[Libguestfs] [PATCH libnbd] generator/Go.ml: Simplify copy_uint32_array

[Nir Soffer]

Create a slice backed up by the entries pointer, and copy the data with
builtin copy(). This can be 3x times faster but I did not measure it.

Eric posted a similar patch[1] last year, but the patch seems to be
stuck with unrelated incomplete work.

[1] https://listman.redhat.com/archives/libguestfs/2021-December/msg00065.html

Signed-off-by: Nir Soffer <nsoffer at redhat.com>
---
 generator/GoLang.ml | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/generator/GoLang.ml b/generator/GoLang.ml
index eb3aa263..73838199 100644
--- a/generator/GoLang.ml
+++ b/generator/GoLang.ml
@@ -1,13 +1,13 @@
 (* hey emacs, this is OCaml code: -*- tuareg -*- *)
 (* nbd client library in userspace: generator
- * Copyright (C) 2013-2020 Red Hat Inc.
+ * Copyright (C) 2013-2022 Red Hat Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Lesser General Public License for more details.
@@ -508,24 +508,24 @@ package libnbd
 #include \"wrappers.h\"
 */
 import \"C\"
 
 import \"unsafe\"
 
 /* Closures. */
 
 func copy_uint32_array (entries *C.uint32_t, count C.size_t) []uint32 {
     ret := make([]uint32, int (count))
-    for i := 0; i < int (count); i++ {
-       entry := (*C.uint32_t) (unsafe.Pointer(uintptr(unsafe.Pointer(entries)) + (unsafe.Sizeof(*entries) * uintptr(i))))
-       ret[i] = uint32 (*entry)
-    }
+    // See https://github.com/golang/go/wiki/cgo#turning-c-arrays-into-go-slices
+    // TODO: Use unsafe.Slice() when we require Go 1.17.
+    s := (*[1<<30]uint32)(unsafe.Pointer(entries))[:count:count]
+    copy(ret, s)
     return ret
 }
 ";
 
   List.iter (
     fun { cbname; cbargs } ->
       let uname = camel_case cbname in
       pr "type %sCallback func (" uname;
       let comma = ref false in
       List.iter (
-- 
2.34.1