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 fixes!
LGTM
 
> 
>>Hi, Maxim!
>>
>>Thanks for the review!
>>
>>On 24.01.23, Maxim Kokryashkin wrote:
>>>
>>> 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/
>>
>>Fixed.
>>
>>> >>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/
>>
>>Fixed.
>>
>>> >>fails.
>>> >>
>>> >>This patch adds the additional check for 0F opcode in `asm_intarith()`.
>>> >Typo: s/for 0F/for the 0F/
>>
>>Fixed, thanks!
>>
>>> >>
>>> >>Sergey Kaplun:
>>> >>* added the description and the test for the problem
>>> >>
>>> >>Part of tarantool/tarantool#8069
>>
>>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?
>PEBKAC, I forgot to add the LUA_USE_ASSERT flag.
>>> >--
>>> >Best regards,
>>> >Maxim Kokryashkin
>>> > 
>>
>>--
>>Best regards,
>>Sergey Kaplun
>--
>Best regards,
>Maxim Kokryashkin
> 

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