[Tarantool-patches] [PATCH luajit] Maintain chain invariant in DCE.

[Sergey Kaplun]

From: Mike Pall <mike>

Thanks to Peter Cawley.

(cherry picked from commit f72c19e482b6f918b7cf42b0436e2b117d160a29)

Instructions with strong guards that are sometimes emitted with a guard
and sometimes emitted without a guard (like HREFK, CONV, or SLOAD) may
be eliminated from the IR chain and replaced with the NOP IR. If the
next IR of the same kind on the trace is not eliminated, it may
reference the IR NOP instead of an instruction of the same type. This
may lead to the corresponding assertion failure in the `rec_check_ir()`.

This patch unconditionally links the IRs during chain maintenance in
DCE.

Sergey Kaplun:
* added the description and the test for the problem

Part of tarantool/tarantool#9924
---

Be aware that the reproducer from the ticket does not lead to the
assertion failure (this is why it is omitted in the test). I suppose it
just illustrates the situation when the IR is left off the chain.
Although the reproducer is clumsy, I can't simplify it or make it less
tricky. Please, ideas are welcome :).

Branch: https://github.com/tarantool/luajit/tree/skaplun/lj-1094-ir-chain-dce
Related Issues:
* https://github.com/tarantool/tarantool/issues/9924
* https://github.com/LuaJIT/LuaJIT/issues/1094

 src/lj_opt_dce.c                              |  2 +-
 .../lj-1094-ir-chain-dce.test.lua             | 51 +++++++++++++++++++
 2 files changed, 52 insertions(+), 1 deletion(-)
 create mode 100644 test/tarantool-tests/lj-1094-ir-chain-dce.test.lua

diff --git a/src/lj_opt_dce.c b/src/lj_opt_dce.c
index 2417f324..6948179c 100644
--- a/src/lj_opt_dce.c
+++ b/src/lj_opt_dce.c
@@ -44,7 +44,6 @@ static void dce_propagate(jit_State *J)
     IRIns *ir = IR(ins);
     if (irt_ismarked(ir->t)) {
       irt_clearmark(ir->t);
-      pchain[ir->o] = &ir->prev;
     } else if (!ir_sideeff(ir)) {
       *pchain[ir->o] = ir->prev;  /* Reroute original instruction chain. */
       ir->t.irt = IRT_NIL;
@@ -53,6 +52,7 @@ static void dce_propagate(jit_State *J)
       ir->prev = 0;
       continue;
     }
+    pchain[ir->o] = &ir->prev;
     if (ir->op1 >= REF_FIRST) irt_setmark(IR(ir->op1)->t);
     if (ir->op2 >= REF_FIRST) irt_setmark(IR(ir->op2)->t);
   }
diff --git a/test/tarantool-tests/lj-1094-ir-chain-dce.test.lua b/test/tarantool-tests/lj-1094-ir-chain-dce.test.lua
new file mode 100644
index 00000000..3faae7d4
--- /dev/null
+++ b/test/tarantool-tests/lj-1094-ir-chain-dce.test.lua
@@ -0,0 +1,51 @@
+local tap = require('tap')
+
+-- Test file to demonstrate LuaJIT's incorrect maintenance of the
+-- IR chain during DCE.
+-- See also: https://github.com/LuaJIT/LuaJIT/issues/1094.
+
+local test = tap.test('lj-1094-ir-chain-dce'):skipcond({
+  ['Test requires JIT enabled'] = not jit.status(),
+})
+
+test:plan(1)
+
+jit.opt.start('hotloop=1')
+
+-- XXX: The code below is generated by the fuzzer (locally) and
+-- simplified as much as possible.
+
+-- Simple noop function.
+local __newindex = function() end
+debug.setmetatable(0, {
+    __newindex = __newindex,
+})
+
+local counter = 0
+-- luacheck: no unused
+local tab = {}
+while true do
+  -- The loop is still not recorded because the guard always
+  -- fails.
+  -- So, just try to compile it and check that there is no
+  -- assertion failure.
+  if counter > 2 then break end
+  counter = counter + 1
+  -- The pattern `-#{}` allows us to get CONV IRs. The first
+  -- appearance of this IR (in the `(-#{}).key`) is considered
+  -- unused by the compiler due to the corresponding "noop"
+  -- `__newindex` function.
+  -- The second usage of conversion (`tab[-#{}]`) is guarded (to
+  -- the int type), so it can't be eliminated.
+  -- As a result, the 0048 CONV references the 0039 NOP IR after
+  -- DCE in the IR chain.
+  -- XXX: TDUP prevents the corresponding second usage from being
+  -- eliminated since the table insert semantics may change.
+  -- XXX: Use some numbers to simplify reading the `jit.dump`
+  -- output.
+  tab, tab[-#{}], (-#{}).key = {tdup = 'tdup'}, 1, 2
+end
+
+test:ok(true, 'no assertion failure')
+
+test:done(true)
-- 
2.45.0