Re: [Tarantool-patches] [PATCH luajit] Fix TNEW load forwarding with instable types.

[Maxim Kokryashkin via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

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Hi again!
I forgot to mention another important nit — the
Tarantool CI is incomplete, there are 20 runs missing.
I think you should trigger them again, just to make sure
everything is ok.
--
Best regards,
Maxim Kokryashkin
 
  
>Понедельник, 15 мая 2023, 13:30 +03:00 от Maxim Kokryashkin via Tarantool-patches <tarantool-patches@dev.tarantool.org>:
> 
>Hi, Sergey!
>Thanks for the patch!
>LGTM, except for several nits below.
> 
>> 
>>>From: Mike Pall <mike>
>>>
>>>(cherry picked from commit d4c0c6e17ef7edf1f2893bc807746b80612e63e9)
>>>
>>>During loop unrolling IR instructions are coped, substituted and
>>>re-emitted for the FOLD/CSE/etc. pipeline. Assume, we have the following
>>>loop-carried dependency: we create a new table or load some stored one
>>>depends on iteration number. In case when we copy the emitted ALOAD IR
>>Typo: s/depends on iteration/depending on the iteration/
>>Typo: s/In case when/If/
>>>instruction (from from the stored table), we copy it as is, including
>>Typo: s/from from/from/
>>>loading type. But the ALOAD from the new table created (on the previous
>>Typo: s/loading/load/
>>>iteration) should have nil type, so the assertion in `fwd_ahload()` is
>>>failed.
>>>
>>>This patch replaces the assertion to `return 0` to avoid the assertion
>>Typo: s/assertion to/assertion with/
>>>failure and stop forwarding in such situations.
>>>
>>>But the emitted type-guarded ALOAD will lead to the persistent failure
>>>of the assertion guard on the trace. Hence, another one fix should be
>>Typo: s/another one/another/
>>>added. Also, TDUP IR is affected, too.
>>>See also the issue mentioned in the test.
>>>
>>>Sergey Kaplun:
>>>* added the description and the test for the problem
>>>
>>>Part of tarantool/tarantool#8516
>>>---
>>>
>>>Branch:  https://github.com/tarantool/luajit/tree/skaplun/lj-994-instable-types-during-loop-unroll
>>>PR:  https://github.com/tarantool/tarantool/pull/8642
>>>Related issues:
>>>*  https://github.com/tarantool/tarantool/issues/8516
>>>*  https://github.com/LuaJIT/LuaJIT/issues/994
>>>
>>>I don't mention the 994 intentionally to avoid Mike bothering. Also, it
>>>isn't the origin of this commit. Quite the opposite: as a result of this
>>>backporting the following issue was created.
>>>
>>>I prefer to backport the patches (this one and the prospective for 994)
>>>separately. So, after this patch is backported, it doesn't add any
>>>critical bugs (always failing guard just creates a new side trace), but
>>>helps to avoid conflicts for future backporting (sadly remembered
>>>"Improve assertions.").
>>>
>>> src/lj_opt_mem.c | 3 +-
>>> ...instable-types-during-loop-unroll.test.lua | 53 +++++++++++++++++++
>>> 2 files changed, 55 insertions(+), 1 deletion(-)
>>> create mode 100644 test/tarantool-tests/lj-994-instable-types-during-loop-unroll.test.lua
>>>
>>>diff --git a/src/lj_opt_mem.c b/src/lj_opt_mem.c
>>>index cc177d39..c8265b4f 100644
>>>--- a/src/lj_opt_mem.c
>>>+++ b/src/lj_opt_mem.c
>>>@@ -180,7 +180,8 @@ static TRef fwd_ahload(jit_State *J, IRRef xref)
>>>  }
>>>  ref = store->prev;
>>>       }
>>>- lua_assert(ir->o != IR_TNEW || irt_isnil(fins->t));
>>>+ if (ir->o == IR_TNEW && !irt_isnil(fins->t))
>>>+ return 0; /* Type instability in loop-carried dependency. */
>>>       if (irt_ispri(fins->t)) {
>>>  return TREF_PRI(irt_type(fins->t));
>>>       } else if (irt_isnum(fins->t) || (LJ_DUALNUM && irt_isint(fins->t)) ||
>>>diff --git a/test/tarantool-tests/lj-994-instable-types-during-loop-unroll.test.lua b/test/tarantool-tests/lj-994-instable-types-during-loop-unroll.test.lua
>>>new file mode 100644
>>>index 00000000..435f6e0e
>>>--- /dev/null
>>>+++ b/test/tarantool-tests/lj-994-instable-types-during-loop-unroll.test.lua
>>>@@ -0,0 +1,53 @@
>>>+local tap = require('tap')
>>>+
>>>+local test = tap.test('lj-994-instable-types-during-loop-unroll'):skipcond({
>>>+ ['Test requires JIT enabled'] = not jit.status(),
>>>+})
>>>+
>>>+-- Test file to demonstrate LuaJIT misbehaviour during loop
>>>+-- unrolling and load forwarding for newly created tables.
>>>+-- See also  https://github.com/LuaJIT/LuaJIT/issues/994 .
>>>+
>>>+-- TODO: test that compiled traces don't always exit by the type
>>>+-- guard. See also the comment for the TDUP test chunk.
>>>+test:plan(1)
>>>+
>>>+-- TNEW.
>>>+local result
>>>+local stored_tab = {1}
>>>+local slot = {}
>>>+local key = 1
>>>+
>>>+jit.opt.start('hotloop=1')
>>>+-- The trouble happens during loop unrolling when we copy
>>>+-- `>+ num ALOAD` IR in the context of the table on the previous
>>>+-- iteration instead of a new one created via TNEW containing no
>>>+-- values (so type nil should be used instead of num).
>>>+for _ = 1, 5 do
>>>+ local t = slot
>>>+ -- Use non-constant key to avoid LJ_TRERR_GFAIL and undoing the
>>>+ -- loop.
>>Typo: s/Use/Use a/
>>>+ result = t[key]
>>>+ -- Swap table loaded by SLOAD to the created via TNEW.
>>>+ slot = _ % 2 ~= 0 and stored_tab or {}
>>>+end
>>>+test:is(result, nil, 'TNEW load forwarding was successful')
>>Nit: not sure about that assertion. AFAICS, we want to check whether we
>>have failed on assertion and not the validity of `result` variable. Feel free
>>to ignore.
>>>+
>>>+-- TDUP.
>>>+--[[
>>>+-- FIXME: Disable test case for now. Enable, with another one
>>Typo: s/another one/another/
>>>+-- backported commit with a fix for the aforementioned issue
>>>+-- (and after patch "Improve assertions.").
>>>+-- Test taken trace exits too.
>>>+for _ = 1, 5 do
>>>+ local t = slot
>>>+ -- Now use constant key slot to get necessary branch.
>>>+ -- LJ_TRERR_GFAIL isn't triggered here.
>>>+ -- See `fwd_ahload()` in <src/lj_opt_mem.c> for details.
>>>+ result = t[1]
>>>+ slot = _ % 2 ~= 0 and stored_tab or {true}
>>>+end
>>>+test:is(result, true, 'TDUP load forwarding was successful')
>>>+]]
>>Nit: Not sure if it is good to add this test here. Maybe it is better
>>to create a ticket in our repo and move that chunk there. Feel free
>>to ignore.
>>>+
>>>+os.exit(test:check() and 0 or 1)
>>>--
>>>2.34.1
>>--
>>Best regards,
>>Maxim Kokryashkin
>> 
 

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