Re: [Tarantool-patches] [PATCH luajit 3/3] x86/x64: Fix loop realignment.

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

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Hi!

Thanks for the patch, it is LGTM.

Sergos


> 
> The new commit message is the following:
> 
> | x86/x64: Fix loop realignment.
> |
> | (cherry picked from commit 522d2073da4be2af79db4728cbb375db0fbdfc48)
> |
> | `asm_intarith()` function may try to drop `test r, r` (where `r` is an
> | allocated register) instruction before the Jcc instruction. However, in
> | cases when Jcc instruction is "Jump short if ..." instruction (i.e. has
> | no 0F opcode prefix like "Jump near if ..."), the `test` instruction is
> | dropped when it shouldn't be, due to usage for the comparison the next
> | byte after instruction itself. As the result, the loop can't be
> | realigned later in `asm_loop_fixup` due to target to jump being
> | misaligned and the assertion fails.
> |
> | This patch adds the additional check for the 0F opcode in
> | `asm_intarith()`.
> |
> | Sergey Kaplun:
> | * added the description and the test for the problem
> |
> | Part of tarantool/tarantool#8069
> 
> Branch is force pushed.
> 
>>>> ---
>>>>  src/lj_asm_x86.h | 5 +++--
>>>>  .../lj-556-fix-loop-realignment.test.lua | 18 ++++++++++++++++++
>>>>  2 files changed, 21 insertions(+), 2 deletions(-)
>>>>  create mode 100644 test/tarantool-tests/lj-556-fix-loop-realignment.test.lua
>>>> 
>>>> diff --git a/src/lj_asm_x86.h b/src/lj_asm_x86.h
>>>> index 8efda8e5..e6c42c6d 100644
>>>> --- a/src/lj_asm_x86.h
>>>> +++ b/src/lj_asm_x86.h
>>>> @@ -2068,8 +2068,9 @@ static void asm_intarith(ASMState *as, IRIns *ir, x86Arith xa)
>>>>    int32_t k = 0;
>>>>    if (as->flagmcp == as->mcp) { /* Drop test r,r instruction. */
>>>>      MCode *p = as->mcp + ((LJ_64 && *as->mcp < XI_TESTb) ? 3 : 2);
>>>> - if ((p[1] & 15) < 14) {
>>>> - if ((p[1] & 15) >= 12) p[1] -= 4; /* L <->S, NL <-> NS */
>>>> + MCode *q = p[0] == 0x0f ? p+1 : p;
>>>> + if ((*q & 15) < 14) {
>>>> + if ((*q & 15) >= 12) *q -= 4; /* L <->S, NL <-> NS */
>>>>        as->flagmcp = NULL;
>>>>        as->mcp = p;
>>>>      } /* else: cannot transform LE/NLE to cc without use of OF. */
>>>> diff --git a/test/tarantool-tests/lj-556-fix-loop-realignment.test.lua b/test/tarantool-tests/lj-556-fix-loop-realignment.test.lua
>>>> new file mode 100644
>>>> index 00000000..9a8e6098
>>>> --- /dev/null
>>>> +++ b/test/tarantool-tests/lj-556-fix-loop-realignment.test.lua
>>>> @@ -0,0 +1,18 @@
>>>> +local tap = require('tap')
>>>> +
>>>> +local test = tap.test('lj-505-fold-icorrect-behavior')
>>>> +test:plan(1)
>>>> +
>>>> +-- Test file to demonstrate JIT misbehaviour for loop realignment
>>>> +-- in LUAJIT_NUMMODE=2. See also
>>>> +--  https://github.com/LuaJIT/LuaJIT/issues/556 .
>>>> +
>>>> +jit.opt.start('hotloop=1')
>>>> +
>>>> +local s = 4
>>>> +while s > 0 do
>>>> + s = s - 1
>>>> +end
>>>> +
>>>> +test:ok(true, 'loop is compiled and ran successfully')
>>>> +os.exit(test:check() and 0 or 1)
>>>> --
>>> The test works just fine with HEAD on 
>>> f7d61d96  ci: introduce workflow for exotic builds.
>>>  
>>> Tested configurations: 
>>> LJ_64: True, LJ_GC64: True, LJ_DUALNUM: True
>>> LJ_64: True, LJ_GC64: False, LJ_DUALNUM: True
> 
> It's strange...
> I use the following build command:
> | $ cmake . -DCMAKE_BUILD_TYPE=Debug -DLUA_USE_APICHECK=ON -DLUA_USE_ASSERT=ON -DLUAJIT_ENABLE_GC64=OFF -DLUAJIT_NUMMODE=2 && make -j
> and get the following assertion:
> | asm_loop_fixup: Assertion `((intptr_t)target & 15) == 0' failed.
> What command do you use to build LuaJIT?
> 
>>> --
>>> Best regards,
>>> Maxim Kokryashkin
>>>  
> 
> -- 
> Best regards,
> Sergey Kaplun


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