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

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

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Hi, Sergey!
Thanks for the patch!
Please consider my comments below.
 
> 
>>From: Mike Pall <mike>
>>
>>(cherry picked from commit 522d2073da4be2af79db4728cbb375db0fbdfc48)
>>
>>`asm_intarith()` function may try to drop `test r, r` instruction before
>Please note that "r" is an allocated register for the instruction.
>>the Jcc instruction. However, in case when Jcc instruction is "Jump
>Typo: s/in case when/in cases where/
>>short if ..." instruction (i.e. has no 0F opcode prefix like "Jump near
>>if ..."), the `test` instruction is dropped when shouldn't be, due to
>Typo: s/when/when it/
>>memory miss. As the result, the loop can't be realigned later in
>Typo: s/memory/a memory/
>Also, that part about the memory miss is unclear, it would be better if you
>could clarify it a bit.
>>`asm_loop_fixup` due to target to jump isn't aligned and the assertion
>Typo: s/isn’t aligned/being misaligned/
>>fails.
>>
>>This patch adds the additional check for 0F opcode in `asm_intarith()`.
>Typo: s/for 0F/for the 0F/
>>
>>Sergey Kaplun:
>>* added the description and the test for the problem
>>
>>Part of tarantool/tarantool#8069
>>---
>> 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
>--
>Best regards,
>Maxim Kokryashkin
> 

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