no_alloc) {
+        ggml_backend_buffer_t buf = ggml_backend_buffer_init(buft, {}, NULL, size);
+        buf->no_alloc = true;
+        return buf;
+    }
+
     return buft->iface.alloc_buffer(buft, size);
 }
 
@@ -89,7 +99,8 @@ ggml_backend_buffer_t ggml_backend_buffer_init(
         /* .buft      = */ buft,
         /* .context   = */ context,
         /* .size      = */ size,
-        /* .usage     = */ GGML_BACKEND_BUFFER_USAGE_ANY
+        /* .usage     = */ GGML_BACKEND_BUFFER_USAGE_ANY,
+        /* .no_alloc  = */ false
     };
 
     return buffer;
@@ -119,6 +130,12 @@ void * ggml_backend_buffer_get_base(ggml_backend_buffer_t buffer) {
         return NULL;
     }
 
+    // If we aren't allocating memory, return a placeholder non-NULL pointer
+    // that meets alignment requirements
+    if (buffer->no_alloc) {
+        return (void *)ggml_backend_buffer_get_alignment(buffer);
+    }
+
     void * base = buffer->iface.get_base(buffer);
 
     GGML_ASSERT(base != NULL && "backend buffer base cannot be NULL");