Hi, Sergey

thanks for the patch! LGTM

From: Mike Pall <mike>

Reported by Eric Gouyer.

(cherry picked from commit 36b2962d400db3981a7d7322f85c469240eb6f3b)

According to C++ Standard (5.3.6.3) [1], the `alignof()` for the
reference should be the same as for the referenced type. This patch
fixes the behaviour by following the reference to get a child id for
`ffi.alignof()`.

Sergey Kaplun:
* added the description and the test for the problem

Part of tarantool/tarantool#9924

[1]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4594.pdf#subsection.5.3.6
---
 src/lib_ffi.c                                 |  2 +-
 src/lj_ctype.c                                |  8 ++++++
 src/lj_ctype.h                                |  1 +
 .../lj-861-ctype-attributes.test.lua          | 26 +++++++++++++++++++
 4 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 test/tarantool-tests/lj-861-ctype-attributes.test.lua

diff --git a/src/lib_ffi.c b/src/lib_ffi.c
index e60e7b19..2bfca25a 100644
--- a/src/lib_ffi.c
+++ b/src/lib_ffi.c
@@ -639,7 +639,7 @@ LJLIB_CF(ffi_alignof)	LJLIB_REC(ffi_xof FF_ffi_alignof)
   CTState *cts = ctype_cts(L);
   CTypeID id = ffi_checkctype(L, cts, NULL);
   CTSize sz = 0;
-  CTInfo info = lj_ctype_info(cts, id, &sz);
+  CTInfo info = lj_ctype_info_raw(cts, id, &sz);
   setintV(L->top-1, 1 << ctype_align(info));
   return 1;
 }
diff --git a/src/lj_ctype.c b/src/lj_ctype.c
index 0874fa61..83042118 100644
--- a/src/lj_ctype.c
+++ b/src/lj_ctype.c
@@ -345,6 +345,14 @@ CTInfo lj_ctype_info(CTState *cts, CTypeID id, CTSize *szp)
   return qual;
 }
 
+/* Ditto, but follow a reference. */
+CTInfo lj_ctype_info_raw(CTState *cts, CTypeID id, CTSize *szp)
+{
+  CType *ct = ctype_get(cts, id);
+  if (ctype_isref(ct->info)) id = ctype_cid(ct->info);
+  return lj_ctype_info(cts, id, szp);
+}
+
 /* Get ctype metamethod. */
 cTValue *lj_ctype_meta(CTState *cts, CTypeID id, MMS mm)
 {
diff --git a/src/lj_ctype.h b/src/lj_ctype.h
index fce29409..8edbd561 100644
--- a/src/lj_ctype.h
+++ b/src/lj_ctype.h
@@ -467,6 +467,7 @@ LJ_FUNC CType *lj_ctype_rawref(CTState *cts, CTypeID id);
 LJ_FUNC CTSize lj_ctype_size(CTState *cts, CTypeID id);
 LJ_FUNC CTSize lj_ctype_vlsize(CTState *cts, CType *ct, CTSize nelem);
 LJ_FUNC CTInfo lj_ctype_info(CTState *cts, CTypeID id, CTSize *szp);
+LJ_FUNC CTInfo lj_ctype_info_raw(CTState *cts, CTypeID id, CTSize *szp);
 LJ_FUNC cTValue *lj_ctype_meta(CTState *cts, CTypeID id, MMS mm);
 LJ_FUNC GCstr *lj_ctype_repr(lua_State *L, CTypeID id, GCstr *name);
 LJ_FUNC GCstr *lj_ctype_repr_int64(lua_State *L, uint64_t n, int isunsigned);
diff --git a/test/tarantool-tests/lj-861-ctype-attributes.test.lua b/test/tarantool-tests/lj-861-ctype-attributes.test.lua
new file mode 100644
index 00000000..d88045a5
--- /dev/null
+++ b/test/tarantool-tests/lj-861-ctype-attributes.test.lua
@@ -0,0 +1,26 @@
+local tap = require('tap')
+
+-- Test file to demonstrate LuaJIT incorrect behaviour during
+-- parsing and working with ctypes with attributes.
+-- See also: https://github.com/LuaJIT/LuaJIT/issues/861.
+
+local test = tap.test('lj-861-ctype-attributes')
+local ffi = require('ffi')
+
+test:plan(2)
+
+local EXPECTED_ALIGN = 4
+
+ffi.cdef([[
+struct __attribute__((aligned($))) s_aligned {
+  uint8_t a;
+};
+]], EXPECTED_ALIGN)
+
+local ref_align = ffi.alignof(ffi.typeof('struct s_aligned &'))
+
+test:is(ref_align, EXPECTED_ALIGN, 'the reference alignment is correct')
+test:is(ref_align, ffi.alignof(ffi.typeof('struct s_aligned')),
+        'the alignment of a reference is the same as for the referenced type')
+
+test:done(true)