[libc] GPU RPC interface: add return value to rpc_host_call (#111288)

libc/newhdrgen/yaml/gpu/rpc.yaml

@@ -16,7 +16,7 @@ functions:
   - name: rpc_host_call
     standards:
       - GPUExtensions
-    return_type: void
+    return_type: unsigned long long
     arguments:
       - type: void *
       - type: void *

libc/spec/gpu_ext.td

@@ -7,7 +7,7 @@ def GPUExtensions : StandardSpec<"GPUExtensions"> {
     [
         FunctionSpec<
             "rpc_host_call",
-            RetValSpec<VoidType>,
+            RetValSpec<UnsignedLongLongType>,
             [ArgSpec<VoidPtr>, ArgSpec<VoidPtr>, ArgSpec<SizeTType>]
         >,
     ]

libc/src/gpu/rpc_host_call.cpp

@@ -17,14 +17,19 @@ namespace LIBC_NAMESPACE_DECL {
 
 // This calls the associated function pointer on the RPC server with the given
 // arguments. We expect that the pointer here is a valid pointer on the server.
-LLVM_LIBC_FUNCTION(void, rpc_host_call, (void *fn, void *data, size_t size)) {
+LLVM_LIBC_FUNCTION(unsigned long long, rpc_host_call,
+                   (void *fn, void *data, size_t size)) {
   rpc::Client::Port port = rpc::client.open<RPC_HOST_CALL>();
   port.send_n(data, size);
   port.send([=](rpc::Buffer *buffer) {
     buffer->data[0] = reinterpret_cast<uintptr_t>(fn);
   });
-  port.recv([](rpc::Buffer *) {});
+  unsigned long long ret;
+  port.recv([&](rpc::Buffer *buffer) {
+    ret = static_cast<unsigned long long>(buffer->data[0]);
+  });
   port.close();
+  return ret;
 }
 
 } // namespace LIBC_NAMESPACE_DECL

libc/src/gpu/rpc_host_call.h

@@ -14,7 +14,7 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
-void rpc_host_call(void *fn, void *buffer, size_t size);
+unsigned long long rpc_host_call(void *fn, void *buffer, size_t size);
 
 } // namespace LIBC_NAMESPACE_DECL
 

libc/utils/gpu/server/rpc_server.cpp

@@ -319,13 +319,18 @@ rpc_status_t handle_server_impl(
   }
   case RPC_HOST_CALL: {
     uint64_t sizes[lane_size] = {0};
+    unsigned long long results[lane_size] = {0};
     void *args[lane_size] = {nullptr};
     port->recv_n(args, sizes,
                  [&](uint64_t size) { return temp_storage.alloc(size); });
     port->recv([&](rpc::Buffer *buffer, uint32_t id) {
-      reinterpret_cast<void (*)(void *)>(buffer->data[0])(args[id]);
+      using func_ptr_t = unsigned long long (*)(void *);
+      auto func = reinterpret_cast<func_ptr_t>(buffer->data[0]);
+      results[id] = func(args[id]);
+    });
+    port->send([&](rpc::Buffer *buffer, uint32_t id) {
+      buffer->data[0] = static_cast<uint64_t>(results[id]);
     });
-    port->send([&](rpc::Buffer *, uint32_t id) {});
     break;
   }
   case RPC_FEOF: {

offload/test/libc/host_call.c

@@ -8,14 +8,14 @@
 
 #pragma omp begin declare variant match(device = {kind(gpu)})
 // Extension provided by the 'libc' project.
-void rpc_host_call(void *fn, void *args, size_t size);
+unsigned long long rpc_host_call(void *fn, void *args, size_t size);
 #pragma omp declare target to(rpc_host_call) device_type(nohost)
 #pragma omp end declare variant
 
 #pragma omp begin declare variant match(device = {kind(cpu)})
 // Dummy host implementation to make this work for all targets.
-void rpc_host_call(void *fn, void *args, size_t size) {
-  ((void (*)(void *))fn)(args);
+unsigned long long rpc_host_call(void *fn, void *args, size_t size) {
+  return ((unsigned long long (*)(void *))fn)(args);
 }
 #pragma omp end declare variant
 
@@ -25,17 +25,26 @@ typedef struct args_s {
 } args_t;
 
 // CHECK-DAG: Thread: 0, Block: 0
+// CHECK-DAG: Result: 42
 // CHECK-DAG: Thread: 1, Block: 0
+// CHECK-DAG: Result: 42
 // CHECK-DAG: Thread: 0, Block: 1
+// CHECK-DAG: Result: 42
 // CHECK-DAG: Thread: 1, Block: 1
+// CHECK-DAG: Result: 42
 // CHECK-DAG: Thread: 0, Block: 2
+// CHECK-DAG: Result: 42
 // CHECK-DAG: Thread: 1, Block: 2
+// CHECK-DAG: Result: 42
 // CHECK-DAG: Thread: 0, Block: 3
+// CHECK-DAG: Result: 42
 // CHECK-DAG: Thread: 1, Block: 3
-void foo(void *data) {
+// CHECK-DAG: Result: 42
+long long foo(void *data) {
   assert(omp_is_initial_device() && "Not executing on host?");
   args_t *args = (args_t *)data;
   printf("Thread: %d, Block: %d\n", args->thread_id, args->block_id);
+  return 42;
 }
 
 void *fn_ptr = NULL;
@@ -49,6 +58,7 @@ int main() {
 #pragma omp parallel num_threads(2)
   {
     args_t args = {omp_get_thread_num(), omp_get_team_num()};
-    rpc_host_call(fn_ptr, &args, sizeof(args_t));
+    unsigned long long res = rpc_host_call(fn_ptr, &args, sizeof(args_t));
+    printf("Result: %d\n", (int)res);
   }
 }