Prevent CSE of a REF_BASE operand across IR_RETF.

Reported by XmiliaH.

(cherry-picked from commit e73916d)

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

Reviewed-by: Sergey Bronnikov <sergeyb@tarantool.org>
Reviewed-by: Maxim Kokryashkin <m.kokryashkin@tarantool.org>
Signed-off-by: Igor Munkin <imun@tarantool.org>
(cherry picked from commit 804f85a)

Author: Mike Pall

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

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 resulted IR 0012 below.
+
+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)