From: Sergey Bronnikov via Tarantool-patches <tarantool-patches@dev.tarantool.org> To: Sergey Kaplun <skaplun@tarantool.org>, Maxim Kokryashkin <m.kokryashkin@tarantool.org> Cc: tarantool-patches@dev.tarantool.org Subject: Re: [Tarantool-patches] [PATCH luajit] Prevent CSE of a REF_BASE operand across IR_RETF. Date: Wed, 29 Nov 2023 17:26:54 +0300 [thread overview] Message-ID: <07734b67-027a-4cc8-9c8a-03cd77a54137@tarantool.org> (raw) In-Reply-To: <20231128122112.16229-1-skaplun@tarantool.org> [-- Attachment #1: Type: text/plain, Size: 6549 bytes --] Hello, Sergey thanks for the patch! LGTM with a three minor comments below On 11/28/23 15:21, Sergey Kaplun wrote: > From: Mike Pall <mike> > > Reported by XmiliaH. > > (cherry-picked from commit e73916d811710ab02a4dfe447d621c99f4e7186c) > > The RETF IR has a side effect: it shifts base when returning to a lower > frame, i.e., it affects `REF_BASE` IR (0000) (thus, we can say that this > IR is violating SSA form). So any optimization of IRs with `REF_BASE` as > an operand across RETF IR may lead to incorrect optimizations (see > details in the test file). > > This patch adds rules to the folding engine to prevent CSE across `IR_RETF` > for all possible IRs containing REF_BASE. > > 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-784-cse-ref-base-over-retf > Tarantool PR:https://github.com/tarantool/tarantool/pull/9421 > Related issues: > *https://github.com/LuaJIT/LuaJIT/issues/784 > *https://github.com/tarantool/tarantool/issues/9145 > > Interested reviewers can mention that only the `SUB any BASE` case is > tested. > The reason is that other cases are impossible to record in LuaJIT: > * EQ any BASE: EQ pgc REF_BASE IR for upvalues is emitted when > the open upvalue aliases a SSA slot, i.e., it belongs to the frame of > the currently executed function. In that case, if we want to emit RETF > IR, we need to leave this function. So we need to record the UCLO > bytecode, which is NIY in JIT. So, such a type of trace is impossible. > * SUB BASE any: SUB BASE fr is emitted for the recording of VARG Nit: fr -> frame or put in backticks if you refer to a variable in source code > bytecode, in case varargs are undefined on trace. We need a vararg > function to call to create an additional frame. But returning to lower > frames from a vararg function isn't implemented in LuaJIT -- either > the trace recording is stopped or the error is rased and the trace > isn't compiled. Also, IINM, fr operands will always be different for Nit: fr -> frame or put in backticks if you refer to a variable in source code > different frames, so there is no possible CSE here. > > So, these cases are needed to prevent any regressions in the future. > > Please correct me if I've missed something. > > src/lj_opt_fold.c | 11 +++ > .../lj-784-cse-ref-base-over-retf.test.lua | 86 +++++++++++++++++++ > 2 files changed, 97 insertions(+) > create mode 100644 test/tarantool-tests/lj-784-cse-ref-base-over-retf.test.lua > > diff --git a/src/lj_opt_fold.c b/src/lj_opt_fold.c > index c5f2232e..750f1c7e 100644 > --- a/src/lj_opt_fold.c > +++ b/src/lj_opt_fold.c > @@ -2313,6 +2313,17 @@ LJFOLDF(xload_kptr) > LJFOLD(XLOAD any any) > LJFOLDX(lj_opt_fwd_xload) > > +/* -- Frame handling ------------------------------------------------------ */ > + > +/* Prevent CSE of a REF_BASE operand across IR_RETF. */ > +LJFOLD(SUB any BASE) > +LJFOLD(SUB BASE any) > +LJFOLD(EQ any BASE) > +LJFOLDF(fold_base) > +{ > + return lj_opt_cselim(J, J->chain[IR_RETF]); > +} > + > /* -- Write barriers ------------------------------------------------------ */ > > /* Write barriers are amenable to CSE, but not across any incremental > diff --git a/test/tarantool-tests/lj-784-cse-ref-base-over-retf.test.lua b/test/tarantool-tests/lj-784-cse-ref-base-over-retf.test.lua > new file mode 100644 > index 00000000..095376fc > --- /dev/null > +++ b/test/tarantool-tests/lj-784-cse-ref-base-over-retf.test.lua > @@ -0,0 +1,86 @@ > +local tap = require('tap') > + > +-- Test file to demonstrate incorrect FOLD optimization for IR > +-- with REF_BASE operand across IR RETF. > +-- See also,https://github.com/LuaJIT/LuaJIT/issues/784. > + > +local test = tap.test('lj-784-cse-ref-base-over-retf'):skipcond({ > + ['Test requires JIT enabled'] = not jit.status(), > +}) > + > +test:plan(1) > + > +-- The RETF IR has a side effect: it shifts base when returning to > +-- a lower frame, i.e., it affects `REF_BASE` IR (0000) (thus, we > +-- can say that this IR is violating SSA form). > +-- So any optimization of IRs with `REF_BASE` as an operand across > +-- RETF IR may lead to incorrect optimizations. > +-- In this test, SUB uref REF_BASE IR was eliminated, so instead > +-- the following trace: > +-- > +-- 0004 p32 SUB 0003 0000 > +-- 0005 > p32 UGT 0004 +32 > +-- ... > +-- 0009 > p32 RETF proto: 0x407dc118 [0x407dc194] > +-- ... > +-- 0012 p32 SUB 0003 0000 > +-- 0013 > p32 UGT 0012 +72 > +-- > +-- We got the following: > +-- > +-- 0004 p32 SUB 0003 0000 > +-- 0005 > p32 UGT 0004 +32 > +-- ... > +-- 0009 > p32 RETF proto: 0x41ffe0c0 [0x41ffe13c] > +-- ... > +-- 0012 > p32 UGT 0004 +72 > +-- > +-- As you can see, the 0012 SUB IR is eliminated because it is the > +-- same as the 0004 IR. This leads to incorrect assertion guards > +-- in the IR below. I would rephrase it to "assertion guards in the resulted IR" because there is no IR below the comment. > + > +local MAGIC = 42 > +-- XXX: simplify `jit.dump()` output. > +local fmod = math.fmod > + > +local function exit_with_retf(closure) > + -- Forcify stitch. Any NYI is OK here. > + fmod(1, 1) > + -- Call the closure so that we have emitted `uref - REF_BASE`. > + closure(0) > + -- Exit with `IR_RETF`. This will change `REF_BASE`. > +end > + > +local function sub_uref_base(closure) > + local open_upvalue > + if closure == nil then > + closure = function(val) > + local old = open_upvalue > + open_upvalue = val > + return old > + end > + -- First, create an additional frame, so we got the trace, > + -- where the open upvalue reference is always < `REF_BASE`. > + sub_uref_base(closure) > + end > + for _ = 1, 4 do > + -- `closure` function is inherited from the previous frame. > + exit_with_retf(closure) > + open_upvalue = MAGIC > + -- The open upvalue guard will use CSE over `IR_RETF` for > + -- `uref - REF_BASE`. `IR_RETF` changed the value of > + -- `REF_BASE`. > + -- Thus, the guards afterwards take the wrong IR as the first > + -- operand, so they are not failed, and the wrong value is > + -- returned from the trace. > + open_upvalue = closure(0) > + end > + return open_upvalue > +end > + > +jit.opt.start('hotloop=1') > + > +local res = sub_uref_base() > +test:is(res, MAGIC, 'no SUB uref REF_BASE CSE across RETF') > + > +test:done(true) [-- Attachment #2: Type: text/html, Size: 8105 bytes --]
next prev parent reply other threads:[~2023-11-29 14:26 UTC|newest] Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-11-28 12:21 Sergey Kaplun via Tarantool-patches 2023-11-29 14:26 ` Sergey Bronnikov via Tarantool-patches [this message] 2023-11-30 7:34 ` Sergey Kaplun via Tarantool-patches 2023-11-30 8:53 ` Sergey Bronnikov via Tarantool-patches 2023-11-30 17:59 ` Maxim Kokryashkin via Tarantool-patches 2024-01-10 8:51 ` Igor Munkin via Tarantool-patches
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