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

[Maxim Kokryashkin]

Hi, Sergey!
Thanks for the patch!
LGTM
--
Best regards,
Maxim Kokryashkin
 
  
>Вторник, 31 октября 2023, 15:49 +03:00 от Sergey Kaplun <skaplun at tarantool.org>:
> 
>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)
>--
>2.42.0
 
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