[Tarantool-patches] [PATCH luajit] FFI: Fix pointer difference operation on 64 bit platforms.

[Sergey Kaplun via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

From: Mike Pall <mike>

Thanks to cuiweixie.

(cherry picked from commit b58b07189521e82c7a8e8bc43fc3c271f89832fd)

The metamethod for FFI pointer subtraction casts the result value to a
32-bit value, which leads to invalid results.

This patch removes the excess cast.

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

Part of tarantool/tarantool#12880
---

Branch: https://github.com/tarantool/luajit/tree/skaplun/lj-1449-fix-ptr-diff-64-bit
Related issues:
* https://github.com/LuaJIT/LuaJIT/issues/1449
* https://github.com/tarantool/tarantool/issues/12880

 src/lj_carith.c                               |  2 +-
 .../lj-1449-fix-ptr-diff-64-bit.test.lua      | 39 +++++++++++++++++++
 2 files changed, 40 insertions(+), 1 deletion(-)
 create mode 100644 test/tarantool-tests/lj-1449-fix-ptr-diff-64-bit.test.lua

diff --git a/src/lj_carith.c b/src/lj_carith.c
index eb56d552..3384c2cd 100644
--- a/src/lj_carith.c
+++ b/src/lj_carith.c
@@ -120,7 +120,7 @@ static int carith_ptr(lua_State *L, CTState *cts, CDArith *ca, MMS mm)
 	/* All valid pointer differences on x64 are in (-2^47, +2^47),
 	** which fits into a double without loss of precision.
 	*/
-	setintptrV(L->top-1, (int32_t)diff);
+	setintptrV(L->top-1, diff);
 	return 1;
       } else if (mm == MM_lt) {  /* Pointer comparison (unsigned). */
 	setboolV(L->top-1, ((uintptr_t)pp < (uintptr_t)pp2));
diff --git a/test/tarantool-tests/lj-1449-fix-ptr-diff-64-bit.test.lua b/test/tarantool-tests/lj-1449-fix-ptr-diff-64-bit.test.lua
new file mode 100644
index 00000000..032b9b4d
--- /dev/null
+++ b/test/tarantool-tests/lj-1449-fix-ptr-diff-64-bit.test.lua
@@ -0,0 +1,39 @@
+local tap = require('tap')
+
+-- Test file to demonstrate LuaJIT's incorrect 64-bit pointer
+-- subtraction.
+-- See also: https://github.com/LuaJIT/LuaJIT/issues/1449.
+
+local test = tap.test('lj-1449-fix-ptr-diff-64-bit')
+
+local ffi = require('ffi')
+
+test:plan(2)
+
+local diff = 0x80000001ULL
+local base = 0x700000000000ULL
+local p0 = ffi.cast('char *', base)
+local p1 = ffi.cast('char *', base + diff)
+
+test:is(p1 - p0, diff, 'correct pointer difference between 64-bit pointers')
+
+test:skipcond({
+  ['Test requires JIT enabled'] = not jit.status(),
+})
+
+local results = {}
+
+jit.opt.start('hotloop=1')
+
+for i = 1, 4 do
+  -- Use constants on trace.
+  local delta = 0x80000001ULL
+  local b = 0x700000000000ULL
+  local pt0 = ffi.cast('char *', b)
+  local pt1 = ffi.cast('char *', b + delta)
+  results[i] = pt1 - pt0
+end
+
+test:samevalues(results, 'consistent JIT and VM behaviour for ptr subtraction')
+
+test:done(true)
-- 
2.55.0