Re: [Tarantool-patches] [PATCH luajit] Fix maxslots when recording BC_VARG, part 3.

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

Hi, Sergey

thanks for the patch! See comment inline.


On 8/15/23 15:32, Sergey Kaplun wrote:
> From: Mike Pall <mike>
>
> Thanks to Peter Cawley.
>
> (cherry-picked from commit abb27c7771947e082c9d919d184ad5f5f03e2e32)
>
> In case, when `BC_VARG` set the VARG slot to the non-top stack slot,
> `maxslot` value was unconditionally set to the destination slot, so some
> top slots may be omitted in the snapshot entry. Since these slots are
> omitted, they are not restored correctly, when restoring from snapshot
> for this side exit.
>
> This patch adds the check for the aforementioned case, to avoid maxslot
> shrinking.
>
> Sergey Kaplun:
> * added the description and the test for the problem
>
> Part of tarantool/tarantool#8825
> ---
>
> Branch: https://github.com/tarantool/luajit/tree/skaplun/lj-1046-fix-bc-varg-recording
> PR: https://github.com/tarantool/tarantool/pull/8986
> Related issues:
> * https://github.com/LuaJIT/LuaJIT/issues/1046
> * https://github.com/tarantool/tarantool/issues/8825
>
>   src/lj_record.c                               | 12 +++-
>   .../lj-1046-fix-bc-varg-recording.test.lua    | 58 +++++++++++++++++++
>   2 files changed, 67 insertions(+), 3 deletions(-)
>   create mode 100644 test/tarantool-tests/lj-1046-fix-bc-varg-recording.test.lua
>
> diff --git a/src/lj_record.c b/src/lj_record.c
> index 34d1210a..6bcdb04c 100644
> --- a/src/lj_record.c
> +++ b/src/lj_record.c
> @@ -1807,8 +1807,12 @@ static void rec_varg(jit_State *J, BCReg dst, ptrdiff_t nresults)
>     if (J->framedepth > 0) {  /* Simple case: varargs defined on-trace. */
>       ptrdiff_t i;
>       if (nvararg < 0) nvararg = 0;
> -    if (nresults == -1) nresults = nvararg;
> -    J->maxslot = dst + (BCReg)nresults;
> +    if (nresults != 1) {
> +      if (nresults == -1) nresults = nvararg;
> +      J->maxslot = dst + (BCReg)nresults;
> +    } else if (dst >= J->maxslot) {
> +      J->maxslot = dst + 1;
> +    }
>       for (i = 0; i < nresults; i++)
>         J->base[dst+i] = i < nvararg ? getslot(J, i - nvararg - 1 - LJ_FR2) : TREF_NIL;
>     } else {  /* Unknown number of varargs passed to trace. */
> @@ -1840,7 +1844,9 @@ static void rec_varg(jit_State *J, BCReg dst, ptrdiff_t nresults)
>         }
>         for (i = nvararg; i < nresults; i++)
>   	J->base[dst+i] = TREF_NIL;
> -      J->maxslot = dst + (BCReg)nresults;
> +      if (nresults != 1 || dst >= J->maxslot) {
> +	J->maxslot = dst + (BCReg)nresults;
> +      }
>       } else if (select_detect(J)) {  /* y = select(x, ...) */
>         TRef tridx = J->base[dst-1];
>         TRef tr = TREF_NIL;
> diff --git a/test/tarantool-tests/lj-1046-fix-bc-varg-recording.test.lua b/test/tarantool-tests/lj-1046-fix-bc-varg-recording.test.lua
> new file mode 100644
> index 00000000..34c5c572
> --- /dev/null
> +++ b/test/tarantool-tests/lj-1046-fix-bc-varg-recording.test.lua
> @@ -0,0 +1,58 @@
> +local tap = require('tap')
> +local test = tap.test('lj-1046-fix-bc-varg-recording'):skipcond({
> +  ['Test requires JIT enabled'] = not jit.status(),
> +})
> +
> +test:plan(2)
> +
> +jit.opt.start('hotloop=1')
> +
> +-- luacheck: ignore
> +local anchor
> +local N_ITER = 5
> +local SIDE_ITER = N_ITER - 1
> +for i = 1, N_ITER do
> +  -- In case, when `BC_VARG` set the VARG slot to the non-top
> +  -- stack slot, `maxslot` value was unconditionally set to the
> +  -- destination slot, so the following snapshot is used:
> +  -- SNAP   #4   [ ---- ---- ---- nil  ]
> +  -- instead of:
> +  -- SNAP   #4   [ ---- nil  ---- ---- 0009 0001 ---- 0009 ]
> +  -- Since these slots are omitted, they are not restored
> +  -- correctly, when restoring from snapshot for this side exit.
> +  anchor = ...
> +  if i > SIDE_ITER then
> +    -- XXX: Don't use `test:ok()` here to avoid double-running of
> +    -- tests in case of `i` incorrect restoring from the snapshot.
> +    assert(i > SIDE_ITER)
> +  end
> +end
> +
> +test:ok(true, 'BC_VARG recording 0th frame depth, 1 result')

This always passed testcase is confused and it will confuse everyone who 
will use test.

I propose to replace test:ok() to test:diag:

diff --git a/test/tarantool-tests/lj-1046-fix-bc-varg-recording.test.lua 
b/test/tarantool-tests/lj-1046-fix-bc-varg-recording.test.lua
index 30a87e54..32566c54 100644
--- a/test/tarantool-tests/lj-1046-fix-bc-varg-recording.test.lua
+++ b/test/tarantool-tests/lj-1046-fix-bc-varg-recording.test.lua
@@ -3,7 +3,7 @@ local test = 
tap.test('lj-1046-fix-bc-varg-recording'):skipcond({
    ['Test requires JIT enabled'] = not jit.status(),
  })

-test:plan(2)
+test:plan(0)

  jit.opt.start('hotloop=1')

@@ -49,7 +49,7 @@ for i = 1, N_ITER do
    end
  end

-test:ok(true, 'BC_VARG recording 0th frame depth, 1 result')
+test:diag('BC_VARG recording 0th frame depth, 1 result')

  -- Now the same case, but with an additional frame, so VARG slots
  -- are defined on the trace.
@@ -67,6 +67,6 @@ for i = 1, N_ITER do
    varg_frame(nil, i, SIDE_ITER)
  end

-test:ok(true, 'BC_VARG recording with VARG slots defined on trace, 1 
result')
+test:diag('BC_VARG recording with VARG slots defined on trace, 1 result')

  test:done(true)


or you can define a global variable and replace assert with assigning 
result to this variable.

then check expected and actual value for global variable using test:ok().

something like this:


  test:ok(true, 'BC_VARG recording 0th frame depth, 1 result')

+local varg_recorded = false
+
  -- Now the same case, but with an additional frame, so VARG slots
  -- are defined on the trace.
  local function varg_frame(anchor, i, side_iter, ...)
    anchor = ...
-  if i > side_iter then
-    -- XXX: `test:ok()` isn't used here to avoid double-running of
-    -- tests in case of `i` incorrect restoration from the
-    -- snapshot.
-    assert(i > side_iter)
-  end
+  varg_recorded = i > side_iter
  end

  for i = 1, N_ITER do
    varg_frame(nil, i, SIDE_ITER)
  end

-test:ok(true, 'BC_VARG recording with VARG slots defined on trace, 1 
result')
+test:ok(varg_recorded, 'BC_VARG recording with VARG slots defined on 
trace, 1 result')

  test:done(true)


> +
> +-- Now the same case, but with an additional frame, so VARG slots
> +-- are defined on the trace.
> +local function varg_frame(anchor, i, side_iter, ...)
> +  anchor = ...
> +  -- In case, when `BC_VARG` set the VARG slot to the non-top
> +  -- stack slot, `maxslot` value was unconditionally set to the
> +  -- destination slot, so the following snapshot is used:
> +  -- SNAP   #4   [ <snipped> | nil  nil  nil  `varg_frame` | nil ]
> +  -- instead of:
> +  -- SNAP   #4   [ <snipped> | nil  nil  nil  `varg_frame` | nil 0009 0005 ]
> +  -- Since these slots are omitted, they are not restored
> +  -- correctly, when restoring from snapshot for this side exit.
> +  if i > side_iter then
> +    -- XXX: Don't use `test:ok()` here to avoid double-running of
> +    -- tests in case of `i` incorrect restoring from the snapshot.
> +    assert(i > side_iter)
> +  end
> +end
> +
> +for i = 1, N_ITER do
> +  varg_frame(nil, i, SIDE_ITER)
> +end
> +
> +test:ok(true, 'BC_VARG recording with VARG slots defined on trace, 1 result')
> +
> +test:done(true)