Re: [Tarantool-patches] [PATCH luajit] Prevent CSE of a REF_BASE operand across IR_RETF.

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

Hi, Sergey!
Thanks for the review!
I've fixed your comment regarding test comment and force-pushed the
branch.

On 29.11.23, Sergey Bronnikov wrote:
> 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

Since this is only a remider for review in the ML, I've not changed it:).
I suppose that to mention this information in the commit message is
excess and important only for clarification on review.

> 
> >    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.

Fixed, branch is force-pushed.

===================================================================
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
index 095376fc..d6442cbb 100644
--- 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
@@ -37,7 +37,7 @@ test:plan(1)
 --
 -- 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.
+-- in the resulted IR 0012 below.
 
 local MAGIC = 42
 -- XXX: simplify `jit.dump()` output.
===================================================================

> 
> > +
> > +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)

-- 
Best regards,
Sergey Kaplun