Re: [Tarantool-patches] [PATCH luajit] Fix FOLD rule for x-0.

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

Hi, Sergey!
Thanks for the patch!
LGTM

On 10/31/23 15:44, Sergey Kaplun wrote:
> From: Mike Pall <mike>
>
> Reported by XmiliaH.
>
> (cherry-picked from commit 7b994e0ee0399caf6319865bbac88ddf62129a36)
>
> Fold optimization x - (-0) ==> x is INVALID for x = -0 in FP arithmetic.
> Its result is -0 instead of +0. This patch allows only x - (+0) ==> x
> optimization.
>
> Sergey Kaplun:
> * added the description and the test for the problem
>
> Part of tarantool/tarantool#9145
> ---
>
> Branch: https://github.com/tarantool/luajit/tree/skaplun/lj-783-fix-fold-x-0
> Tarantool PR: https://github.com/tarantool/tarantool/pull/9320
> Related Issues:
> * https://github.com/tarantool/tarantool/issues/9145
> * https://github.com/LuaJIT/LuaJIT/pull/783
>
>   src/lj_opt_fold.c                            |  3 +--
>   test/tarantool-tests/lj-783-fold--0.test.lua | 28 ++++++++++++++++++++
>   2 files changed, 29 insertions(+), 2 deletions(-)
>   create mode 100644 test/tarantool-tests/lj-783-fold--0.test.lua
>
> diff --git a/src/lj_opt_fold.c b/src/lj_opt_fold.c
> index 09e6c87b..944a9ecc 100644
> --- a/src/lj_opt_fold.c
> +++ b/src/lj_opt_fold.c
> @@ -1005,8 +1005,7 @@ LJFOLDF(simplify_numadd_xneg)
>   LJFOLD(SUB any KNUM)
>   LJFOLDF(simplify_numsub_k)
>   {
> -  lua_Number n = knumright;
> -  if (n == 0.0)  /* x - (+-0) ==> x */
> +  if (ir_knum(fright)->u64 == 0)  /* x - (+0) ==> x */
>       return LEFTFOLD;
>     return NEXTFOLD;
>   }
> diff --git a/test/tarantool-tests/lj-783-fold--0.test.lua b/test/tarantool-tests/lj-783-fold--0.test.lua
> new file mode 100644
> index 00000000..d6b4b493
> --- /dev/null
> +++ b/test/tarantool-tests/lj-783-fold--0.test.lua
> @@ -0,0 +1,28 @@
> +local tap = require('tap')
> +
> +-- Test file to demonstrate LuaJIT's incorrect fold optimization
> +-- x - (-0) ==> x.
> +-- See also https://github.com/LuaJIT/LuaJIT/issues/783.
> +local test = tap.test('lj-783-fold--0'):skipcond({
> +  ['Test requires JIT enabled'] = not jit.status(),
> +})
> +
> +test:plan(2)
> +
> +-- XXX: Use the variable to avoid folding during parsing.
> +local minus_zero = -0
> +local results = {}
> +
> +jit.opt.start('hotloop=1')
> +
> +for i = 1, 4 do
> +  results[i] = tostring(minus_zero - (-0))
> +end
> +
> +-- Fold optimization x - (-0) ==> x is INVALID for x = -0 in FP
> +-- arithmetic. Its result is -0 instead of +0.
> +
> +test:is(results[1], '0', 'correct VM value for -0 - (-0)')
> +test:samevalues(results, '-0 folding in simplify_numsub_k')
> +
> +test:done(true)