Fix bit op coercion in DUALNUM builds.

Thanks to Sergey Kaplun.

(cherry picked from commit f5fd222)

The `lj_carith_check64()` function coerces the given number value to the
32-bit wide value. In this case, the 64-bit-wide operands will lose
upper bits.

This patch removes the excess coercion for the DUALNUM mode and drops
the corresponding skipcond introduced for the test in the previous
commit.

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

Part of tarantool/tarantool#10199

src/lj_carith.c

@@ -349,9 +349,7 @@ uint64_t lj_carith_check64(lua_State *L, int narg, CTypeID *id)
   if (LJ_LIKELY(tvisint(o))) {
     return (uint32_t)intV(o);
   } else {
-    int32_t i = lj_num2bit(numV(o));
-    if (LJ_DUALNUM) setintV(o, i);
-    return (uint32_t)i;
+    return (uint32_t)lj_num2bit(numV(o));
   }
 }
 

test/tarantool-tests/lj-1079-fix-64-bitshift-folds.test.lua

@@ -69,9 +69,6 @@ end
 test:test('rol', test_64bitness, bitop_rotation, bit.rol)
 test:test('ror', test_64bitness, bitop_rotation, bit.ror)
 test:test('rshift signed', test_64bitness, bitop_rshift_signed)
--- XXX: This skipcond will be dropped in the next commit.
-local IS_DUALNUM = tostring(tonumber('-0')) ~= tostring(-0)
-test:skipcond({['Has inconsistent behaviour for DUALNUM mode'] = IS_DUALNUM})
 test:test('rshift huge',   test_64bitness, bitop_rshift_huge)
 
 test:done(true)

test/tarantool-tests/lj-1273-dualnum-bit-coercion.test.lua

@@ -0,0 +1,19 @@
+local tap = require('tap')
+
+-- Test file to demonstrate LuaJIT misbehaviour on operating
+-- for 64-bit operands in built-in `bit` library in DUALNUM mode.
+-- See also, https://github.com/LuaJIT/LuaJIT/issues/1273.
+
+local test = tap.test('lj-1273-dualnum-bit-coercion')
+test:plan(1)
+
+local bit = require('bit')
+
+-- The cdata value for 2 ^ 33.
+local EXPECTED = 2 ^ 33 + 0LL
+-- Same value is used as mask for `bit.band()`.
+local MASK = EXPECTED
+
+test:is(bit.band(2 ^ 33, MASK), EXPECTED, 'correct bit.band result')
+
+test:done(true)