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

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

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?

> >--
> >Best regards,
> >Maxim Kokryashkin
> > 

-- 
Best regards,
Sergey Kaplun