Re: [Tarantool-patches] [PATCH luajit] Fix bytecode register allocation for comparisons.

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

Sergey,

Thanks for the fixes! LGTM now.

On 16.08.21, Sergey Kaplun wrote:
> Igor,
> 
> Thanks for the review!
> 
> See the new comment message above:
> ===================================================================
> Fix bytecode register allocation for comparisons.
> 
> (cherry picked from commit 2f3f07882fb4ad9c64967d7088461b1ca0a25d3a)
> 
> When LuaJIT is built with LJ_FR2 (e.g. with GC64 mode enabled),
> information about frame takes two slots -- the first takes the TValue
> with the function to be called, the second takes the framelink. The JIT
> recording machinery does pretty the same -- the function IR_KGC is
> loaded in the first slot, and the second is set to TREF_FRAME value.
> This value should be rewritten after return from a callee. This slot is
> cleared either by return values or manually (set to zero), when there
> are no values to return. The latter case is done by the next bytecode
> with RA dst mode. This obliges that the destination of RA takes the next
> slot after TREF_FRAME. Hence, this an earlier instruction must use the
> smallest possible destination register (see `lj_record_ins()` for the
> details).
> 
> Bytecode emitter swaps operands for ISGT and ISGE comparisons. As a
> result, the aforementioned rule for registers allocations may be
> violated. When it happens for a chunk being recorded, the slot with
> TREF_FRAME is not rewritten (but the next empty slot after TREF_FRAME
> is). This leads to JIT slots inconsistency and assertion failure in
> `rec_check_slots()` during recording of the next bytecode instruction.
> 
> This patch fixes bytecode register allocation by changing the VM
> register allocation order in case of ISGT and ISGE bytecodes.
> 
> Sergey Kaplun:
> * added the description and the test for the problem
> 
> Resolves tarantool/tarantool#6227
> Part of tarantool/tarantool#5629
> ===================================================================
> 
> On 16.08.21, Igor Munkin wrote:

<snipped>

> 
> > 
> > Furthermore, what does stop you from using local variables?
> 
> They occupy new slots and make it harder to maintain, see the new
> comment below.

Meh, OK anyway :)

> 
> > 

<snipped>

> 
> ===================================================================
> diff --git a/test/tarantool-tests/gh-6227-bytecode-allocator-for-comparisons.test.lua b/test/tarantool-tests/gh-6227-bytecode-allocator-for-comparisons.test.lua
> index 66f6885e..9788923a 100644
> --- a/test/tarantool-tests/gh-6227-bytecode-allocator-for-comparisons.test.lua
> +++ b/test/tarantool-tests/gh-6227-bytecode-allocator-for-comparisons.test.lua
> @@ -14,26 +14,39 @@ local function empty() end
>  local uv = 0
>  
>  -- This function needs to reset register enumerating.
> --- Also set `J->maxslot` to zero.
> --- The upvalue function to call is loaded to 0 slot.
> +-- `J->maxslot` is initialized with `nargs` (i.e. zero in this
> +-- case) in `rec_call_setup()`.
>  local function bump_frame()
>    -- First call function with RET0 to set TREF_FRAME in the
>    -- last slot.
>    empty()
> +  -- The old bytecode to be recorded looks like the following:
> +  -- 0000  . FUNCF    4
> +  -- 0001  . UGET     0   0      ; empty
> +  -- 0002  . CALL     0   1   1
> +  -- 0000  . . JFUNCF   1   1
> +  -- 0001  . . RET0     0   1
> +  -- 0002  . CALL     0   1   1
> +  -- 0003  . UGET     0   0      ; empty
> +  -- 0004  . UGET     3   1      ; uv
> +  -- 0005  . KSHORT   2   1
> +  -- 0006  . ISLT     3   2
>    -- Test ISGE or ISGT bytecode. These bytecodes swap their
> -  -- operands. Also, a constant is always loaded into the slot
> -  -- smaller than upvalue. So, if upvalue loads before KSHORT,
> -  -- then the difference between registers is more than 2 (2 is
> -  -- needed for LJ_FR2) and TREF_FRAME slot is not rewriting by
> -  -- the bytecode after call and return as expected. That leads
> -  -- to recording slots inconsistency and assertion failure at
> -  -- `rec_check_slots()`.
> +  -- operands (consider ISLT above).
> +  -- Two calls of `empty()` function in a row is necessary for 2
> +  -- slot gap in LJ_FR2 mode.
> +  -- Upvalue loads before KSHORT, so the difference between slot
> +  -- for upvalue `empty` (function to be called) and slot for
> +  -- upvalue `uv` is more than 2. Hence, TREF_FRAME slot is not
> +  -- rewritten by the bytecode after return from `empty()`
> +  -- function as expected. That leads to recording slots
> +  -- inconsistency and assertion failure at `rec_check_slots()`.
>    empty(1>uv)
>  end
>  
>  jit.opt.start('hotloop=1')
>  
> -for _ = 1,3 do
> +for _ = 1, 3 do
>    bump_frame()
>  end
> ===================================================================
> 

<snipped>

> 
> -- 
> Best regards,
> Sergey Kaplun

-- 
Best regards,
IM