FFI: Fix recording of union initialization.

Thanks to Alex Shpilkin.

(cherry-picked from commit 56c04ac)

As stated here[1], only the first field of a union can be
initialized with a flat initializer. However, before this
patch, on-trace initialization instructions were emitted
for other union members too, overwriting the previous
initialization values.

This patch fixes the mentioned behavior by preventing
initialization of members other than the first one.

[1]: https://luajit.org/ext_ffi_semantics.html#init

Maxim Kokryashkin:
* added the description and the test for the problem

Part of tarantool/tarantool#8825

src/lj_crecord.c

@@ -1065,6 +1065,11 @@ static void crec_alloc(jit_State *J, RecordFFData *rd, CTypeID id)
 	  dp = emitir(IRT(IR_ADD, IRT_PTR), trcd,
 		      lj_ir_kintp(J, df->size + sizeof(GCcdata)));
 	  crec_ct_tv(J, dc, dp, sp, sval);
+	  if ((d->info & CTF_UNION)) {
+	    if (d->size != dc->size)  /* NYI: partial init of union. */
+	      lj_trace_err(J, LJ_TRERR_NYICONV);
+	    break;
+	  }
 	} else if (!ctype_isconstval(df->info)) {
 	  /* NYI: init bitfields and sub-structures. */
 	  lj_trace_err(J, LJ_TRERR_NYICONV);

test/tarantool-tests/lj-128-fix-union-init.test.lua

@@ -0,0 +1,18 @@
+local tap = require('tap')
+local test = tap.test('lj-128-fix-union-init'):skipcond({
+  ['Test requires JIT enabled'] = not jit.status(),
+})
+
+local NITERATIONS = 4
+
+test:plan(NITERATIONS)
+
+local ffi = require('ffi')
+local union_type = ffi.typeof('union { uint32_t u; float f; }')
+
+jit.opt.start('hotloop=1')
+for i = 1, NITERATIONS do
+  test:ok(union_type(i).u == i)
+end
+
+os.exit(test:check() and 0 or 1)