Add ALLOC_ONLY MemType variant for out bufs

runtime/cpp/include/fletcher/context.h

@@ -49,7 +49,18 @@ enum class MemType {
    * Selecting CACHE may result in higher performance if there is data reuse by the kernel, but may result in lower
    * performance if the data is not reused by the kernel (for example fully streamable kernels).
    */
-      CACHE
+      CACHE,
+
+  /**
+   * @brief Allocate a buffer for the data on the on-board memory of the device, but don't copy the buffer contents to it.
+   *
+   * If available, this forces a buffer to be allocated on device on-board memory (e.g. some seperate DRAM chips sitting
+   * on the accelerator PCB next to the FPGA, but it could be HBM on top of the FPGA fabric in the same chip, or
+   * BRAM. This depends on the platform). However, the data currently in the provided buffer is not copied to the device;
+   * the device memory is left uninitialized. This is useful for output buffers, or if you want to handle initialization
+   * manually.
+   */
+      ALLOC_ONLY
 };
 
 /// A buffer on the device

runtime/cpp/src/fletcher/context.cc

@@ -67,6 +67,10 @@ Status Context::Enable() {
                                               device_buf.size);
           // Cache always allocates on device.
           device_buf.was_alloced = true;
+        } else if (type == MemType::ALLOC_ONLY) {
+          status = platform_->DeviceMalloc(&device_buf.device_address,
+                                           device_buf.size);
+          device_buf.was_alloced = true;
         } else {
           status = Status::ERROR("Invalid / unsupported MemType.");
         }

runtime/python/pyfletcher/context.pxi

@@ -51,6 +51,7 @@ cdef class Context():
             record_batch : Arrow RecordBatch to queue
             memtype (str): Memory type: - 'any' results in least effort to make data available to FPGA (depending on the platform implementation).
                                         - 'cache' force copy to accelerator on-board DRAM memory, if available.
+                                        - 'alloc_only' force allocation on accelerator on-board DRAM memory (if available), but don't copy.
 
         """
 
@@ -60,6 +61,8 @@ cdef class Context():
             queue_mem_type = MemType.ANY
         elif mem_type == "cache":
             queue_mem_type = MemType.CACHE
+        elif mem_type == "alloc_only":
+            queue_mem_type = MemType.ALLOC_ONLY
         else:
             raise ValueError("mem_type argument can be only 'any' or 'cache'")
 

runtime/python/pyfletcher/includes/libfletcher.pxd

@@ -37,8 +37,9 @@ cdef extern from "fletcher/arrow-utils.h" namespace "fletcher":
 
 cdef extern from "fletcher/context.h" namespace "fletcher":
     cdef enum MemType:
-        ANY   "fletcher::MemType::ANY",
-        CACHE "fletcher::MemType::CACHE"
+        ANY         "fletcher::MemType::ANY",
+        CACHE       "fletcher::MemType::CACHE"
+        ALLOC_ONLY  "fletcher::MemType::ALLOC_ONLY"
 
 
 cdef extern from "fletcher/api.h" namespace "fletcher" nogil: