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

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

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Hi, Sergey,

thanks for the patch! LGTM

Sergey

On 7/20/26 17:24, Sergey Kaplun wrote:
> 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)

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