Fix BC_UCLO insertion for returns.

Contributed by XmiliaH.

(cherry-picked from commit 93a65d3)

Patch fixes a problem when LuaJIT generates a wrong bytecode with a
missed BC_UCLO instruction. When some of BC_RET bytecode instructions are
not fixup-ed, due to an early return, if UCLO is obtained before, those
leads to VM inconsistency after return from the function.

Patch makes the following changes in bytecode (thats it, emits extra
BC_UCLO instruction that closes upvalues):

@@ -11,11 +11,12 @@
 0006 => LOOP     1 => 0012
 0007    ISF          0
 0008    JMP      1 => 0010
-0009    RET1     0   2
+0009    UCLO     0 => 0014
 0010 => FNEW     0   0      ; uclo.lua:56
 0011    JMP      1 => 0006
 0012 => UCLO     0 => 0001
 0013 => RET0     0   1
+0014 => RET1     0   2

NOTE: After emitting the bytecode instruction BC_FNEW fixup is not
required, because FuncState will set a flag PROTO_CHILD that will
trigger emitting a pair of instructions BC_UCLO and BC_RET (see
<src/lj_parse.c:2355>) and BC_RET will close all upvalues from a base
equal to 0.

JIT compilation of missing_uclo() function without a patch with fix is failed:
src/lj_record.c:135: rec_check_slots: Assertion `((((((tr))>>24) & IRT_TYPE) - (TRef)(IRT_NUM) <= (TRef)(IRT_INT-IRT_NUM)))' failed.
(Thanks to Sergey Kaplun for discovering this!)
Thus second testcase in a test covers a case with compilation as well.

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

Part of tarantool/tarantool#8825

Helped-by: Sergey Kaplun <skaplun@tarantool.org>
Signed-off-by: Sergey Bronnikov <sergeyb@tarantool.org>
Reviewed-by: Maxim Kokryashkin <m.kokryashkin@tarantool.org>
Reviewed-by: Sergey Kaplun <skaplun@tarantool.org>
Signed-off-by: Igor Munkin <imun@tarantool.org>

src/lj_parse.c

@@ -1546,7 +1546,7 @@ static void fs_fixup_ret(FuncState *fs)
 	/* Replace with UCLO plus branch. */
 	fs->bcbase[pc].ins = BCINS_AD(BC_UCLO, 0, offset);
 	break;
-      case BC_UCLO:
+      case BC_FNEW:
 	return;  /* We're done. */
       default:
 	break;

test/tarantool-tests/lj-819-fix-missing-uclo.test.lua

@@ -0,0 +1,115 @@
+local tap = require('tap')
+-- Test contains a reproducer for a problem when LuaJIT generates a wrong
+-- bytecode with a missed BC_UCLO instruction.
+local test = tap.test('lj-819-fix-missing-uclo'):skipcond({
+  ['Test requires JIT enabled'] = not jit.status(),
+})
+
+test:plan(2)
+
+-- Let's take a look at listings Listing 1 and Listing 2 below with bytecode
+-- generated for a function missing_uclo() with and without a patch.
+-- Both listings contains two BC_UCLO instructions:
+-- - first one with id 0004 is generated for a statement 'break' inside
+--   condition, see label BC_UCLO1;
+-- - second one with id 0009 is generated for a statement 'return' inside
+--   a nested loop, see label BC_UCLO2;
+-- Both BC_UCLO's closes upvalues after leaving a function's scope.
+--
+-- The problem is happen when fs_fixup_ret() traverses bytecode instructions in
+-- a function prototype, meets first BC_UCLO instruction (break) and forgives a
+-- second one (return). This leads to a wrong result produced by a function
+-- returned by missing_uclo() function. This also explains why do we need a
+-- dead code in reproducer - without first BC_UCLO fs_fixup_ret() successfully
+-- fixup BC_UCLO and problem does not appear.
+--
+-- Listing 1. Bytecode with a fix.
+--
+-- -- BYTECODE -- uclo.lua:1-59
+-- 0001 => LOOP     0 => 0013
+-- 0002    JMP      0 => 0003
+-- 0003 => JMP      0 => 0005
+-- 0004    UCLO     0 => 0013
+-- 0005 => KPRI     0   0
+-- 0006 => LOOP     1 => 0012
+-- 0007    ISF          0
+-- 0008    JMP      1 => 0010
+-- 0009    UCLO     0 => 0014
+-- 0010 => FNEW     0   0      ; uclo.lua:54
+-- 0011    JMP      1 => 0006
+-- 0012 => UCLO     0 => 0001
+-- 0013 => RET0     0   1
+-- 0014 => RET1     0   2
+--
+-- Listing 2. Bytecode without a fix.
+--
+-- BYTECODE -- uclo.lua:1-59
+-- 0001 => LOOP     0 => 0013
+-- 0002    JMP      0 => 0003
+-- 0003 => JMP      0 => 0005
+-- 0004    UCLO     0 => 0013
+-- 0005 => KPRI     0   0
+-- 0006 => LOOP     1 => 0012
+-- 0007    ISF          0
+-- 0008    JMP      1 => 0010
+-- 0009    UCLO     0 => 0014
+-- 0010 => FNEW     0   0      ; uclo.lua:54
+-- 0011    JMP      1 => 0006
+-- 0012 => UCLO     0 => 0001
+-- 0013 => RET0     0   1
+-- 0014 => RET1     0   2
+--
+-- Listing 3. Changes in bytecode before and after a fix.
+--
+-- @@ -11,11 +11,12 @@
+--  0006 => LOOP     1 => 0012
+--  0007    ISF          0
+--  0008    JMP      1 => 0010
+-- -0009    RET1     0   2
+-- +0009    UCLO     0 => 0014
+--  0010 => FNEW     0   0      ; uclo.lua:56
+--  0011    JMP      1 => 0006
+--  0012 => UCLO     0 => 0001
+--  0013 => RET0     0   1
+-- +0014 => RET1     0   2
+--
+-- First testcase checks a correct bytecode generation by frontend
+-- and the second testcase checks consistency on a JIT compilation.
+
+local function missing_uclo()
+  while true do -- luacheck: ignore
+    -- Attention: it is not a dead code, it is a part of reproducer.
+    -- label: BC_UCLO1
+    if false then
+      break
+    end
+    local f
+    while true do
+      if f then
+        -- label: BC_UCLO2
+        return f
+      end
+      f = function()
+        return f
+      end
+    end
+  end
+end
+
+local f = missing_uclo()
+local res = f()
+-- Without a patch we don't get here a function, because upvalue isn't closed
+-- as desirable.
+test:ok(type(res) == 'function', 'virtual machine consistency: type of returned value is correct')
+
+-- Make JIT compiler aggressive.
+jit.opt.start('hotloop=1')
+
+f = missing_uclo()
+f()
+f = missing_uclo()
+local _
+_, res = pcall(f)
+test:ok(type(res) == 'function', 'consistency on compilation: type of returned value is correct')
+
+os.exit(test:check() and 0 or 1)