Re: [Tarantool-patches] [PATCH luajit] Fix string.char() recording with no arguments.

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

Hi! Thanks for the patch!

Some readability comments below.

regards,
Sergos


> On 20 Aug 2021, at 18:48, Sergey Kaplun <skaplun@tarantool.org> wrote:
> 
> From: Mike Pall <mike>
> 
> (cherry picked from commit dfa692b746c9de067857d5fc992a41730be3d99a)
> 
> `string.char()` call without arguments yields an empty string. When JIT
> machinery records the aforementioned call it doesn't handle this case.
JIT recording machinery doesn’t handle this case.

> Each recording fast function expects 1 result by default.  Hence, when
                ^
               of a 

> return from this call is recorded the framelink slot is used as a
> result. It is loaded into the corresponding slot as an IR with `IRT_NUM`

I have a question here: is this number denotes the number of results?
Then, perhaps reword the previous sentence that this very number is 
considered as result.

> type. It leads to assertion failure in `rec_check_slots()`, when a next
> bytecode is recorded, because type of TValue on the stack (`LJ_STR`)
> isn't the same as IR (and TRef) type.
> 
> This patch handles the case without arguments by the loading of IR with
> empty string reference into the corresponding slot.
> 
I would add that code reuses assumption of one result by default,
hence no case for ‘i == 1’ in the code.

> Sergey Kaplun:
> * added the description and the test for the problem
> 
> Resolves tarantool/tarantool#6371
> ---
> 
> Branch: https://github.com/tarantool/luajit/tree/skaplun/gh-6371-string-char-no-arg
> Issue: https://github.com/tarantool/tarantool/issues/6371
> Tarantool branch: https://github.com/tarantool/tarantool/tree/skaplun/gh-6371-string-char-no-arg
> Side note: CI is totally red, but AFAICS it's unrelated with my patch.
> Side note: See also Changelog at the Tarantool branch.
> 
> src/lj_ffrecord.c                             |  2 ++
> .../gh-6371-string-char-no-arg.test.lua       | 28 +++++++++++++++++++
> 2 files changed, 30 insertions(+)
> create mode 100644 test/tarantool-tests/gh-6371-string-char-no-arg.test.lua
> 
> diff --git a/src/lj_ffrecord.c b/src/lj_ffrecord.c
> index 8dfa80ed..be890a93 100644
> --- a/src/lj_ffrecord.c
> +++ b/src/lj_ffrecord.c
> @@ -866,6 +866,8 @@ static void LJ_FASTCALL recff_string_char(jit_State *J, RecordFFData *rd)
>     for (i = 0; J->base[i] != 0; i++)
>       tr = emitir(IRT(IR_BUFPUT, IRT_PGC), tr, J->base[i]);
>     J->base[0] = emitir(IRT(IR_BUFSTR, IRT_STR), tr, hdr);
> +  } else if (i == 0) {
> +    J->base[0] = lj_ir_kstr(J, &J2G(J)->strempty);
>   }
>   UNUSED(rd);
> }
> diff --git a/test/tarantool-tests/gh-6371-string-char-no-arg.test.lua b/test/tarantool-tests/gh-6371-string-char-no-arg.test.lua
> new file mode 100644
> index 00000000..6df93f07
> --- /dev/null
> +++ b/test/tarantool-tests/gh-6371-string-char-no-arg.test.lua
> @@ -0,0 +1,28 @@
> +local tap = require('tap')
> +
> +-- Test file to demonstrate assertion after `string.char()`
> +-- recording.
> +-- See also, https://github.com/tarantool/tarantool/issues/6371.
> +
> +local test = tap.test('gh-6371-string-char-no-arg')
> +-- XXX: Number of loop iterations.
> +-- 1, 2 -- instruction becomes hot
> +-- 3 -- trace is recorded (considering loop recording specifics),
> +-- but bytecodes are still executed via VM
> +-- 4 -- trace is executed, need to check that emitted mcode is
> +--      correct
> +local NTEST = 4
> +test:plan(NTEST)
> +
> +-- Storage for the results to avoid trace aborting by `test:ok()`.
> +local results = {}
> +jit.opt.start('hotloop=1')
> +for _ = 1, NTEST do
> +  table.insert(results, string.char())
> +end
> +
> +for i = 1, NTEST do
> +  test:ok(results[i] == '', 'correct recording of string.char() without args')
> +end
> +
> +os.exit(test:check() and 0 or 1)
> -- 
> 2.31.0
>